Merge remote-tracking branch 'refs/remotes/drebs/feature/consistent-metadata' into develop

17 files changed, 1695 insertions, 787 deletions

diff --git a/common/src/leap/soledad/common/.gitignore b/common/src/leap/soledad/common/.gitignore
new file mode 100644
index 00000000..3378c78a
--- /dev/null
+++ b/common/src/leap/soledad/common/.gitignore
@@ -0,0 +1 @@
+ddocs.py
diff --git a/common/src/leap/soledad/common/README.txt b/common/src/leap/soledad/common/README.txt
new file mode 100644
index 00000000..106efb5e
--- /dev/null
+++ b/common/src/leap/soledad/common/README.txt
@@ -0,0 +1,79 @@
+Soledad common package
+======================
+
+This package contains Soledad bits used by both server and client.
+
+Couch U1DB Backend
+------------------
+
+U1DB backends rely on some atomic operations that modify documents contents
+and metadata (conflicts, transaction ids and indexes). The only atomic
+operation in Couch is a document put, so every u1db atomic operation has to be
+mapped to a couch document put.
+
+The atomic operations in the U1DB SQLite reference backend implementation may
+be identified by the use of a context manager to access the underlying
+database. A listing of the methods involved in each atomic operation are
+depiced below. The top-level elements correpond to the atomic operations that
+have to be mapped, and items on deeper levels of the list have to be
+implemented in a way that all changes will be pushed with just one operation.
+
+    * _set_replica_uid
+    * put_doc:
+        * _get_doc
+        * _put_and_update_indexes
+            * insert/update the document
+            * insert into transaction log
+    * delete_doc
+        * _get_doc
+        * _put_and_update_indexes
+    * get_doc_conflicts
+        * _get_conflicts
+    * _set_replica_gen_and_trans_id
+        * _do_set_replica_gen_and_trans_id
+    * _put_doc_if_newer
+        * _get_doc
+        * _validate_source (**)
+            * _get_replica_gen_and_trans_id
+        * cases:
+            * is newer:
+                * _prune_conflicts (**)
+                    * _has_conflicts
+                    * _delete_conflicts
+                * _put_and_update_indexes
+            * same content as:
+                * _put_and_update_indexes
+            * conflicted:
+                * _force_doc_sync_conflict
+                    * _prune_conflicts
+                    * _add_conflict
+                    * _put_and_update_indexes
+        * _do_set_replica_gen_and_trans_id
+    * resolve_doc
+        * _get_doc
+        * cases:
+            * doc is superseded
+                * _put_and_update_indexes
+            * else
+                * _add_conflict
+        * _delete_conflicts
+    * delete_index
+    * create_index
+
+Couch views and update functions are used in order to achieve atomicity on the
+Couch backend. Transactions are stored in the `u1db_transactions` field of the
+couch document. Document's content and conflicted versions are stored as couch
+document attachments with names, respectivelly, `u1db_content` and
+`u1db_conflicts`.
+
+A map of methods and couch query URI can be found on the `./ddocs/README.txt`
+document.
+
+Notes:
+
+  * Currently, the couch backend does not implement indexing, so what is
+    depicted as `_put_and_update_indexes` above will be found as `_put_doc` in
+    the backend.
+
+  * Conflict updates are part of document put using couch update functions,
+    and as such are part of the same atomic operation as document put.
diff --git a/common/src/leap/soledad/common/couch.py b/common/src/leap/soledad/common/couch.py
index 1396f4d7..d2414477 100644
--- a/common/src/leap/soledad/common/couch.py
+++ b/common/src/leap/soledad/common/couch.py
@@ -18,26 +18,24 @@
 
 """A U1DB backend that uses CouchDB as its persistence layer."""
 
-import re
 import simplejson as json
-import socket
+import re
+import uuid
 import logging
+import binascii
+import socket
 
 
-from u1db import errors
-from u1db.sync import Synchronizer
-from u1db.backends.inmemory import InMemoryIndex
-from u1db.remote.server_state import ServerState
-from u1db.errors import DatabaseDoesNotExist
-from couchdb.client import Server, Document as CouchDocument
+from couchdb.client import Server
 from couchdb.http import ResourceNotFound, Unauthorized
+from u1db import errors, query_parser, vectorclock
+from u1db.backends import CommonBackend, CommonSyncTarget
+from u1db.remote import http_app
+from u1db.remote.server_state import ServerState
 
 
-from leap.soledad.common import USER_DB_PREFIX
-from leap.soledad.common.objectstore import (
-    ObjectStoreDatabase,
-    ObjectStoreSyncTarget,
-)
+from leap.soledad.common import USER_DB_PREFIX, ddocs
+from leap.soledad.common.document import SoledadDocument
 
 
 logger = logging.getLogger(__name__)
@@ -49,158 +47,129 @@ class InvalidURLError(Exception):
     """
 
 
-def persistent_class(cls):
+class CouchDocument(SoledadDocument):
     """
-    Decorator that modifies a class to ensure u1db metadata persists on
-    underlying storage.
+    This is the document used for maintaining the Couch backend.
 
-    @param cls: The class that will be modified.
-    @type cls: type
+    A CouchDocument can fetch and manipulate conflicts and also holds a
+    reference to the couch document revision. This data is used to ensure an
+    atomic and consistent update of the database.
     """
 
-    def _create_persistent_method(old_method_name, key, load_method_name,
-                                  dump_method_name, store):
-        """
-        Create a persistent method to replace C{old_method_name}.
-
-        The new method will load C{key} using C{load_method_name} and stores
-        it using C{dump_method_name} depending on the value of C{store}.
-        """
-        # get methods
-        old_method = getattr(cls, old_method_name)
-        load_method = getattr(cls, load_method_name) \
-            if load_method_name is not None \
-            else lambda self, data: setattr(self, key, data)
-        dump_method = getattr(cls, dump_method_name) \
-            if dump_method_name is not None \
-            else lambda self: getattr(self, key)
-
-        def _new_method(self, *args, **kwargs):
-            # get u1db data from couch db
-            doc = self._get_doc('%s%s' %
-                                (self.U1DB_DATA_DOC_ID_PREFIX, key))
-            load_method(self, doc.content['content'])
-            # run old method
-            retval = old_method(self, *args, **kwargs)
-            # store u1db data on couch
-            if store:
-                doc.content = {'content': dump_method(self)}
-                self._put_doc(doc)
-            return retval
-
-        return _new_method
-
-    # ensure the class has a persistency map
-    if not hasattr(cls, 'PERSISTENCY_MAP'):
-        logger.error('Class %s has no PERSISTENCY_MAP attribute, skipping '
-                     'persistent methods substitution.' % cls)
-        return cls
-    # replace old methods with new persistent ones
-    for key, ((load_method_name, dump_method_name),
-              persistent_methods) in cls.PERSISTENCY_MAP.iteritems():
-        for (method_name, store) in persistent_methods:
-            setattr(cls, method_name,
-                    _create_persistent_method(
-                        method_name,
-                        key,
-                        load_method_name,
-                        dump_method_name,
-                        store))
-    return cls
-
-
-@persistent_class
-class CouchDatabase(ObjectStoreDatabase):
+    def __init__(self, doc_id=None, rev=None, json='{}', has_conflicts=False,
+                 syncable=True):
+        """
+        Container for handling a document that is stored in couch backend.
+
+        :param doc_id: The unique document identifier.
+        :type doc_id: str
+        :param rev: The revision identifier of the document.
+        :type rev: str
+        :param json: The JSON string for this document.
+        :type json: str
+        :param has_conflicts: Boolean indicating if this document has conflicts
+        :type has_conflicts: bool
+        :param syncable: Should this document be synced with remote replicas?
+        :type syncable: bool
+        """
+        SoledadDocument.__init__(self, doc_id, rev, json, has_conflicts)
+        self._couch_rev = None
+        self._conflicts = None
+        self._modified_conflicts = False
+
+    def ensure_fetch_conflicts(self, get_conflicts_fun):
+        """
+        Ensure conflict data has been fetched from the server.
+
+        :param get_conflicts_fun: A function which, given the document id and
+                                  the couch revision, return the conflicted
+                                  versions of the current document.
+        :type get_conflicts_fun: function
+        """
+        if self._conflicts is None:
+            self._conflicts = get_conflicts_fun(self.doc_id,
+                                                couch_rev=self.couch_rev)
+        self.has_conflicts = len(self._conflicts) > 0
+
+    def get_conflicts(self):
+        """
+        Get the conflicted versions of the document.
+
+        :return: The conflicted versions of the document.
+        :rtype: [CouchDocument]
+        """
+        return self._conflicts
+
+    def add_conflict(self, doc):
+        """
+        Add a conflict to this document.
+
+        :param doc: The conflicted version to be added.
+        :type doc: CouchDocument
+        """
+        if self._conflicts is None:
+            raise Exception("Run self.ensure_fetch_conflicts first!")
+        self._modified_conflicts = True
+        self._conflicts.append(doc)
+        self.has_conflicts = len(self._conflicts) > 0
+
+    def delete_conflicts(self, conflict_revs):
+        """
+        Delete conflicted versions of this document.
+
+        :param conflict_revs: The conflicted revisions to be deleted.
+        :type conflict_revs: [str]
+        """
+        if self._conflicts is None:
+            raise Exception("Run self.ensure_fetch_conflicts first!")
+        conflicts_len = len(self._conflicts)
+        self._conflicts = filter(
+            lambda doc: doc.rev not in conflict_revs,
+            self._conflicts)
+        if len(self._conflicts) < conflicts_len:
+            self._modified_conflicts = True
+        self.has_conflicts = len(self._conflicts) > 0
+
+    def modified_conflicts(self):
+        """
+        Return whether this document's conflicts have been modified.
+
+        :return: Whether this document's conflicts have been modified.
+        :rtype: bool
+        """
+        return self._conflicts is not None and \
+            self._modified_conflicts is True
+
+    def _get_couch_rev(self):
+        return self._couch_rev
+
+    def _set_couch_rev(self, rev):
+        self._couch_rev = rev
+
+    couch_rev = property(_get_couch_rev, _set_couch_rev)
+
+
+# monkey-patch the u1db http app to use CouchDocument
+http_app.Document = CouchDocument
+
+
+class CouchDatabase(CommonBackend):
     """
-    A U1DB backend that uses Couch as its persistence layer.
+    A U1DB implementation that uses CouchDB as its persistence layer.
     """
 
-    U1DB_TRANSACTION_LOG_KEY = '_transaction_log'
-    U1DB_CONFLICTS_KEY = '_conflicts'
-    U1DB_OTHER_GENERATIONS_KEY = '_other_generations'
-    U1DB_INDEXES_KEY = '_indexes'
-    U1DB_REPLICA_UID_KEY = '_replica_uid'
-
-    U1DB_DATA_KEYS = [
-        U1DB_TRANSACTION_LOG_KEY,
-        U1DB_CONFLICTS_KEY,
-        U1DB_OTHER_GENERATIONS_KEY,
-        U1DB_INDEXES_KEY,
-        U1DB_REPLICA_UID_KEY,
-    ]
-
-    COUCH_ID_KEY = '_id'
-    COUCH_REV_KEY = '_rev'
-    COUCH_U1DB_ATTACHMENT_KEY = 'u1db_json'
-    COUCH_U1DB_REV_KEY = 'u1db_rev'
-
-    # the following map describes information about methods usage of
-    # properties that have to persist on the underlying database. The format
-    # of the map is assumed to be:
-    #
-    #     {
-    #         'property_name': [
-    #             ('property_load_method_name', 'property_dump_method_name'),
-    #             [('method_1_name', bool),
-    #              ...
-    #              ('method_N_name', bool)]],
-    #         ...
-    #     }
-    #
-    # where the booleans indicate if the property should be stored after
-    # each method execution (i.e. if the method alters the property). Property
-    # load/dump methods will be run after/before properties are read/written
-    # to the underlying db.
-    PERSISTENCY_MAP = {
-        U1DB_TRANSACTION_LOG_KEY: [
-            ('_load_transaction_log_from_json', None),
-            [('_get_transaction_log', False),
-             ('_get_generation', False),
-             ('_get_generation_info', False),
-             ('_get_trans_id_for_gen', False),
-             ('whats_changed', False),
-             ('_put_and_update_indexes', True)]],
-        U1DB_CONFLICTS_KEY: [
-            (None, None),
-            [('_has_conflicts', False),
-             ('get_doc_conflicts', False),
-             ('_prune_conflicts', False),
-             ('resolve_doc', False),
-             ('_replace_conflicts', True),
-             ('_force_doc_sync_conflict', True)]],
-        U1DB_OTHER_GENERATIONS_KEY: [
-            ('_load_other_generations_from_json', None),
-            [('_get_replica_gen_and_trans_id', False),
-             ('_do_set_replica_gen_and_trans_id', True)]],
-        U1DB_INDEXES_KEY: [
-            ('_load_indexes_from_json', '_dump_indexes_as_json'),
-            [('list_indexes', False),
-             ('get_from_index', False),
-             ('get_range_from_index', False),
-             ('get_index_keys', False),
-             ('_put_and_update_indexes', True),
-             ('create_index', True),
-             ('delete_index', True)]],
-        U1DB_REPLICA_UID_KEY: [
-            (None, None),
-            [('_allocate_doc_rev', False),
-             ('_put_doc_if_newer', False),
-             ('_ensure_maximal_rev', False),
-             ('_prune_conflicts', False),
-             ('_set_replica_uid', True)]]}
-
     @classmethod
     def open_database(cls, url, create):
         """
         Open a U1DB database using CouchDB as backend.
 
-        @param url: the url of the database replica
-        @type url: str
-        @param create: should the replica be created if it does not exist?
-        @type create: bool
+        :param url: the url of the database replica
+        :type url: str
+        :param create: should the replica be created if it does not exist?
+        :type create: bool
 
-        @return: the database instance
-        @rtype: CouchDatabase
+        :return: the database instance
+        :rtype: CouchDatabase
         """
         # get database from url
         m = re.match('(^https?://[^/]+)/(.+)$', url)
@@ -213,301 +182,819 @@ class CouchDatabase(ObjectStoreDatabase):
             server[dbname]
         except ResourceNotFound:
             if not create:
-                raise DatabaseDoesNotExist()
+                raise errors.DatabaseDoesNotExist()
         return cls(url, dbname)
 
     def __init__(self, url, dbname, replica_uid=None, full_commit=True,
-                 session=None):
+                 session=None, ensure_ddocs=True):
         """
         Create a new Couch data container.
 
-        @param url: the url of the couch database
-        @type url: str
-        @param dbname: the database name
-        @type dbname: str
-        @param replica_uid: an optional unique replica identifier
-        @type replica_uid: str
-        @param full_commit: turn on the X-Couch-Full-Commit header
-        @type full_commit: bool
-        @param session: an http.Session instance or None for a default session
-        @type session: http.Session
+        :param url: the url of the couch database
+        :type url: str
+        :param dbname: the database name
+        :type dbname: str
+        :param replica_uid: an optional unique replica identifier
+        :type replica_uid: str
+        :param full_commit: turn on the X-Couch-Full-Commit header
+        :type full_commit: bool
+        :param session: an http.Session instance or None for a default session
+        :type session: http.Session
+        :param ensure_ddocs: Ensure that the design docs exist on server.
+        :type ensure_ddocs: bool
         """
         # save params
         self._url = url
         self._full_commit = full_commit
         self._session = session
+        self._factory = CouchDocument
+        self._real_replica_uid = None
         # configure couch
         self._server = Server(url=self._url,
                               full_commit=self._full_commit,
                               session=self._session)
         self._dbname = dbname
-        # this will ensure that transaction and sync logs exist and are
-        # up-to-date.
         try:
             self._database = self._server[self._dbname]
         except ResourceNotFound:
             self._server.create(self._dbname)
             self._database = self._server[self._dbname]
-        ObjectStoreDatabase.__init__(self, replica_uid=replica_uid)
+        if replica_uid is not None:
+            self._set_replica_uid(replica_uid)
+        if ensure_ddocs:
+            self.ensure_ddocs_on_db()
+
+    def ensure_ddocs_on_db(self):
+        """
+        Ensure that the design documents used by the backend exist on the
+        couch database.
+        """
+        # we check for existence of one of the files, and put all of them if
+        # that one does not exist
+        try:
+            self._database['_design/docs']
+            return
+        except ResourceNotFound:
+            for ddoc_name in ['docs', 'syncs', 'transactions']:
+                ddoc = json.loads(
+                    binascii.a2b_base64(
+                        getattr(ddocs, ddoc_name)))
+                self._database.save(ddoc)
+
+    def get_sync_target(self):
+        """
+        Return a SyncTarget object, for another u1db to synchronize with.
+
+        :return: The sync target.
+        :rtype: CouchSyncTarget
+        """
+        return CouchSyncTarget(self)
+
+    def delete_database(self):
+        """
+        Delete a U1DB CouchDB database.
+        """
+        del(self._server[self._dbname])
+
+    def close(self):
+        """
+        Release any resources associated with this database.
+
+        :return: True if db was succesfully closed.
+        :rtype: bool
+        """
+        self._url = None
+        self._full_commit = None
+        self._session = None
+        self._server = None
+        self._database = None
+        return True
+
+    def _set_replica_uid(self, replica_uid):
+        """
+        Force the replica uid to be set.
+
+        :param replica_uid: The new replica uid.
+        :type replica_uid: str
+        """
+        try:
+            # set on existent config document
+            doc = self._database['u1db_config']
+            doc['replica_uid'] = replica_uid
+        except ResourceNotFound:
+            # or create the config document
+            doc = {
+                '_id': 'u1db_config',
+                'replica_uid': replica_uid,
+            }
+        self._database.save(doc)
+        self._real_replica_uid = replica_uid
+
+    def _get_replica_uid(self):
+        """
+        Get the replica uid.
+
+        :return: The replica uid.
+        :rtype: str
+        """
+        if self._real_replica_uid is not None:
+            return self._real_replica_uid
+        try:
+            # grab replica_uid from server
+            doc = self._database['u1db_config']
+            self._real_replica_uid = doc['replica_uid']
+            return self._real_replica_uid
+        except ResourceNotFound:
+            # create a unique replica_uid
+            self._real_replica_uid = uuid.uuid4().hex
+            self._set_replica_uid(self._real_replica_uid)
+            return self._real_replica_uid
+
+    _replica_uid = property(_get_replica_uid, _set_replica_uid)
+
+    def _get_generation(self):
+        """
+        Return the current generation.
 
-    #-------------------------------------------------------------------------
-    # methods from Database
-    #-------------------------------------------------------------------------
+        :return: The current generation.
+        :rtype: int
+        """
+        # query a couch list function
+        res = self._database.resource(
+            '_design', 'transactions', '_list', 'generation', 'log')
+        response = res.get_json()
+        return response[2]['generation']
+
+    def _get_generation_info(self):
+        """
+        Return the current generation.
+
+        :return: A tuple containing the current generation and transaction id.
+        :rtype: (int, str)
+        """
+        # query a couch list function
+        res = self._database.resource(
+            '_design', 'transactions', '_list', 'generation', 'log')
+        response = res.get_json()
+        return (response[2]['generation'], response[2]['transaction_id'])
+
+    def _get_trans_id_for_gen(self, generation):
+        """
+        Get the transaction id corresponding to a particular generation.
+
+        :param generation: The generation for which to get the transaction id.
+        :type generation: int
+
+        :return: The transaction id for C{generation}.
+        :rtype: str
+
+        :raise InvalidGeneration: Raised when the generation does not exist.
+        """
+        if generation == 0:
+            return ''
+        # query a couch list function
+        res = self._database.resource(
+            '_design', 'transactions', '_list', 'trans_id_for_gen', 'log')
+        response = res.get_json(gen=generation)
+        if response[2] == {}:
+            raise errors.InvalidGeneration
+        return response[2]['transaction_id']
+
+    def _get_transaction_log(self):
+        """
+        This is only for the test suite, it is not part of the api.
+
+        :return: The complete transaction log.
+        :rtype: [(str, str)]
+        """
+        # query a couch view
+        res = self._database.resource(
+            '_design', 'transactions', '_view', 'log')
+        response = res.get_json()
+        return map(lambda row: (row['id'], row['value']), response[2]['rows'])
 
     def _get_doc(self, doc_id, check_for_conflicts=False):
         """
-        Get just the document content, without fancy handling.
+        Extract the document from storage.
+
+        This can return None if the document doesn't exist.
+
+        :param doc_id: The unique document identifier
+        :type doc_id: str
+        :param check_for_conflicts: If set to False, then the conflict check
+                                    will be skipped.
+        :type check_for_conflicts: bool
+
+        :return: The document.
+        :rtype: CouchDocument
+        """
+        # get document with all attachments (u1db content and eventual
+        # conflicts)
+        try:
+            result = \
+                self._database.resource(doc_id).get_json(
+                    attachments=True)[2]
+        except ResourceNotFound:
+            return None
+        # restrict to u1db documents
+        if 'u1db_rev' not in result:
+            return None
+        doc = self._factory(doc_id, result['u1db_rev'])
+        # set contents or make tombstone
+        if '_attachments' not in result \
+                or 'u1db_content' not in result['_attachments']:
+            doc.make_tombstone()
+        else:
+            doc.content = json.loads(
+                binascii.a2b_base64(
+                    result['_attachments']['u1db_content']['data']))
+        # determine if there are conflicts
+        if check_for_conflicts \
+                and '_attachments' in result \
+                and 'u1db_conflicts' in result['_attachments']:
+            doc.has_conflicts = True
+        # store couch revision
+        doc.couch_rev = result['_rev']
+        return doc
+
+    def get_doc(self, doc_id, include_deleted=False):
+        """
+        Get the JSON string for the given document.
 
-        @param doc_id: The unique document identifier
-        @type doc_id: str
-        @param include_deleted: If set to True, deleted documents will be
+        :param doc_id: The unique document identifier
+        :type doc_id: str
+        :param include_deleted: If set to True, deleted documents will be
             returned with empty content. Otherwise asking for a deleted
             document will return None.
-        @type include_deleted: bool
+        :type include_deleted: bool
 
-        @return: a Document object.
-        @type: u1db.Document
+        :return: A document object.
+        :rtype: CouchDocument.
         """
-        cdoc = self._database.get(doc_id)
-        if cdoc is None:
+        doc = self._get_doc(doc_id, check_for_conflicts=True)
+        if doc is None:
+            return None
+        if doc.is_tombstone() and not include_deleted:
             return None
-        has_conflicts = False
-        if check_for_conflicts:
-            has_conflicts = self._has_conflicts(doc_id)
-        doc = self._factory(
-            doc_id=doc_id,
-            rev=cdoc[self.COUCH_U1DB_REV_KEY],
-            has_conflicts=has_conflicts)
-        contents = self._database.get_attachment(
-            cdoc,
-            self.COUCH_U1DB_ATTACHMENT_KEY)
-        if contents:
-            doc.content = json.loads(contents.read())
-        else:
-            doc.make_tombstone()
         return doc
 
     def get_all_docs(self, include_deleted=False):
         """
         Get the JSON content for all documents in the database.
 
-        @param include_deleted: If set to True, deleted documents will be
-            returned with empty content. Otherwise deleted documents will not
-            be included in the results.
-        @type include_deleted: bool
+        :param include_deleted: If set to True, deleted documents will be
+                                returned with empty content. Otherwise deleted
+                                documents will not be included in the results.
+        :type include_deleted: bool
 
-        @return: (generation, [Document])
+        :return: (generation, [CouchDocument])
             The current generation of the database, followed by a list of all
             the documents in the database.
-        @rtype: tuple
+        :rtype: (int, [CouchDocument])
         """
+
         generation = self._get_generation()
         results = []
-        for doc_id in self._database:
-            if doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-                continue
-            doc = self._get_doc(doc_id, check_for_conflicts=True)
-            if doc.content is None and not include_deleted:
-                continue
-            results.append(doc)
+        for row in self._database.view('_all_docs'):
+            doc = self.get_doc(row.id, include_deleted=include_deleted)
+            if doc is not None:
+                results.append(doc)
         return (generation, results)
 
-    def _put_doc(self, doc):
+    def _put_doc(self, old_doc, doc):
         """
-        Update a document.
+        Put the document in the Couch backend database.
 
-        This is called everytime we just want to do a raw put on the db (i.e.
-        without index updates, document constraint checks, and conflict
-        checks).
-
-        @param doc: The document to update.
-        @type doc: u1db.Document
-
-        @return: The new revision identifier for the document.
-        @rtype: str
-        """
-        # prepare couch's Document
-        cdoc = CouchDocument()
-        cdoc[self.COUCH_ID_KEY] = doc.doc_id
-        # we have to guarantee that couch's _rev is consistent
-        old_cdoc = self._database.get(doc.doc_id)
-        if old_cdoc is not None:
-            cdoc[self.COUCH_REV_KEY] = old_cdoc[self.COUCH_REV_KEY]
-        # store u1db's rev
-        cdoc[self.COUCH_U1DB_REV_KEY] = doc.rev
-        # save doc in db
-        self._database.save(cdoc)
-        # store u1db's content as json string
-        if not doc.is_tombstone():
-            self._database.put_attachment(
-                cdoc, doc.get_json(),
-                filename=self.COUCH_U1DB_ATTACHMENT_KEY)
-        else:
-            self._database.delete_attachment(
-                cdoc,
-                self.COUCH_U1DB_ATTACHMENT_KEY)
+        :param old_doc: The old document version.
+        :type old_doc: CouchDocument
+        :param doc: The document to be put.
+        :type doc: CouchDocument
 
-    def get_sync_target(self):
+        :raise RevisionConflict: Raised when trying to update a document but
+                                 couch revisions mismatch.
         """
-        Return a SyncTarget object, for another u1db to synchronize with.
-
-        @return: The sync target.
-        @rtype: CouchSyncTarget
+        trans_id = self._allocate_transaction_id()
+        # encode content
+        content = doc.get_json()
+        if content is not None:
+            content = binascii.b2a_base64(content)[:-1]  # exclude trailing \n
+        # encode conflicts
+        conflicts = None
+        update_conflicts = doc.modified_conflicts()
+        if update_conflicts is True:
+            if doc.has_conflicts:
+                conflicts = binascii.b2a_base64(
+                    json.dumps(
+                        map(lambda cdoc: (cdoc.rev, cdoc.content),
+                            doc.get_conflicts()))
+            )[:-1]  # exclude \n
+        # perform the request
+        resource = self._database.resource(
+            '_design', 'docs', '_update', 'put', doc.doc_id)
+        response = resource.put_json(
+            body={
+                'couch_rev': old_doc.couch_rev
+                    if old_doc is not None else None,
+                'u1db_rev': doc.rev,
+                'content': content,
+                'trans_id': trans_id,
+                'conflicts': conflicts,
+                'update_conflicts': update_conflicts,
+            },
+            headers={'content-type': 'application/json'})
+        # the document might have been updated in between, so we check for the
+        # return message
+        msg = response[2].read()
+        if msg == 'ok':
+            return
+        elif msg == 'revision conflict':
+            raise errors.RevisionConflict()
+
+    def put_doc(self, doc):
         """
-        return CouchSyncTarget(self)
+        Update a document.
 
-    def create_index(self, index_name, *index_expressions):
-        """
-        Create a named index, which can then be queried for future lookups.
+        If the document currently has conflicts, put will fail.
+        If the database specifies a maximum document size and the document
+        exceeds it, put will fail and raise a DocumentTooBig exception.
 
-        @param index_name: A unique name which can be used as a key prefix.
-        @param index_expressions: Index expressions defining the index
-            information.
-        """
-        if index_name in self._indexes:
-            if self._indexes[index_name]._definition == list(
-                    index_expressions):
-                return
-            raise errors.IndexNameTakenError
-        index = InMemoryIndex(index_name, list(index_expressions))
-        for doc_id in self._database:
-            if doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-                continue  # skip special files
-            doc = self._get_doc(doc_id)
-            if doc.content is not None:
-                index.add_json(doc_id, doc.get_json())
-        self._indexes[index_name] = index
+        :param doc: A Document with new content.
+        :return: new_doc_rev - The new revision identifier for the document.
+            The Document object will also be updated.
 
-    def close(self):
+        :raise errors.InvalidDocId: Raised if the document's id is invalid.
+        :raise errors.DocumentTooBig: Raised if the document size is too big.
+        :raise errors.ConflictedDoc: Raised if the document has conflicts.
         """
-        Release any resources associated with this database.
-
-        @return: True if db was succesfully closed.
-        @rtype: bool
+        if doc.doc_id is None:
+            raise errors.InvalidDocId()
+        self._check_doc_id(doc.doc_id)
+        self._check_doc_size(doc)
+        old_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
+        if old_doc and old_doc.has_conflicts:
+            raise errors.ConflictedDoc()
+        if old_doc and doc.rev is None and old_doc.is_tombstone():
+            new_rev = self._allocate_doc_rev(old_doc.rev)
+        else:
+            if old_doc is not None:
+                    if old_doc.rev != doc.rev:
+                        raise errors.RevisionConflict()
+            else:
+                if doc.rev is not None:
+                    raise errors.RevisionConflict()
+            new_rev = self._allocate_doc_rev(doc.rev)
+        doc.rev = new_rev
+        self._put_doc(old_doc, doc)
+        return new_rev
+
+    def whats_changed(self, old_generation=0):
         """
-        # TODO: fix this method so the connection is properly closed and
-        # test_close (+tearDown, which deletes the db) works without problems.
-        self._url = None
-        self._full_commit = None
-        self._session = None
-        #self._server = None
-        self._database = None
-        return True
-
-    def sync(self, url, creds=None, autocreate=True):
+        Return a list of documents that have changed since old_generation.
+
+        :param old_generation: The generation of the database in the old
+                               state.
+        :type old_generation: int
+
+        :return: (generation, trans_id, [(doc_id, generation, trans_id),...])
+                 The current generation of the database, its associated
+                 transaction id, and a list of of changed documents since
+                 old_generation, represented by tuples with for each document
+                 its doc_id and the generation and transaction id corresponding
+                 to the last intervening change and sorted by generation (old
+                 changes first)
+        :rtype: (int, str, [(str, int, str)])
         """
-        Synchronize documents with remote replica exposed at url.
+        # query a couch list function
+        res = self._database.resource(
+            '_design', 'transactions', '_list', 'whats_changed', 'log')
+        response = res.get_json(old_gen=old_generation)
+        results = map(
+            lambda row:
+                (row['generation'], row['doc_id'], row['transaction_id']),
+            response[2]['transactions'])
+        results.reverse()
+        cur_gen = old_generation
+        seen = set()
+        changes = []
+        newest_trans_id = ''
+        for generation, doc_id, trans_id in results:
+            if doc_id not in seen:
+                changes.append((doc_id, generation, trans_id))
+                seen.add(doc_id)
+        if changes:
+            cur_gen = changes[0][1]  # max generation
+            newest_trans_id = changes[0][2]
+            changes.reverse()
+        else:
+            cur_gen, newest_trans_id = self._get_generation_info()
 
-        @param url: The url of the target replica to sync with.
-        @type url: str
-        @param creds: optional dictionary giving credentials.
-            to authorize the operation with the server.
-        @type creds: dict
-        @param autocreate: Ask the target to create the db if non-existent.
-        @type autocreate: bool
+        return cur_gen, newest_trans_id, changes
 
-        @return: The local generation before the synchronisation was performed.
-        @rtype: int
+    def delete_doc(self, doc):
         """
-        return Synchronizer(self, CouchSyncTarget(url, creds=creds)).sync(
-            autocreate=autocreate)
+        Mark a document as deleted.
+
+        Will abort if the current revision doesn't match doc.rev.
+        This will also set doc.content to None.
 
-    #-------------------------------------------------------------------------
-    # methods from ObjectStoreDatabase
-    #-------------------------------------------------------------------------
+        :param doc: The document to mark as deleted.
+        :type doc: CouchDocument.
 
-    def _init_u1db_data(self):
+        :raise errors.DocumentDoesNotExist: Raised if the document does not
+                                            exist.
+        :raise errors.RevisionConflict: Raised if the revisions do not match.
+        :raise errors.DocumentAlreadyDeleted: Raised if the document is
+                                              already deleted.
+        :raise errors.ConflictedDoc: Raised if the doc has conflicts.
+        """
+        old_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
+        if old_doc is None:
+            raise errors.DocumentDoesNotExist
+        if old_doc.rev != doc.rev:
+            raise errors.RevisionConflict()
+        if old_doc.is_tombstone():
+            raise errors.DocumentAlreadyDeleted
+        if old_doc.has_conflicts:
+            raise errors.ConflictedDoc()
+        new_rev = self._allocate_doc_rev(doc.rev)
+        doc.rev = new_rev
+        doc.make_tombstone()
+        self._put_doc(old_doc, doc)
+        return new_rev
+
+    def _get_conflicts(self, doc_id, couch_rev=None):
         """
-        Initialize u1db configuration data on backend storage.
+        Get the conflicted versions of a document.
 
-        A U1DB database needs to keep track of all database transactions,
-        document conflicts, the generation of other replicas it has seen,
-        indexes created by users and so on.
+        If the C{couch_rev} parameter is not None, conflicts for a specific
+        document's couch revision are returned.
 
-        In this implementation, all this information is stored in special
-        documents stored in the underlying with doc_id prefix equal to
-        U1DB_DATA_DOC_ID_PREFIX. Those documents ids are reserved: put_doc(),
-        get_doc() and delete_doc() will not allow documents with a doc_id with
-        that prefix to be accessed or modified.
+        :param couch_rev: The couch document revision.
+        :type couch_rev: str
+
+        :return: A list of conflicted versions of the document.
+        :rtype: list
         """
-        for key in self.U1DB_DATA_KEYS:
-            doc_id = '%s%s' % (self.U1DB_DATA_DOC_ID_PREFIX, key)
-            doc = self._get_doc(doc_id)
-            if doc is None:
-                doc = self._factory(doc_id)
-                doc.content = {'content': getattr(self, key)}
-                self._put_doc(doc)
+        # request conflicts attachment from server
+        params = {}
+        if couch_rev is not None:
+            params['rev'] = couch_rev  # restric document's couch revision
+        resource = self._database.resource(doc_id, 'u1db_conflicts')
+        try:
+            response = resource.get_json(**params)
+            conflicts = []
+            # build the conflicted versions
+            for doc_rev, content in json.loads(response[2].read()):
+                doc = self._factory(doc_id, doc_rev)
+                if content is None:
+                    doc.make_tombstone()
+                else:
+                    doc.content = content
+                conflicts.append(doc)
+            return conflicts
+        except ResourceNotFound:
+            return []
 
-    #-------------------------------------------------------------------------
-    # Couch specific methods
-    #-------------------------------------------------------------------------
+    def get_doc_conflicts(self, doc_id):
+        """
+        Get the list of conflicts for the given document.
 
-    INDEX_NAME_KEY = 'name'
-    INDEX_DEFINITION_KEY = 'definition'
-    INDEX_VALUES_KEY = 'values'
+        The order of the conflicts is such that the first entry is the value
+        that would be returned by "get_doc".
 
-    def delete_database(self):
-        """
-        Delete a U1DB CouchDB database.
+        :return: A list of the document entries that are conflicted.
+        :rtype: [CouchDocument]
         """
-        del(self._server[self._dbname])
+        conflict_docs = self._get_conflicts(doc_id)
+        if len(conflict_docs) == 0:
+            return []
+        this_doc = self._get_doc(doc_id, check_for_conflicts=True)
+        return [this_doc] + conflict_docs
 
-    def _dump_indexes_as_json(self):
+    def _get_replica_gen_and_trans_id(self, other_replica_uid):
+        """
+        Return the last known generation and transaction id for the other db
+        replica.
+
+        When you do a synchronization with another replica, the Database keeps
+        track of what generation the other database replica was at, and what
+        the associated transaction id was.  This is used to determine what data
+        needs to be sent, and if two databases are claiming to be the same
+        replica.
+
+        :param other_replica_uid: The identifier for the other replica.
+        :type other_replica_uid: str
+
+        :return: A tuple containing the generation and transaction id we
+                 encountered during synchronization. If we've never
+                 synchronized with the replica, this is (0, '').
+        :rtype: (int, str)
+        """
+        # query a couch view
+        result = self._database.view('syncs/log')
+        if len(result[other_replica_uid].rows) == 0:
+            return (0, '')
+        return (
+            result[other_replica_uid].rows[0]['value']['known_generation'],
+            result[other_replica_uid].rows[0]['value']['known_transaction_id']
+        )
+
+    def _set_replica_gen_and_trans_id(self, other_replica_uid,
+                                      other_generation, other_transaction_id):
         """
-        Dump index definitions as JSON.
+        Set the last-known generation and transaction id for the other
+        database replica.
+
+        We have just performed some synchronization, and we want to track what
+        generation the other replica was at. See also
+        _get_replica_gen_and_trans_id.
+
+        :param other_replica_uid: The U1DB identifier for the other replica.
+        :type other_replica_uid: str
+        :param other_generation: The generation number for the other replica.
+        :type other_generation: int
+        :param other_transaction_id: The transaction id associated with the
+            generation.
+        :type other_transaction_id: str
         """
-        indexes = {}
-        for name, idx in self._indexes.iteritems():
-            indexes[name] = {}
-            for attr in [self.INDEX_NAME_KEY, self.INDEX_DEFINITION_KEY,
-                         self.INDEX_VALUES_KEY]:
-                indexes[name][attr] = getattr(idx, '_' + attr)
-        return indexes
+        self._do_set_replica_gen_and_trans_id(
+            other_replica_uid, other_generation, other_transaction_id)
 
-    def _load_indexes_from_json(self, indexes):
+    def _do_set_replica_gen_and_trans_id(
+            self, other_replica_uid, other_generation, other_transaction_id):
+        """
+        Set the last-known generation and transaction id for the other
+        database replica.
+
+        We have just performed some synchronization, and we want to track what
+        generation the other replica was at. See also
+        _get_replica_gen_and_trans_id.
+
+        :param other_replica_uid: The U1DB identifier for the other replica.
+        :type other_replica_uid: str
+        :param other_generation: The generation number for the other replica.
+        :type other_generation: int
+        :param other_transaction_id: The transaction id associated with the
+                                     generation.
+        :type other_transaction_id: str
+        """
+        # query a couch update function
+        res = self._database.resource(
+            '_design', 'syncs', '_update', 'put', 'u1db_sync_log')
+        res.put_json(
+            body={
+                'other_replica_uid': other_replica_uid,
+                'other_generation': other_generation,
+                'other_transaction_id': other_transaction_id,
+            },
+            headers={'content-type': 'application/json'})
+
+    def _add_conflict(self, doc, my_doc_rev, my_content):
         """
-        Load index definitions from stored JSON.
+        Add a conflict to the document.
+
+        Note that this method does not actually update the backend; rather, it
+        updates the CouchDocument object which will provide the conflict data
+        when the atomic document update is made.
+
+        :param doc: The document to have conflicts added to.
+        :type doc: CouchDocument
+        :param my_doc_rev: The revision of the conflicted document.
+        :type my_doc_rev: str
+        :param my_content: The content of the conflicted document as a JSON
+                           serialized string.
+        :type my_content: str
+        """
+        doc.ensure_fetch_conflicts(self._get_conflicts)
+        doc.add_conflict(
+            self._factory(doc_id=doc.doc_id, rev=my_doc_rev,
+                          json=my_content))
 
-        @param indexes: A JSON representation of indexes as
-            [('index-name', ['field', 'field2', ...]), ...].
-        @type indexes: str
+    def _delete_conflicts(self, doc, conflict_revs):
         """
-        self._indexes = {}
-        for name, idx_dict in indexes.iteritems():
-            idx = InMemoryIndex(name, idx_dict[self.INDEX_DEFINITION_KEY])
-            idx._values = idx_dict[self.INDEX_VALUES_KEY]
-            self._indexes[name] = idx
+        Delete the conflicted revisions from the list of conflicts of C{doc}.
+
+        Note that thie method does not actually update the backed; rather, it
+        updates the CouchDocument object which will provide the conflict data
+        when the atomic document update is made.
 
-    def _load_transaction_log_from_json(self, transaction_log):
+        :param doc: The document to have conflicts deleted.
+        :type doc: CouchDocument
+        :param conflict_revs: A list of the revisions to be deleted.
+        :param conflict_revs: [str]
         """
-        Load transaction log from stored JSON.
+        doc.ensure_fetch_conflicts(self._get_conflicts)
+        doc.delete_conflicts(conflict_revs)
 
-        @param transaction_log: A JSON representation of transaction_log as
-            [('generation', 'transaction_id'), ...].
-        @type transaction_log: list
+    def _prune_conflicts(self, doc, doc_vcr):
+        """
+        Prune conflicts that are older then the current document's revision, or
+        whose content match to the current document's content.
+
+        :param doc: The document to have conflicts pruned.
+        :type doc: CouchDocument
+        :param doc_vcr: A vector clock representing the current document's
+                        revision.
+        :type doc_vcr: u1db.vectorclock.VectorClock
         """
-        self._transaction_log = []
-        for gen, trans_id in transaction_log:
-            self._transaction_log.append((gen, trans_id))
+        if doc.has_conflicts is True:
+            autoresolved = False
+            c_revs_to_prune = []
+            for c_doc in doc.get_conflicts():
+                c_vcr = vectorclock.VectorClockRev(c_doc.rev)
+                if doc_vcr.is_newer(c_vcr):
+                    c_revs_to_prune.append(c_doc.rev)
+                elif doc.same_content_as(c_doc):
+                    c_revs_to_prune.append(c_doc.rev)
+                    doc_vcr.maximize(c_vcr)
+                    autoresolved = True
+            if autoresolved:
+                doc_vcr.increment(self._replica_uid)
+                doc.rev = doc_vcr.as_str()
+            self._delete_conflicts(doc, c_revs_to_prune)
+
+    def _force_doc_sync_conflict(self, doc):
+        """
+        Add a conflict and force a document put.
 
-    def _load_other_generations_from_json(self, other_generations):
+        :param doc: The document to be put.
+        :type doc: CouchDocument
         """
-        Load other generations from stored JSON.
+        my_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
+        self._prune_conflicts(doc, vectorclock.VectorClockRev(doc.rev))
+        self._add_conflict(doc, my_doc.rev, my_doc.get_json())
+        doc.has_conflicts = True
+        self._put_doc(my_doc, doc)
 
-        @param other_generations: A JSON representation of other_generations
-            as {'replica_uid': ('generation', 'transaction_id'), ...}.
-        @type other_generations: dict
+    def resolve_doc(self, doc, conflicted_doc_revs):
+        """
+        Mark a document as no longer conflicted.
+
+        We take the list of revisions that the client knows about that it is
+        superseding. This may be a different list from the actual current
+        conflicts, in which case only those are removed as conflicted.  This
+        may fail if the conflict list is significantly different from the
+        supplied information. (sync could have happened in the background from
+        the time you GET_DOC_CONFLICTS until the point where you RESOLVE)
+
+        :param doc: A Document with the new content to be inserted.
+        :type doc: CouchDocument
+        :param conflicted_doc_revs: A list of revisions that the new content
+                                    supersedes.
+        :type conflicted_doc_revs: [str]
         """
-        self._other_generations = {}
-        for replica_uid, [gen, trans_id] in other_generations.iteritems():
-            self._other_generations[replica_uid] = (gen, trans_id)
+        cur_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
+        new_rev = self._ensure_maximal_rev(cur_doc.rev,
+                                           conflicted_doc_revs)
+        superseded_revs = set(conflicted_doc_revs)
+        doc.rev = new_rev
+        if cur_doc.rev in superseded_revs:
+            self._delete_conflicts(doc, superseded_revs)
+            self._put_doc(cur_doc, doc)
+        else:
+            self._add_conflict(doc, new_rev, doc.get_json())
+            self._delete_conflicts(doc, superseded_revs)
+            # perform request to resolve document in server
+            resource = self._database.resource(
+                '_design', 'docs', '_update', 'resolve_doc', doc.doc_id)
+            conflicts = None
+            if doc.has_conflicts:
+                conflicts = binascii.b2a_base64(
+                    json.dumps(
+                        map(lambda cdoc: (cdoc.rev, cdoc.content),
+                            doc.get_conflicts()))
+                )[:-1]  # exclude \n
+            response = resource.put_json(
+                body={
+                    'couch_rev': cur_doc.couch_rev,
+                    'conflicts': conflicts,
+                },
+                headers={'content-type': 'application/json'})
+
+    def _put_doc_if_newer(self, doc, save_conflict, replica_uid, replica_gen,
+                          replica_trans_id=''):
+        """
+        Insert/update document into the database with a given revision.
+
+        This api is used during synchronization operations.
+
+        If a document would conflict and save_conflict is set to True, the
+        content will be selected as the 'current' content for doc.doc_id,
+        even though doc.rev doesn't supersede the currently stored revision.
+        The currently stored document will be added to the list of conflict
+        alternatives for the given doc_id.
+
+        This forces the new content to be 'current' so that we get convergence
+        after synchronizing, even if people don't resolve conflicts. Users can
+        then notice that their content is out of date, update it, and
+        synchronize again. (The alternative is that users could synchronize and
+        think the data has propagated, but their local copy looks fine, and the
+        remote copy is never updated again.)
+
+        :param doc: A document object
+        :type doc: CouchDocument
+        :param save_conflict: If this document is a conflict, do you want to
+                              save it as a conflict, or just ignore it.
+        :type save_conflict: bool
+        :param replica_uid: A unique replica identifier.
+        :type replica_uid: str
+        :param replica_gen: The generation of the replica corresponding to the
+                            this document. The replica arguments are optional,
+                            but are used during synchronization.
+        :type replica_gen: int
+        :param replica_trans_id: The transaction_id associated with the
+                                 generation.
+        :type replica_trans_id: str
+
+        :return: (state, at_gen) -  If we don't have doc_id already, or if
+                 doc_rev supersedes the existing document revision, then the
+                 content will be inserted, and state is 'inserted'.  If
+                 doc_rev is less than or equal to the existing revision, then
+                 the put is ignored and state is respecitvely 'superseded' or
+                 'converged'.  If doc_rev is not strictly superseded or
+                 supersedes, then state is 'conflicted'. The document will not
+                 be inserted if save_conflict is False.  For 'inserted' or
+                 'converged', at_gen is the insertion/current generation.
+        :rtype: (str, int)
+        """
+        cur_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
+        # at this point, `doc` has arrived from the other syncing party, and
+        # we will decide what to do with it.
+        # First, we prepare the arriving doc to update couch database.
+        old_doc = doc
+        doc = self._factory(doc.doc_id, doc.rev, doc.get_json())
+        if cur_doc is not None:
+            doc.couch_rev = cur_doc.couch_rev
+        # fetch conflicts because we will eventually manipulate them
+        doc.ensure_fetch_conflicts(self._get_conflicts)
+        # from now on, it works just like u1db sqlite backend
+        doc_vcr = vectorclock.VectorClockRev(doc.rev)
+        if cur_doc is None:
+            cur_vcr = vectorclock.VectorClockRev(None)
+        else:
+            cur_vcr = vectorclock.VectorClockRev(cur_doc.rev)
+        self._validate_source(replica_uid, replica_gen, replica_trans_id)
+        if doc_vcr.is_newer(cur_vcr):
+            rev = doc.rev
+            self._prune_conflicts(doc, doc_vcr)
+            if doc.rev != rev:
+                # conflicts have been autoresolved
+                state = 'superseded'
+            else:
+                state = 'inserted'
+            self._put_doc(cur_doc, doc)
+        elif doc.rev == cur_doc.rev:
+            # magical convergence
+            state = 'converged'
+        elif cur_vcr.is_newer(doc_vcr):
+            # Don't add this to seen_ids, because we have something newer,
+            # so we should send it back, and we should not generate a
+            # conflict
+            state = 'superseded'
+        elif cur_doc.same_content_as(doc):
+            # the documents have been edited to the same thing at both ends
+            doc_vcr.maximize(cur_vcr)
+            doc_vcr.increment(self._replica_uid)
+            doc.rev = doc_vcr.as_str()
+            self._put_doc(cur_doc, doc)
+            state = 'superseded'
+        else:
+            state = 'conflicted'
+            if save_conflict:
+                self._force_doc_sync_conflict(doc)
+        if replica_uid is not None and replica_gen is not None:
+            self._do_set_replica_gen_and_trans_id(
+                replica_uid, replica_gen, replica_trans_id)
+        # update info
+        old_doc.rev = doc.rev
+        if doc.is_tombstone():
+            old_doc.is_tombstone()
+        else:
+            old_doc.content = doc.content
+        old_doc.has_conflicts = doc.has_conflicts
+        return state, self._get_generation()
 
 
-class CouchSyncTarget(ObjectStoreSyncTarget):
+class CouchSyncTarget(CommonSyncTarget):
     """
     Functionality for using a CouchDatabase as a synchronization target.
     """
-    pass
+
+    def get_sync_info(self, source_replica_uid):
+        source_gen, source_trans_id = self._db._get_replica_gen_and_trans_id(
+            source_replica_uid)
+        my_gen, my_trans_id = self._db._get_generation_info()
+        return (
+            self._db._replica_uid, my_gen, my_trans_id, source_gen,
+            source_trans_id)
+
+    def record_sync_info(self, source_replica_uid, source_replica_generation,
+                         source_replica_transaction_id):
+        if self._trace_hook:
+            self._trace_hook('record_sync_info')
+        self._db._set_replica_gen_and_trans_id(
+            source_replica_uid, source_replica_generation,
+            source_replica_transaction_id)
 
 
 class NotEnoughCouchPermissions(Exception):
@@ -527,12 +1014,12 @@ class CouchServerState(ServerState):
         """
         Initialize the couch server state.
 
-        @param couch_url: The URL for the couch database.
-        @type couch_url: str
-        @param shared_db_name: The name of the shared database.
-        @type shared_db_name: str
-        @param tokens_db_name: The name of the tokens database.
-        @type tokens_db_name: str
+        :param couch_url: The URL for the couch database.
+        :type couch_url: str
+        :param shared_db_name: The name of the shared database.
+        :type shared_db_name: str
+        :param tokens_db_name: The name of the tokens database.
+        :type tokens_db_name: str
         """
         self._couch_url = couch_url
         self._shared_db_name = shared_db_name
@@ -563,12 +1050,12 @@ class CouchServerState(ServerState):
         couch library to ensure that Soledad Server can do everything it needs
         on the underlying couch database.
 
-        @param couch_url: The URL of the couch database.
-        @type couch_url: str
+        :param couch_url: The URL of the couch database.
+        :type couch_url: str
 
         @raise NotEnoughCouchPermissions: Raised in case there are not enough
             permissions to read/write/create the needed couch databases.
-        @rtype: bool
+        :rtype: bool
         """
 
         def _open_couch_db(dbname):
@@ -601,11 +1088,11 @@ class CouchServerState(ServerState):
         """
         Open a couch database.
 
-        @param dbname: The name of the database to open.
-        @type dbname: str
+        :param dbname: The name of the database to open.
+        :type dbname: str
 
-        @return: The CouchDatabase object.
-        @rtype: CouchDatabase
+        :return: The CouchDatabase object.
+        :rtype: CouchDatabase
         """
         # TODO: open couch
         return CouchDatabase.open_database(
@@ -616,11 +1103,11 @@ class CouchServerState(ServerState):
         """
         Ensure couch database exists.
 
-        @param dbname: The name of the database to ensure.
-        @type dbname: str
+        :param dbname: The name of the database to ensure.
+        :type dbname: str
 
-        @return: The CouchDatabase object and the replica uid.
-        @rtype: (CouchDatabase, str)
+        :return: The CouchDatabase object and the replica uid.
+        :rtype: (CouchDatabase, str)
         """
         db = CouchDatabase.open_database(
             self._couch_url + '/' + dbname,
@@ -631,8 +1118,8 @@ class CouchServerState(ServerState):
         """
         Delete couch database.
 
-        @param dbname: The name of the database to delete.
-        @type dbname: str
+        :param dbname: The name of the database to delete.
+        :type dbname: str
         """
         CouchDatabase.delete_database(self._couch_url + '/' + dbname)
 
@@ -640,8 +1127,8 @@ class CouchServerState(ServerState):
         """
         Set the couchdb URL
 
-        @param url: CouchDB URL
-        @type url: str
+        :param url: CouchDB URL
+        :type url: str
         """
         self._couch_url = url
 
@@ -649,7 +1136,7 @@ class CouchServerState(ServerState):
         """
         Return CouchDB URL
 
-        @rtype: str
+        :rtype: str
         """
         return self._couch_url
 
diff --git a/common/src/leap/soledad/common/ddocs/README.txt b/common/src/leap/soledad/common/ddocs/README.txt
new file mode 100644
index 00000000..5569d929
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/README.txt
@@ -0,0 +1,34 @@
+This directory holds a folder structure containing javascript files that
+represent the design documents needed by the CouchDB U1DB backend. These files
+are compiled into the `../ddocs.py` file by setuptools when creating the
+source distribution.
+
+The following table depicts the U1DB CouchDB backend method and the URI that
+is queried to obtain/update data from/to the server.
+
+   +----------------------------------+------------------------------------------------------------------+
+   | u1db backend method              | URI                                                              |
+   |----------------------------------+------------------------------------------------------------------|
+   | _get_generation                  | _design/transactions/_list/generation/log                        |
+   | _get_generation_info             | _design/transactions/_list/generation/log                        |
+   | _get_trans_id_for_gen            | _design/transactions/_list/trans_id_for_gen/log                  |
+   | _get_transaction_log             | _design/transactions/_view/log                                   |
+   | _get_doc (*)                     | _design/docs/_view/get?key=<doc_id>                              |
+   | _has_conflicts                   | _design/docs/_view/get?key=<doc_id>                              |
+   | get_all_docs                     | _design/docs/_view/get                                           |
+   | _put_doc                         | _design/docs/_update/put/<doc_id>                                |
+   | _whats_changed                   | _design/transactions/_list/whats_changed/log?old_gen=<gen>       |
+   | _get_conflicts (*)               | _design/docs/_view/conflicts?key=<doc_id>                        |
+   | _get_replica_gen_and_trans_id    | _design/syncs/_view/log?other_replica_uid=<uid>                  |
+   | _do_set_replica_gen_and_trans_id | _design/syncs/_update/put/u1db_sync_log                          |
+   | _add_conflict                    | _design/docs/_update/add_conflict/<doc_id>                       |
+   | _delete_conflicts                | _design/docs/_update/delete_conflicts/<doc_id>?doc_rev=<doc_rev> |
+   | list_indexes                     | not implemented                                                  |
+   | _get_index_definition            | not implemented                                                  |
+   | delete_index                     | not implemented                                                  |
+   | _get_indexed_fields              | not implemented                                                  |
+   | _put_and_update_indexes          | not implemented                                                  |
+   +----------------------------------+------------------------------------------------------------------+
+
+(*) These methods also request CouchDB document attachments that store U1DB
+    document contents.
diff --git a/common/src/leap/soledad/common/ddocs/docs/updates/put.js b/common/src/leap/soledad/common/ddocs/docs/updates/put.js
new file mode 100644
index 00000000..5a4647de
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/docs/updates/put.js
@@ -0,0 +1,64 @@
+function(doc, req){
+    /* we expect to receive the following in `req.body`:
+     * {
+     *     'couch_rev': '<couch_rev>',
+     *     'u1db_rev': '<u1db_rev>',
+     *     'content': '<base64 encoded content>',
+     *     'trans_id': '<reansaction_id>'
+     *     'conflicts': '<base64 encoded conflicts>',
+     *     'update_conflicts': <boolean>
+     * }
+     */
+    var body = JSON.parse(req.body);
+
+    // create a new document document
+    if (!doc) {
+        doc = {}
+        doc['_id'] = req['id'];
+    }
+    // or fail if couch revisions do not match
+    else if (doc['_rev'] != body['couch_rev']) {
+        // of fail if revisions do not match
+        return [null, 'revision conflict']
+    }
+
+    // store u1db rev
+    doc.u1db_rev = body['u1db_rev'];
+
+    // save content as attachment
+    if (body['content'] != null) {
+        // save u1db content as attachment
+        if (!doc._attachments)
+            doc._attachments = {};
+        doc._attachments.u1db_content =  {
+            content_type: "application/octet-stream",
+            data: body['content']  // should be base64 encoded
+        };
+    }
+    // or delete the attachment if document is tombstone
+    else if (doc._attachments &&
+             doc._attachments.u1db_content)
+        delete doc._attachments.u1db_content;
+
+    // store the transaction id
+    if (!doc.u1db_transactions)
+        doc.u1db_transactions = [];
+    var d = new Date();
+    doc.u1db_transactions.push([d.getTime(), body['trans_id']]);
+
+    // save conflicts as attachment if they were sent
+    if (body['update_conflicts'])
+        if (body['conflicts'] != null) {
+            if (!doc._attachments)
+                doc._attachments = {};
+            doc._attachments.u1db_conflicts = {
+                content_type: "application/octet-stream",
+                data: body['conflicts']  // should be base64 encoded
+            }
+        } else {
+            if(doc._attachments && doc._attachments.u1db_conflicts)
+                delete doc._attachments.u1db_conflicts
+        }
+
+    return [doc, 'ok'];
+}
diff --git a/common/src/leap/soledad/common/ddocs/docs/updates/resolve_doc.js b/common/src/leap/soledad/common/ddocs/docs/updates/resolve_doc.js
new file mode 100644
index 00000000..7ba66cf8
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/docs/updates/resolve_doc.js
@@ -0,0 +1,39 @@
+function(doc, req){
+    /* we expect to receive the following in `req.body`:
+     * {
+     *     'couch_rev': '<couch_rev>',
+     *     'conflicts': '<base64 encoded conflicts>',
+     * }
+     */
+    var body = JSON.parse(req.body);
+
+    // fail if no document was given
+    if (!doc) {
+        return [null, 'document does not exist']
+    } 
+
+    // fail if couch revisions do not match
+    if (body['couch_rev'] != null
+        && doc['_rev'] != body['couch_rev']) {
+        return [null, 'revision conflict']
+    }
+
+    // fail if conflicts were not sent
+    if (body['conflicts'] == null)
+        return [null, 'missing conflicts']
+
+    // save conflicts as attachment if they were sent
+    if (body['conflicts'] != null) {
+        if (!doc._attachments)
+            doc._attachments = {};
+        doc._attachments.u1db_conflicts = {
+            content_type: "application/octet-stream",
+            data: body['conflicts']  // should be base64 encoded
+        }
+    }
+    // or delete attachment if there are no conflicts
+    else if (doc._attachments && doc._attachments.u1db_conflicts)
+        delete doc._attachments.u1db_conflicts;
+
+    return [doc, 'ok'];
+}
diff --git a/common/src/leap/soledad/common/ddocs/docs/views/get/map.js b/common/src/leap/soledad/common/ddocs/docs/views/get/map.js
new file mode 100644
index 00000000..ae08d9e9
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/docs/views/get/map.js
@@ -0,0 +1,20 @@
+function(doc) {
+    if (doc.u1db_rev) {
+        var is_tombstone = true;
+        var has_conflicts = false;
+        if (doc._attachments) {
+            if (doc._attachments.u1db_content)
+                is_tombstone = false;
+            if (doc._attachments.u1db_conflicts)
+                has_conflicts = true;
+        }
+        emit(doc._id,
+            {
+                "couch_rev": doc._rev,
+                "u1db_rev": doc.u1db_rev,
+                "is_tombstone": is_tombstone,
+                "has_conflicts": has_conflicts,
+            }
+        );
+    }
+}
diff --git a/common/src/leap/soledad/common/ddocs/syncs/updates/put.js b/common/src/leap/soledad/common/ddocs/syncs/updates/put.js
new file mode 100644
index 00000000..722f695a
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/syncs/updates/put.js
@@ -0,0 +1,22 @@
+function(doc, req){
+    if (!doc) {
+        doc = {}
+        doc['_id'] = 'u1db_sync_log';
+        doc['syncs'] = [];
+    }
+    body = JSON.parse(req.body);
+    // remove outdated info
+    doc['syncs'] = doc['syncs'].filter(
+        function (entry) {
+            return entry[0] != body['other_replica_uid'];
+        }
+    );
+    // store u1db rev
+    doc['syncs'].push([
+        body['other_replica_uid'],
+        body['other_generation'],
+        body['other_transaction_id']
+    ]);
+    return [doc, 'ok'];
+}
+
diff --git a/common/src/leap/soledad/common/ddocs/syncs/views/log/map.js b/common/src/leap/soledad/common/ddocs/syncs/views/log/map.js
new file mode 100644
index 00000000..a63c7cf4
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/syncs/views/log/map.js
@@ -0,0 +1,12 @@
+function(doc) {
+    if (doc._id == 'u1db_sync_log') {
+        if (doc.syncs)
+            doc.syncs.forEach(function (entry) {
+                emit(entry[0],
+                    {
+                        'known_generation': entry[1],
+                        'known_transaction_id': entry[2]
+                    });
+            });
+    }
+}
diff --git a/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js b/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js
new file mode 100644
index 00000000..dbdfff0d
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js
@@ -0,0 +1,20 @@
+function(head, req) {
+    var row;
+    var rows=[];
+    // fetch all rows
+    while(row = getRow()) {
+        rows.push(row);
+    }
+    if (rows.length > 0)
+        send(JSON.stringify({
+            "generation": rows.length,
+            "doc_id": rows[rows.length-1]['id'],
+            "transaction_id": rows[rows.length-1]['value']
+        }));
+    else
+        send(JSON.stringify({
+            "generation": 0,
+            "doc_id": "",
+            "transaction_id": "",
+        }));
+}
diff --git a/common/src/leap/soledad/common/ddocs/transactions/lists/trans_id_for_gen.js b/common/src/leap/soledad/common/ddocs/transactions/lists/trans_id_for_gen.js
new file mode 100644
index 00000000..2ec91794
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/transactions/lists/trans_id_for_gen.js
@@ -0,0 +1,19 @@
+function(head, req) {
+    var row;
+    var rows=[];
+    var i = 1;
+    var gen = 1;
+    if (req.query.gen)
+        gen = parseInt(req.query['gen']);
+    // fetch all rows
+    while(row = getRow())
+        rows.push(row);
+    if (gen <= rows.length)
+        send(JSON.stringify({
+            "generation": gen,
+            "doc_id": rows[gen-1]['id'],
+            "transaction_id": rows[gen-1]['value'],
+        }));
+    else
+        send('{}');
+}
diff --git a/common/src/leap/soledad/common/ddocs/transactions/lists/whats_changed.js b/common/src/leap/soledad/common/ddocs/transactions/lists/whats_changed.js
new file mode 100644
index 00000000..b35cdf51
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/transactions/lists/whats_changed.js
@@ -0,0 +1,22 @@
+function(head, req) {
+    var row;
+    var gen = 1;
+    var old_gen = 0;
+    if (req.query.old_gen)
+        old_gen = parseInt(req.query['old_gen']);
+    send('{"transactions":[\n');
+    // fetch all rows
+    while(row = getRow()) {
+        if (gen > old_gen) {
+            if (gen > old_gen+1)
+                send(',\n');
+            send(JSON.stringify({
+                "generation": gen,
+                "doc_id": row["id"],
+                "transaction_id": row["value"]
+            }));
+        }
+        gen++;
+    }
+    send('\n]}');
+}
diff --git a/common/src/leap/soledad/common/ddocs/transactions/views/log/map.js b/common/src/leap/soledad/common/ddocs/transactions/views/log/map.js
new file mode 100644
index 00000000..94ef63ca
--- /dev/null
+++ b/common/src/leap/soledad/common/ddocs/transactions/views/log/map.js
@@ -0,0 +1,7 @@
+function(doc) {
+    if (doc.u1db_transactions)
+        doc.u1db_transactions.forEach(function(t) {
+            emit(t[0],  // use timestamp as key so the results are ordered
+                 t[1]); // value is the transaction_id
+        });
+}
diff --git a/common/src/leap/soledad/common/objectstore.py b/common/src/leap/soledad/common/objectstore.py
deleted file mode 100644
index 7aff3e32..00000000
--- a/common/src/leap/soledad/common/objectstore.py
+++ /dev/null
@@ -1,282 +0,0 @@
-# -*- coding: utf-8 -*-
-# objectstore.py
-# Copyright (C) 2013 LEAP
-#
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program. If not, see <http://www.gnu.org/licenses/>.
-
-
-"""
-Abstract U1DB backend to handle storage using object stores (like CouchDB, for
-example).
-
-This backend uses special documents to store all U1DB data (replica uid,
-indexes, transaction logs and info about other dbs). The id of these documents
-are reserved and have prefix equal to ObjectStore.U1DB_DATA_DOC_ID_PREFIX.
-
-Right now, this is only used by CouchDatabase backend, but can also be
-extended to implement OpenStack or Amazon S3 storage, for example.
-
-See U1DB documentation for more information on how to use databases.
-"""
-
-
-from base64 import b64encode, b64decode
-
-
-import uuid
-import simplejson as json
-
-
-from u1db import errors
-from u1db.backends.inmemory import (
-    InMemoryDatabase,
-    InMemorySyncTarget,
-)
-
-
-class ObjectStoreDatabase(InMemoryDatabase):
-    """
-    A backend for storing u1db data in an object store.
-    """
-
-    U1DB_DATA_DOC_ID_PREFIX = 'u1db/'
-
-    @classmethod
-    def open_database(cls, url, create, document_factory=None):
-        """
-        Open a U1DB database using an object store as backend.
-
-        @param url: the url of the database replica
-        @type url: str
-        @param create: should the replica be created if it does not exist?
-        @type create: bool
-        @param document_factory: A function that will be called with the same
-            parameters as Document.__init__.
-        @type document_factory: callable
-
-        @return: the database instance
-        @rtype: CouchDatabase
-        """
-        raise NotImplementedError(cls.open_database)
-
-    def __init__(self, replica_uid=None, document_factory=None):
-        """
-        Initialize the object store database.
-
-        @param replica_uid: an optional unique replica identifier
-        @type replica_uid: str
-        @param document_factory: A function that will be called with the same
-            parameters as Document.__init__.
-        @type document_factory: callable
-        """
-        InMemoryDatabase.__init__(
-            self,
-            replica_uid,
-            document_factory=document_factory)
-        if self._replica_uid is None:
-            self._replica_uid = uuid.uuid4().hex
-        self._init_u1db_data()
-
-    def _init_u1db_data(self):
-        """
-        Initialize u1db configuration data on backend storage.
-
-        A U1DB database needs to keep track of all database transactions,
-        document conflicts, the generation of other replicas it has seen,
-        indexes created by users and so on.
-
-        In this implementation, all this information is stored in special
-        documents stored in the couch db with id prefix equal to
-        U1DB_DATA_DOC_ID_PREFIX.  Those documents ids are reserved:
-        put_doc(), get_doc() and delete_doc() will not allow documents with
-        a doc_id with that prefix to be accessed or modified.
-        """
-        raise NotImplementedError(self._init_u1db_data)
-
-    #-------------------------------------------------------------------------
-    # methods from Database
-    #-------------------------------------------------------------------------
-
-    def put_doc(self, doc):
-        """
-        Update a document.
-
-        If the document currently has conflicts, put will fail.
-        If the database specifies a maximum document size and the document
-        exceeds it, put will fail and raise a DocumentTooBig exception.
-
-        This method prevents from updating the document with doc_id equals to
-        self.U1DB_DATA_DOC_ID, which contains U1DB data.
-
-        @param doc: A Document with new content.
-        @type doc: Document
-
-        @return: new_doc_rev - The new revision identifier for the document.
-            The Document object will also be updated.
-        @rtype: str
-        """
-        if doc.doc_id is not None and \
-                doc.doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-            raise errors.InvalidDocId()
-        return InMemoryDatabase.put_doc(self, doc)
-
-    def _put_doc(self, doc):
-        """
-        Update a document.
-
-        This is called everytime we just want to do a raw put on the db (i.e.
-        without index updates, document constraint checks, and conflict
-        checks).
-
-        @param doc: The document to update.
-        @type doc: u1db.Document
-
-        @return: The new revision identifier for the document.
-        @rtype: str
-        """
-        raise NotImplementedError(self._put_doc)
-
-    def get_doc(self, doc_id, include_deleted=False):
-        """
-        Get the JSON string for the given document.
-
-        This method prevents from getting the document with doc_id equals to
-        self.U1DB_DATA_DOC_ID, which contains U1DB data.
-
-        @param doc_id: The unique document identifier
-        @type doc_id: str
-        @param include_deleted: If set to True, deleted documents will be
-            returned with empty content. Otherwise asking for a deleted
-            document will return None.
-        @type include_deleted: bool
-
-        @return: a Document object.
-        @rtype: Document
-        """
-        if doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-            raise errors.InvalidDocId()
-        return InMemoryDatabase.get_doc(self, doc_id, include_deleted)
-
-    def _get_doc(self, doc_id):
-        """
-        Get just the document content, without fancy handling.
-
-        @param doc_id: The unique document identifier
-        @type doc_id: str
-        @param include_deleted: If set to True, deleted documents will be
-            returned with empty content. Otherwise asking for a deleted
-            document will return None.
-        @type include_deleted: bool
-
-        @return: a Document object.
-        @type: u1db.Document
-        """
-        raise NotImplementedError(self._get_doc)
-
-    def get_all_docs(self, include_deleted=False):
-        """
-        Get the JSON content for all documents in the database.
-
-        @param include_deleted: If set to True, deleted documents will be
-            returned with empty content. Otherwise deleted documents will not
-            be included in the results.
-        @type include_deleted: bool
-
-        @return: (generation, [Document])
-            The current generation of the database, followed by a list of all
-            the documents in the database.
-        @rtype: tuple
-        """
-        generation = self._get_generation()
-        results = []
-        for doc_id in self._database:
-            if doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-                continue
-            doc = self._get_doc(doc_id, check_for_conflicts=True)
-            if doc.content is None and not include_deleted:
-                continue
-            results.append(doc)
-        return (generation, results)
-
-    def delete_doc(self, doc):
-        """
-        Mark a document as deleted.
-
-        This method prevents from deleting the document with doc_id equals to
-        self.U1DB_DATA_DOC_ID, which contains U1DB data.
-
-        @param doc: The document to mark as deleted.
-        @type doc: u1db.Document
-
-        @return: The new revision id of the document.
-        @type: str
-        """
-        if doc.doc_id.startswith(self.U1DB_DATA_DOC_ID_PREFIX):
-            raise errors.InvalidDocId()
-        old_doc = self._get_doc(doc.doc_id, check_for_conflicts=True)
-        if old_doc is None:
-            raise errors.DocumentDoesNotExist
-        if old_doc.rev != doc.rev:
-            raise errors.RevisionConflict()
-        if old_doc.is_tombstone():
-            raise errors.DocumentAlreadyDeleted
-        if old_doc.has_conflicts:
-            raise errors.ConflictedDoc()
-        new_rev = self._allocate_doc_rev(doc.rev)
-        doc.rev = new_rev
-        doc.make_tombstone()
-        self._put_and_update_indexes(old_doc, doc)
-        return new_rev
-
-    # index-related methods
-
-    def create_index(self, index_name, *index_expressions):
-        """
-        Create a named index, which can then be queried for future lookups.
-
-        See U1DB documentation for more information.
-
-        @param index_name: A unique name which can be used as a key prefix.
-        @param index_expressions: Index expressions defining the index
-            information.
-        """
-        raise NotImplementedError(self.create_index)
-
-    #-------------------------------------------------------------------------
-    # implemented methods from CommonBackend
-    #-------------------------------------------------------------------------
-
-    def _put_and_update_indexes(self, old_doc, doc):
-        """
-        Update a document and all indexes related to it.
-
-        @param old_doc: The old version of the document.
-        @type old_doc: u1db.Document
-        @param doc: The new version of the document.
-        @type doc: u1db.Document
-        """
-        for index in self._indexes.itervalues():
-            if old_doc is not None and not old_doc.is_tombstone():
-                index.remove_json(old_doc.doc_id, old_doc.get_json())
-            if not doc.is_tombstone():
-                index.add_json(doc.doc_id, doc.get_json())
-        trans_id = self._allocate_transaction_id()
-        self._put_doc(doc)
-        self._transaction_log.append((doc.doc_id, trans_id))
-
-
-class ObjectStoreSyncTarget(InMemorySyncTarget):
-    """
-    Functionality for using an ObjectStore as a synchronization target.
-    """
diff --git a/common/src/leap/soledad/common/tests/couchdb.ini.template b/common/src/leap/soledad/common/tests/couchdb.ini.template
index 7d0316f0..217ae201 100644
--- a/common/src/leap/soledad/common/tests/couchdb.ini.template
+++ b/common/src/leap/soledad/common/tests/couchdb.ini.template
@@ -74,6 +74,8 @@ use_users_db = false
 
 [query_servers]
 ; javascript = %(tempdir)s/server/main.js
+javascript = /usr/bin/couchjs /usr/share/couchdb/server/main.js
+coffeescript = /usr/bin/couchjs /usr/share/couchdb/server/main-coffee.js
 
 
 ; Changing reduce_limit to false will disable reduce_limit.
@@ -219,4 +221,4 @@ min_file_size = 131072
 
 ;[admins]
 ;testuser = -hashed-f50a252c12615697c5ed24ec5cd56b05d66fe91e,b05471ba260132953930cf9f97f327f5
-; pass for above user is 'testpass'
\ No newline at end of file
+; pass for above user is 'testpass'
diff --git a/common/src/leap/soledad/common/tests/test_couch.py b/common/src/leap/soledad/common/tests/test_couch.py
index 42edf9fe..72346333 100644
--- a/common/src/leap/soledad/common/tests/test_couch.py
+++ b/common/src/leap/soledad/common/tests/test_couch.py
@@ -25,9 +25,11 @@ import copy
 import shutil
 from base64 import b64decode
 
+from couchdb.client import Server
+from u1db import errors
+
 from leap.common.files import mkdir_p
 
-from leap.soledad.common.document import SoledadDocument
 from leap.soledad.common.tests import u1db_tests as tests
 from leap.soledad.common.tests.u1db_tests import test_backends
 from leap.soledad.common.tests.u1db_tests import test_sync
@@ -148,7 +150,7 @@ class TestCouchBackendImpl(CouchDBTestCase):
 
     def test__allocate_doc_id(self):
         db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
+                                 'u1db_tests', ensure_ddocs=True)
         doc_id1 = db._allocate_doc_id()
         self.assertTrue(doc_id1.startswith('D-'))
         self.assertEqual(34, len(doc_id1))
@@ -163,32 +165,51 @@ class TestCouchBackendImpl(CouchDBTestCase):
 def make_couch_database_for_test(test, replica_uid):
     port = str(test.wrapper.port)
     return couch.CouchDatabase('http://localhost:' + port, replica_uid,
-                               replica_uid=replica_uid or 'test')
+                               replica_uid=replica_uid or 'test',
+                               ensure_ddocs=True)
 
 
 def copy_couch_database_for_test(test, db):
     port = str(test.wrapper.port)
-    new_db = couch.CouchDatabase('http://localhost:' + port,
-                                 db._replica_uid + '_copy',
+    couch_url = 'http://localhost:' + port
+    new_dbname = db._replica_uid + '_copy'
+    new_db = couch.CouchDatabase(couch_url,
+                                 new_dbname,
                                  replica_uid=db._replica_uid or 'test')
-    gen, docs = db.get_all_docs(include_deleted=True)
-    for doc in docs:
-        new_db._put_doc(doc)
-    new_db._transaction_log = copy.deepcopy(db._transaction_log)
-    new_db._conflicts = copy.deepcopy(db._conflicts)
-    new_db._other_generations = copy.deepcopy(db._other_generations)
-    new_db._indexes = copy.deepcopy(db._indexes)
-    # save u1db data on couch
-    for key in new_db.U1DB_DATA_KEYS:
-        doc_id = '%s%s' % (new_db.U1DB_DATA_DOC_ID_PREFIX, key)
-        doc = new_db._get_doc(doc_id)
-        doc.content = {'content': getattr(new_db, key)}
-        new_db._put_doc(doc)
+    # copy all docs
+    old_couch_db = Server(couch_url)[db._replica_uid]
+    new_couch_db = Server(couch_url)[new_dbname]
+    for doc_id in old_couch_db:
+        doc = old_couch_db.get(doc_id)
+        # copy design docs
+        if ('u1db_rev' not in doc):
+            new_couch_db.save(doc)
+        # copy u1db docs
+        else:
+            new_doc = {
+                '_id': doc['_id'],
+                'u1db_transactions': doc['u1db_transactions'],
+                'u1db_rev': doc['u1db_rev']
+            }
+            attachments = []
+            if ('u1db_conflicts' in doc):
+                new_doc['u1db_conflicts'] = doc['u1db_conflicts']
+                for c_rev in doc['u1db_conflicts']:
+                    attachments.append('u1db_conflict_%s' % c_rev)
+            new_couch_db.save(new_doc)
+            # save conflict data
+            attachments.append('u1db_content')
+            for att_name in attachments:
+                att = old_couch_db.get_attachment(doc_id, att_name)
+                if (att is not None):
+                    new_couch_db.put_attachment(new_doc, att,
+                                                filename=att_name)
     return new_db
 
 
 def make_document_for_test(test, doc_id, rev, content, has_conflicts=False):
-    return SoledadDocument(doc_id, rev, content, has_conflicts=has_conflicts)
+    return couch.CouchDocument(
+        doc_id, rev, content, has_conflicts=has_conflicts)
 
 
 COUCH_SCENARIOS = [
@@ -202,8 +223,22 @@ class CouchTests(test_backends.AllDatabaseTests, CouchDBTestCase):
 
     scenarios = COUCH_SCENARIOS
 
+    def setUp(self):
+        test_backends.AllDatabaseTests.setUp(self)
+        # save db info because of test_close
+        self._server = self.db._server
+        self._dbname = self.db._dbname
+
     def tearDown(self):
-        self.db.delete_database()
+        # if current test is `test_close` we have to use saved objects to
+        # delete the database because the close() method will have removed the
+        # references needed to do it using the CouchDatabase.
+        if self.id() == \
+                'leap.soledad.common.tests.test_couch.CouchTests.' \
+                'test_close(couch)':
+            del(self._server[self._dbname])
+        else:
+            self.db.delete_database()
         test_backends.AllDatabaseTests.tearDown(self)
 
 
@@ -246,17 +281,16 @@ class CouchWithConflictsTests(
         test_backends.LocalDatabaseWithConflictsTests.tearDown(self)
 
 
-# Notice: the CouchDB backend is currently used for storing encrypted data in
-# the server, so indexing makes no sense. Thus, we ignore index testing for
-# now.
-
-class CouchIndexTests(test_backends.DatabaseIndexTests, CouchDBTestCase):
+# Notice: the CouchDB backend does not have indexing capabilities, so we do
+# not test indexing now.
 
-    scenarios = COUCH_SCENARIOS
-
-    def tearDown(self):
-        self.db.delete_database()
-        test_backends.DatabaseIndexTests.tearDown(self)
+#class CouchIndexTests(test_backends.DatabaseIndexTests, CouchDBTestCase):
+#
+#    scenarios = COUCH_SCENARIOS
+#
+#    def tearDown(self):
+#        self.db.delete_database()
+#        test_backends.DatabaseIndexTests.tearDown(self)
 
 
 #-----------------------------------------------------------------------------
@@ -311,6 +345,89 @@ class CouchDatabaseSyncTargetTests(test_sync.DatabaseSyncTargetTests,
                  [(doc.doc_id, doc.rev), (doc2.doc_id, doc2.rev)]})
 
 
+# The following tests need that the database have an index, so we fake one.
+old_class = couch.CouchDatabase
+
+from u1db.backends.inmemory import InMemoryIndex
+
+
+class IndexedCouchDatabase(couch.CouchDatabase):
+
+    def __init__(self, url, dbname, replica_uid=None, full_commit=True,
+                     session=None, ensure_ddocs=True):
+        old_class.__init__(self, url, dbname, replica_uid, full_commit,
+                           session, ensure_ddocs=True)
+        self._indexes = {}
+
+    def _put_doc(self, old_doc, doc):
+        for index in self._indexes.itervalues():
+            if old_doc is not None and not old_doc.is_tombstone():
+                index.remove_json(old_doc.doc_id, old_doc.get_json())
+            if not doc.is_tombstone():
+                index.add_json(doc.doc_id, doc.get_json())
+        old_class._put_doc(self, old_doc, doc)
+
+    def create_index(self, index_name, *index_expressions):
+        if index_name in self._indexes:
+            if self._indexes[index_name]._definition == list(
+                    index_expressions):
+                return
+            raise errors.IndexNameTakenError
+        index = InMemoryIndex(index_name, list(index_expressions))
+        _, all_docs = self.get_all_docs()
+        for doc in all_docs:
+            index.add_json(doc.doc_id, doc.get_json())
+        self._indexes[index_name] = index
+
+    def delete_index(self, index_name):
+        del self._indexes[index_name]
+
+    def list_indexes(self):
+        definitions = []
+        for idx in self._indexes.itervalues():
+            definitions.append((idx._name, idx._definition))
+        return definitions
+
+    def get_from_index(self, index_name, *key_values):
+        try:
+            index = self._indexes[index_name]
+        except KeyError:
+            raise errors.IndexDoesNotExist
+        doc_ids = index.lookup(key_values)
+        result = []
+        for doc_id in doc_ids:
+            result.append(self._get_doc(doc_id, check_for_conflicts=True))
+        return result
+
+    def get_range_from_index(self, index_name, start_value=None,
+                             end_value=None):
+        """Return all documents with key values in the specified range."""
+        try:
+            index = self._indexes[index_name]
+        except KeyError:
+            raise errors.IndexDoesNotExist
+        if isinstance(start_value, basestring):
+            start_value = (start_value,)
+        if isinstance(end_value, basestring):
+            end_value = (end_value,)
+        doc_ids = index.lookup_range(start_value, end_value)
+        result = []
+        for doc_id in doc_ids:
+            result.append(self._get_doc(doc_id, check_for_conflicts=True))
+        return result
+
+    def get_index_keys(self, index_name):
+        try:
+            index = self._indexes[index_name]
+        except KeyError:
+            raise errors.IndexDoesNotExist
+        keys = index.keys()
+        # XXX inefficiency warning
+        return list(set([tuple(key.split('\x01')) for key in keys]))
+
+
+couch.CouchDatabase = IndexedCouchDatabase
+
 sync_scenarios = []
 for name, scenario in COUCH_SCENARIOS:
     scenario = dict(scenario)
@@ -344,98 +461,4 @@ class CouchDatabaseSyncTests(test_sync.DatabaseSyncTests, CouchDBTestCase):
         test_sync.DatabaseSyncTests.tearDown(self)
 
 
-#-----------------------------------------------------------------------------
-# The following tests test extra functionality introduced by our backends
-#-----------------------------------------------------------------------------
-
-class CouchDatabaseStorageTests(CouchDBTestCase):
-
-    def _listify(self, l):
-        if type(l) is dict:
-            return {
-                self._listify(a): self._listify(b) for a, b in l.iteritems()}
-        if hasattr(l, '__iter__'):
-            return [self._listify(i) for i in l]
-        return l
-
-    def _fetch_u1db_data(self, db, key):
-        doc = db._get_doc("%s%s" % (db.U1DB_DATA_DOC_ID_PREFIX, key))
-        return doc.content['content']
-
-    def test_transaction_log_storage_after_put(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        db.create_doc({'simple': 'doc'})
-        content = self._fetch_u1db_data(db, db.U1DB_TRANSACTION_LOG_KEY)
-        self.assertEqual(
-            self._listify(db._transaction_log),
-            self._listify(content))
-
-    def test_conflict_log_storage_after_put_if_newer(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        doc = db.create_doc({'simple': 'doc'})
-        doc.set_json(nested_doc)
-        doc.rev = db._replica_uid + ':2'
-        db._force_doc_sync_conflict(doc)
-        content = self._fetch_u1db_data(db, db.U1DB_CONFLICTS_KEY)
-        self.assertEqual(
-            self._listify(db._conflicts),
-            self._listify(content))
-
-    def test_other_gens_storage_after_set(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        doc = db.create_doc({'simple': 'doc'})
-        db._set_replica_gen_and_trans_id('a', 'b', 'c')
-        content = self._fetch_u1db_data(db, db.U1DB_OTHER_GENERATIONS_KEY)
-        self.assertEqual(
-            self._listify(db._other_generations),
-            self._listify(content))
-
-    def test_index_storage_after_create(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        doc = db.create_doc({'name': 'john'})
-        db.create_index('myindex', 'name')
-        content = self._fetch_u1db_data(db, db.U1DB_INDEXES_KEY)
-        myind = db._indexes['myindex']
-        index = {
-            'myindex': {
-                'definition': myind._definition,
-                'name': myind._name,
-                'values': myind._values,
-            }
-        }
-        self.assertEqual(
-            self._listify(index),
-            self._listify(content))
-
-    def test_index_storage_after_delete(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        doc = db.create_doc({'name': 'john'})
-        db.create_index('myindex', 'name')
-        db.create_index('myindex2', 'name')
-        db.delete_index('myindex')
-        content = self._fetch_u1db_data(db, db.U1DB_INDEXES_KEY)
-        myind = db._indexes['myindex2']
-        index = {
-            'myindex2': {
-                'definition': myind._definition,
-                'name': myind._name,
-                'values': myind._values,
-            }
-        }
-        self.assertEqual(
-            self._listify(index),
-            self._listify(content))
-
-    def test_replica_uid_storage_after_db_creation(self):
-        db = couch.CouchDatabase('http://localhost:' + str(self.wrapper.port),
-                                 'u1db_tests')
-        content = self._fetch_u1db_data(db, db.U1DB_REPLICA_UID_KEY)
-        self.assertEqual(db._replica_uid, content)
-
-
 load_tests = tests.load_with_scenarios
diff --git a/common/src/leap/soledad/common/tests/test_couch_operations_atomicity.py b/common/src/leap/soledad/common/tests/test_couch_operations_atomicity.py
new file mode 100644
index 00000000..a0c473b1
--- /dev/null
+++ b/common/src/leap/soledad/common/tests/test_couch_operations_atomicity.py
@@ -0,0 +1,339 @@
+# -*- coding: utf-8 -*-
+# test_soledad.py
+# Copyright (C) 2013 LEAP
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+
+"""
+"""
+
+import os
+import mock
+import tempfile
+import threading
+
+from leap.soledad.client import Soledad
+from leap.soledad.common.couch import CouchDatabase, CouchServerState
+from leap.soledad.common.tests.test_couch import CouchDBTestCase
+from leap.soledad.common.tests.u1db_tests import TestCaseWithServer
+from leap.soledad.common.tests.test_target import (
+    make_token_soledad_app,
+    make_leap_document_for_test,
+    token_leap_sync_target,
+)
+
+
+REPEAT_TIMES = 20
+
+
+class CouchAtomicityTestCase(CouchDBTestCase, TestCaseWithServer):
+
+    @staticmethod
+    def make_app_after_state(state):
+        return make_token_soledad_app(state)
+
+    make_document_for_test = make_leap_document_for_test
+
+    sync_target = token_leap_sync_target
+
+    def _soledad_instance(self, user='user-uuid', passphrase=u'123',
+                          prefix='',
+                          secrets_path=Soledad.STORAGE_SECRETS_FILE_NAME,
+                          local_db_path='soledad.u1db', server_url='',
+                          cert_file=None, auth_token=None, secret_id=None):
+        """
+        Instantiate Soledad.
+        """
+
+        # this callback ensures we save a document which is sent to the shared
+        # db.
+        def _put_doc_side_effect(doc):
+            self._doc_put = doc
+
+        # we need a mocked shared db or else Soledad will try to access the
+        # network to find if there are uploaded secrets.
+        class MockSharedDB(object):
+
+            get_doc = mock.Mock(return_value=None)
+            put_doc = mock.Mock(side_effect=_put_doc_side_effect)
+            lock = mock.Mock(return_value=('atoken', 300))
+            unlock = mock.Mock()
+
+            def __call__(self):
+                return self
+
+        Soledad._shared_db = MockSharedDB()
+        return Soledad(
+            user,
+            passphrase,
+            secrets_path=os.path.join(self.tempdir, prefix, secrets_path),
+            local_db_path=os.path.join(
+                self.tempdir, prefix, local_db_path),
+            server_url=server_url,
+            cert_file=cert_file,
+            auth_token=auth_token,
+            secret_id=secret_id)
+
+    def make_app(self):
+        self.request_state = CouchServerState(self._couch_url, 'shared',
+                                              'tokens')
+        return self.make_app_after_state(self.request_state)
+
+    def setUp(self):
+        TestCaseWithServer.setUp(self)
+        CouchDBTestCase.setUp(self)
+        self._couch_url = 'http://localhost:' + str(self.wrapper.port)
+        self.db = CouchDatabase(
+            self._couch_url, 'user-user-uuid', replica_uid='replica')
+        self.tempdir = tempfile.mkdtemp(prefix="leap_tests-")
+
+    def tearDown(self):
+        CouchDBTestCase.tearDown(self)
+        TestCaseWithServer.tearDown(self)
+
+    #
+    # Sequential tests
+    #
+
+    def test_correct_transaction_log_after_sequential_puts(self):
+        """
+        Assert that the transaction_log increases accordingly with sequential
+        puts.
+        """
+        doc = self.db.create_doc({'ops': 0})
+        ops = 1
+        docs = [doc.doc_id]
+        for i in range(0, REPEAT_TIMES):
+            self.assertEqual(
+                i+1, len(self.db._get_transaction_log()))
+            doc.content['ops'] += 1
+            self.db.put_doc(doc)
+            docs.append(doc.doc_id)
+
+        # assert length of transaction_log
+        transaction_log = self.db._get_transaction_log()
+        self.assertEqual(
+            REPEAT_TIMES+1, len(transaction_log))
+
+        # assert that all entries in the log belong to the same doc
+        self.assertEqual(REPEAT_TIMES+1, len(docs))
+        for doc_id in docs:
+            self.assertEqual(
+                REPEAT_TIMES+1,
+                len(filter(lambda t: t[0] == doc_id, transaction_log)))
+
+    def test_correct_transaction_log_after_sequential_deletes(self):
+        """
+        Assert that the transaction_log increases accordingly with sequential
+        puts and deletes.
+        """
+        docs = []
+        for i in range(0, REPEAT_TIMES):
+            doc = self.db.create_doc({'ops': 0})
+            self.assertEqual(
+                2*i+1, len(self.db._get_transaction_log()))
+            docs.append(doc.doc_id)
+            self.db.delete_doc(doc)
+            self.assertEqual(
+                2*i+2, len(self.db._get_transaction_log()))
+
+        # assert length of transaction_log
+        transaction_log = self.db._get_transaction_log()
+        self.assertEqual(
+            2*REPEAT_TIMES, len(transaction_log))
+
+        # assert that each doc appears twice in the transaction_log
+        self.assertEqual(REPEAT_TIMES, len(docs))
+        for doc_id in docs:
+            self.assertEqual(
+                2,
+                len(filter(lambda t: t[0] == doc_id, transaction_log)))
+
+    def test_correct_sync_log_after_sequential_syncs(self):
+        """
+        Assert that the sync_log increases accordingly with sequential syncs.
+        """
+        self.startServer()
+        sol = self._soledad_instance(
+            auth_token='auth-token',
+            server_url=self.getURL())
+
+        def _create_docs_and_sync(sol, syncs):
+            # create a lot of documents
+            for i in range(0, REPEAT_TIMES):
+                sol.create_doc({})
+            # assert sizes of transaction and sync logs
+            self.assertEqual(
+                syncs*REPEAT_TIMES,
+                len(self.db._get_transaction_log()))
+            self.assertEqual(
+                1 if syncs > 0 else 0,
+                len(self.db._database.view('syncs/log').rows))
+            # sync to the remote db
+            sol.sync()
+            gen, docs = self.db.get_all_docs()
+            self.assertEqual((syncs+1)*REPEAT_TIMES, gen)
+            self.assertEqual((syncs+1)*REPEAT_TIMES, len(docs))
+            # assert sizes of transaction and sync logs
+            self.assertEqual((syncs+1)*REPEAT_TIMES,
+                             len(self.db._get_transaction_log()))
+            sync_log_rows = self.db._database.view('syncs/log').rows
+            sync_log = sync_log_rows[0].value
+            replica_uid = sync_log_rows[0].key
+            known_gen = sync_log['known_generation']
+            known_trans_id = sync_log['known_transaction_id']
+            # assert sync_log has exactly 1 row
+            self.assertEqual(1, len(sync_log_rows))
+            # assert it has the correct replica_uid, gen and trans_id
+            self.assertEqual(sol._db._replica_uid, replica_uid)
+            sol_gen, sol_trans_id = sol._db._get_generation_info()
+            self.assertEqual(sol_gen, known_gen)
+            self.assertEqual(sol_trans_id, known_trans_id)
+
+        _create_docs_and_sync(sol, 0)
+        _create_docs_and_sync(sol, 1)
+
+    #
+    # Concurrency tests
+    #
+    
+    class _WorkerThread(threading.Thread):
+        
+        def __init__(self, params, run_method):
+            threading.Thread.__init__(self)
+            self._params = params
+            self._run_method = run_method
+
+        def run(self):
+            self._run_method(self)
+
+    def test_correct_transaction_log_after_concurrent_puts(self):
+        """
+        Assert that the transaction_log increases accordingly with concurrent
+        puts.
+        """
+        pool = threading.BoundedSemaphore(value=1)
+        threads = []
+        docs = []
+
+        def _run_method(self):
+            doc = self._params['db'].create_doc({})
+            pool.acquire()
+            self._params['docs'].append(doc.doc_id)
+            pool.release()
+
+
+        for i in range(0, REPEAT_TIMES):
+            thread = self._WorkerThread(
+                {'docs': docs, 'db': self.db},
+                _run_method)
+            thread.start()
+            threads.append(thread)
+
+        for thread in threads:
+            thread.join()
+        
+        # assert length of transaction_log
+        transaction_log = self.db._get_transaction_log()
+        self.assertEqual(
+            REPEAT_TIMES, len(transaction_log))
+
+        # assert all documents are in the log
+        self.assertEqual(REPEAT_TIMES, len(docs))
+        for doc_id in docs:
+            self.assertEqual(
+                1,
+                len(filter(lambda t: t[0] == doc_id, transaction_log)))
+
+    def test_correct_transaction_log_after_concurrent_deletes(self):
+        """
+        Assert that the transaction_log increases accordingly with concurrent
+        puts and deletes.
+        """
+        threads = []
+        docs = []
+        pool = threading.BoundedSemaphore(value=1)
+
+        # create/delete method that will be run concurrently
+        def _run_method(self):
+            doc = self._params['db'].create_doc({})
+            pool.acquire()
+            docs.append(doc.doc_id)
+            pool.release()
+            self._params['db'].delete_doc(doc)
+
+        # launch concurrent threads
+        for i in range(0, REPEAT_TIMES):
+            thread = self._WorkerThread({'db': self.db}, _run_method)
+            thread.start()
+            threads.append(thread)
+
+        # wait for threads to finish
+        for thread in threads:
+            thread.join()
+
+        # assert transaction log
+        transaction_log = self.db._get_transaction_log()
+        self.assertEqual(
+            2*REPEAT_TIMES, len(transaction_log))
+        # assert that each doc appears twice in the transaction_log
+        self.assertEqual(REPEAT_TIMES, len(docs))
+        for doc_id in docs:
+            self.assertEqual(
+                2,
+                len(filter(lambda t: t[0] == doc_id, transaction_log)))
+
+    def test_correct_sync_log_after_concurrent_puts_and_sync(self):
+        """
+        Assert that the sync_log is correct after concurrent syncs.
+        """
+        threads = []
+        docs = []
+        pool = threading.BoundedSemaphore(value=1)
+        self.startServer()
+        sol = self._soledad_instance(
+            auth_token='auth-token',
+            server_url=self.getURL())
+
+        def _run_method(self):
+            # create a lot of documents
+            doc = self._params['sol'].create_doc({})
+            pool.acquire()
+            docs.append(doc.doc_id)
+            pool.release()
+
+        # launch threads to create documents in parallel
+        for i in range(0, REPEAT_TIMES):
+            thread = self._WorkerThread(
+                {'sol': sol, 'syncs': i},
+                _run_method)
+            thread.start()
+            threads.append(thread)
+
+        # wait for threads to finish
+        for thread in threads:
+            thread.join()
+        
+        # do the sync!
+        sol.sync()
+
+        transaction_log = self.db._get_transaction_log()
+        self.assertEqual(REPEAT_TIMES, len(transaction_log))
+        # assert all documents are in the remote log
+        self.assertEqual(REPEAT_TIMES, len(docs))
+        for doc_id in docs:
+            self.assertEqual(
+                1,
+                len(filter(lambda t: t[0] == doc_id, transaction_log)))