Merge tag '0.9.0'

Tag version 0.9.0

28 files changed, 298 insertions, 647 deletions

diff --git a/common/src/leap/soledad/common/.gitignore b/common/src/leap/soledad/common/.gitignore
deleted file mode 100644
index 3378c78a..00000000
--- a/common/src/leap/soledad/common/.gitignore
+++ /dev/null
@@ -1 +0,0 @@
-ddocs.py
diff --git a/common/src/leap/soledad/common/README.txt b/common/src/leap/soledad/common/README.txt
index 38b9858e..0a252650 100644
--- a/common/src/leap/soledad/common/README.txt
+++ b/common/src/leap/soledad/common/README.txt
@@ -60,15 +60,6 @@ implemented in a way that all changes will be pushed with just one operation.
     * 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
diff --git a/common/src/leap/soledad/common/couch/__init__.py b/common/src/leap/soledad/common/couch/__init__.py
index 523a50a0..0f4102db 100644
--- a/common/src/leap/soledad/common/couch/__init__.py
+++ b/common/src/leap/soledad/common/couch/__init__.py
@@ -23,21 +23,17 @@ import json
 import re
 import uuid
 import binascii
-import time
-import functools
 
 
 from StringIO import StringIO
 from urlparse import urljoin
 from contextlib import contextmanager
-from multiprocessing.pool import ThreadPool
 
 
 from couchdb.client import Server, Database
 from couchdb.http import (
     ResourceConflict,
     ResourceNotFound,
-    ServerError,
     Session,
     urljoin as couch_urljoin,
     Resource,
@@ -50,9 +46,6 @@ from leap.soledad.common.l2db.errors import (
 from leap.soledad.common.l2db.remote import http_app
 
 
-from leap.soledad.common import ddocs
-from .errors import raise_server_error
-from .errors import raise_missing_design_doc_error
 from .support import MultipartWriter
 from leap.soledad.common.errors import InvalidURLError
 from leap.soledad.common.document import ServerDocument
@@ -100,7 +93,19 @@ def couch_server(url):
     yield server
 
 
-THREAD_POOL = ThreadPool(20)
+def _get_gen_doc_id(gen):
+    return 'gen-%s' % str(gen).zfill(10)
+
+
+GENERATION_KEY = 'gen'
+TRANSACTION_ID_KEY = 'trans_id'
+REPLICA_UID_KEY = 'replica_uid'
+DOC_ID_KEY = 'doc_id'
+SCHEMA_VERSION_KEY = 'schema_version'
+
+CONFIG_DOC_ID = '_local/config'
+SYNC_DOC_ID_PREFIX = '_local/sync_'
+SCHEMA_VERSION = 1
 
 
 class CouchDatabase(object):
@@ -111,7 +116,7 @@ class CouchDatabase(object):
     """
 
     @classmethod
-    def open_database(cls, url, create, ensure_ddocs=False, replica_uid=None,
+    def open_database(cls, url, create, replica_uid=None,
                       database_security=None):
         """
         Open a U1DB database using CouchDB as backend.
@@ -122,8 +127,6 @@ class CouchDatabase(object):
         :type create: bool
         :param replica_uid: an optional unique replica identifier
         :type replica_uid: str
-        :param ensure_ddocs: Ensure that the design docs exist on server.
-        :type ensure_ddocs: bool
         :param database_security: security rules as CouchDB security doc
         :type database_security: dict
 
@@ -144,21 +147,20 @@ class CouchDatabase(object):
                     server.create(dbname)
                 else:
                     raise DatabaseDoesNotExist()
-        db = cls(url,
-                 dbname, ensure_ddocs=ensure_ddocs,
+        db = cls(url, dbname, ensure_security=create,
                  database_security=database_security)
         return SoledadBackend(
             db, replica_uid=replica_uid)
 
-    def __init__(self, url, dbname, ensure_ddocs=True,
+    def __init__(self, url, dbname, ensure_security=False,
                  database_security=None):
         """
         :param url: Couch server URL with necessary credentials
         :type url: string
         :param dbname: Couch database name
         :type dbname: string
-        :param ensure_ddocs: Ensure that the design docs exist on server.
-        :type ensure_ddocs: bool
+        :param ensure_security: will PUT a _security ddoc if set
+        :type ensure_security: bool
         :param database_security: security rules as CouchDB security doc
         :type database_security: dict
         """
@@ -169,8 +171,7 @@ class CouchDatabase(object):
         self.batching = False
         self.batch_generation = None
         self.batch_docs = {}
-        if ensure_ddocs:
-            self.ensure_ddocs_on_db()
+        if ensure_security:
             self.ensure_security_ddoc(database_security)
 
     def batch_start(self):
@@ -205,22 +206,6 @@ class CouchDatabase(object):
         except ResourceNotFound:
             raise DatabaseDoesNotExist()
 
-    def ensure_ddocs_on_db(self):
-        """
-        Ensure that the design documents used by the backend exist on the
-        couch database.
-        """
-        for ddoc_name in ['docs', 'syncs', 'transactions']:
-            try:
-                self.json_from_resource(['_design'] +
-                                        ddoc_name.split('/') + ['_info'],
-                                        check_missing_ddoc=False)
-            except ResourceNotFound:
-                ddoc = json.loads(
-                    binascii.a2b_base64(
-                        getattr(ddocs, ddoc_name)))
-                self._database.save(ddoc)
-
     def ensure_security_ddoc(self, security_config=None):
         """
         Make sure that only soledad user is able to access this database as
@@ -261,13 +246,14 @@ class CouchDatabase(object):
         """
         try:
             # set on existent config document
-            doc = self._database['u1db_config']
-            doc['replica_uid'] = replica_uid
+            doc = self._database[CONFIG_DOC_ID]
+            doc[REPLICA_UID_KEY] = replica_uid
         except ResourceNotFound:
             # or create the config document
             doc = {
-                '_id': 'u1db_config',
-                'replica_uid': replica_uid,
+                '_id': CONFIG_DOC_ID,
+                REPLICA_UID_KEY: replica_uid,
+                SCHEMA_VERSION_KEY: SCHEMA_VERSION,
             }
         self._database.save(doc)
 
@@ -280,8 +266,8 @@ class CouchDatabase(object):
         """
         try:
             # grab replica_uid from server
-            doc = self._database['u1db_config']
-            replica_uid = doc['replica_uid']
+            doc = self._database[CONFIG_DOC_ID]
+            replica_uid = doc[REPLICA_UID_KEY]
             return replica_uid
         except ResourceNotFound:
             # create a unique replica_uid
@@ -308,8 +294,8 @@ class CouchDatabase(object):
         """
 
         generation, _ = self.get_generation_info()
-        results = list(self.get_docs(self._database,
-                                     include_deleted=include_deleted))
+        results = list(
+            self._get_docs(None, True, include_deleted))
         return (generation, results)
 
     def get_docs(self, doc_ids, check_for_conflicts=True,
@@ -330,24 +316,37 @@ class CouchDatabase(object):
                  in matching doc_ids order.
         :rtype: iterable
         """
-        # Workaround for:
-        #
-        #   http://bugs.python.org/issue7980
-        #   https://leap.se/code/issues/5449
-        #
-        # python-couchdb uses time.strptime, which is not thread safe. In
-        # order to avoid the problem described on the issues above, we preload
-        # strptime here by evaluating the conversion of an arbitrary date.
-        # This will not be needed when/if we switch from python-couchdb to
-        # paisley.
-        time.strptime('Mar 8 1917', '%b %d %Y')
-        get_one = functools.partial(
-            self.get_doc, check_for_conflicts=check_for_conflicts)
-        docs = [THREAD_POOL.apply_async(get_one, [doc_id])
-                for doc_id in doc_ids]
-        for doc in docs:
-            doc = doc.get()
-            if not doc or not include_deleted and doc.is_tombstone():
+        return self._get_docs(doc_ids, check_for_conflicts, include_deleted)
+
+    def _get_docs(self, doc_ids, check_for_conflicts, include_deleted):
+        """
+        Use couch's `_all_docs` view to get the documents indicated in
+        `doc_ids`,
+
+        :param doc_ids: A list of document identifiers or None for all.
+        :type doc_ids: list
+        :param check_for_conflicts: If set to False, then the conflict check
+                                    will be skipped, and 'None' will be
+                                    returned instead of True/False.
+        :type check_for_conflicts: 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.
+
+        :return: iterable giving the Document object for each document id
+                 in matching doc_ids order.
+        :rtype: iterable
+        """
+        params = {'include_docs': 'true', 'attachments': 'true'}
+        if doc_ids is not None:
+            params['keys'] = doc_ids
+        view = self._database.view("_all_docs", **params)
+        for row in view.rows:
+            result = row['doc']
+            doc = self.__parse_doc_from_couch(
+                result, result['_id'], check_for_conflicts=check_for_conflicts)
+            # filter out non-u1db or deleted documents
+            if not doc or (not include_deleted and doc.is_tombstone()):
                 continue
             yield doc
 
@@ -434,8 +433,6 @@ class CouchDatabase(object):
                         result['_attachments']['u1db_conflicts']['data']))))
         # store couch revision
         doc.couch_rev = result['_rev']
-        # store transactions
-        doc.transactions = result['u1db_transactions']
         return doc
 
     def _build_conflicts(self, doc_id, attached_conflicts):
@@ -471,14 +468,11 @@ class CouchDatabase(object):
         """
         if generation == 0:
             return ''
-        # query a couch list function
-        ddoc_path = [
-            '_design', 'transactions', '_list', 'trans_id_for_gen', 'log'
-        ]
-        response = self.json_from_resource(ddoc_path, gen=generation)
-        if response == {}:
+        log = self._get_transaction_log(start=generation, end=generation)
+        if not log:
             raise InvalidGeneration
-        return response['transaction_id']
+        _, _, trans_id = log[0]
+        return trans_id
 
     def get_replica_gen_and_trans_id(self, other_replica_uid):
         """
@@ -499,18 +493,19 @@ class CouchDatabase(object):
                  synchronized with the replica, this is (0, '').
         :rtype: (int, str)
         """
-        doc_id = 'u1db_sync_%s' % other_replica_uid
+        doc_id = '%s%s' % (SYNC_DOC_ID_PREFIX, other_replica_uid)
         try:
             doc = self._database[doc_id]
         except ResourceNotFound:
             doc = {
                 '_id': doc_id,
-                'generation': 0,
-                'transaction_id': '',
+                GENERATION_KEY: 0,
+                REPLICA_UID_KEY: str(other_replica_uid),
+                TRANSACTION_ID_KEY: '',
             }
             self._database.save(doc)
-        result = doc['generation'], doc['transaction_id']
-        return result
+        gen, trans_id = doc[GENERATION_KEY], doc[TRANSACTION_ID_KEY]
+        return gen, trans_id
 
     def get_doc_conflicts(self, doc_id, couch_rev=None):
         """
@@ -537,7 +532,6 @@ class CouchDatabase(object):
 
         try:
             response = self.json_from_resource([doc_id, 'u1db_conflicts'],
-                                               check_missing_ddoc=False,
                                                **params)
             return conflicts + self._build_conflicts(
                 doc_id, json.loads(response.read()))
@@ -562,13 +556,13 @@ class CouchDatabase(object):
                                      generation.
         :type other_transaction_id: str
         """
-        doc_id = 'u1db_sync_%s' % other_replica_uid
+        doc_id = '%s%s' % (SYNC_DOC_ID_PREFIX, other_replica_uid)
         try:
             doc = self._database[doc_id]
         except ResourceNotFound:
             doc = {'_id': doc_id}
-        doc['generation'] = other_generation
-        doc['transaction_id'] = other_transaction_id
+        doc[GENERATION_KEY] = other_generation
+        doc[TRANSACTION_ID_KEY] = other_transaction_id
         self._database.save(doc)
 
     def get_transaction_log(self):
@@ -578,12 +572,35 @@ class CouchDatabase(object):
         :return: The complete transaction log.
         :rtype: [(str, str)]
         """
-        # query a couch view
-        ddoc_path = ['_design', 'transactions', '_view', 'log']
-        response = self.json_from_resource(ddoc_path)
-        return map(
-            lambda row: (row['id'], row['value']),
-            response['rows'])
+        log = self._get_transaction_log()
+        return map(lambda i: (i[1], i[2]), log)
+
+    def _get_gen_docs(
+            self, start=0, end=9999999999, descending=None, limit=None):
+        params = {}
+        if descending:
+            params['descending'] = 'true'
+            # honor couch way of traversing the view tree in reverse order
+            start, end = end, start
+        params['startkey'] = _get_gen_doc_id(start)
+        params['endkey'] = _get_gen_doc_id(end)
+        params['include_docs'] = 'true'
+        if limit:
+            params['limit'] = limit
+        view = self._database.view("_all_docs", **params)
+        return view.rows
+
+    def _get_transaction_log(self, start=0, end=9999999999):
+        # get current gen and trans_id
+        rows = self._get_gen_docs(start=start, end=end)
+        log = []
+        for row in rows:
+            doc = row['doc']
+            log.append((
+                doc[GENERATION_KEY],
+                doc[DOC_ID_KEY],
+                doc[TRANSACTION_ID_KEY]))
+        return log
 
     def whats_changed(self, old_generation=0):
         """
@@ -602,32 +619,16 @@ class CouchDatabase(object):
                  changes first)
         :rtype: (int, str, [(str, int, str)])
         """
-        # query a couch list function
-        ddoc_path = [
-            '_design', 'transactions', '_list', 'whats_changed', 'log'
-        ]
-        response = self.json_from_resource(ddoc_path, old_gen=old_generation)
-        results = map(
-            lambda row:
-                (row['generation'], row['doc_id'], row['transaction_id']),
-            response['transactions'])
-        results.reverse()
-        cur_gen = old_generation
-        seen = set()
         changes = []
-        newest_trans_id = ''
-        for generation, doc_id, trans_id in results:
+        cur_generation, last_trans_id = self.get_generation_info()
+        relevant_tail = self._get_transaction_log(start=old_generation + 1)
+        seen = set()
+        for generation, doc_id, trans_id in reversed(relevant_tail):
             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()
-
-        return cur_gen, newest_trans_id, changes
+        changes.reverse()
+        return (cur_generation, last_trans_id, changes)
 
     def get_generation_info(self):
         """
@@ -638,53 +639,74 @@ class CouchDatabase(object):
         """
         if self.batching and self.batch_generation:
             return self.batch_generation
-        # query a couch list function
-        ddoc_path = ['_design', 'transactions', '_list', 'generation', 'log']
-        info = self.json_from_resource(ddoc_path)
-        return (info['generation'], info['transaction_id'])
+        rows = self._get_gen_docs(descending=True, limit=1)
+        if not rows:
+            return 0, ''
+        gen_doc = rows.pop()['doc']
+        return gen_doc[GENERATION_KEY], gen_doc[TRANSACTION_ID_KEY]
 
-    def json_from_resource(self, ddoc_path, check_missing_ddoc=True,
-                           **kwargs):
+    def json_from_resource(self, doc_path, **kwargs):
         """
         Get a resource from it's path and gets a doc's JSON using provided
-        parameters, also checking for missing design docs by default.
+        parameters.
 
-        :param ddoc_path: The path to resource.
-        :type ddoc_path: [str]
-        :param check_missing_ddoc: Raises info on what design doc is missing.
-        :type check_missin_ddoc: bool
+        :param doc_path: The path to resource.
+        :type doc_path: [str]
 
         :return: The request's data parsed from JSON to a dict.
         :rtype: dict
-
-        :raise MissingDesignDocError: Raised when tried to access a missing
-                                      design document.
-        :raise MissingDesignDocListFunctionError: Raised when trying to access
-                                                  a missing list function on a
-                                                  design document.
-        :raise MissingDesignDocNamedViewError: Raised when trying to access a
-                                               missing named view on a design
-                                               document.
-        :raise MissingDesignDocDeletedError: Raised when trying to access a
-                                             deleted design document.
-        :raise MissingDesignDocUnknownError: Raised when failed to access a
-                                             design document for an yet
-                                             unknown reason.
-        """
-        if ddoc_path is not None:
-            resource = self._database.resource(*ddoc_path)
+        """
+        if doc_path is not None:
+            resource = self._database.resource(*doc_path)
         else:
             resource = self._database.resource()
-        try:
-            _, _, data = resource.get_json(**kwargs)
-            return data
-        except ResourceNotFound as e:
-            if check_missing_ddoc:
-                raise_missing_design_doc_error(e, ddoc_path)
-            else:
-                raise e
-        except ServerError as e:
-            raise_server_error(e, ddoc_path)
+        _, _, data = resource.get_json(**kwargs)
+        return data
+
+    def _allocate_new_generation(self, doc_id, transaction_id):
+        """
+        Allocate a new generation number for a document modification.
+
+        We need to allocate a new generation to this document modification by
+        creating a new gen doc. In order to avoid concurrent database updates
+        from allocating the same new generation, we will try to create the
+        document until we succeed, meaning that no other piece of code holds
+        the same generation number as ours.
+
+        The loop below would only be executed more than once if:
+
+          1. there's more than one thread trying to modify the user's database,
+             and
+
+          2. the execution of getting the current generation and saving the gen
+             doc different threads get interleaved (one of them will succeed
+             and the others will fail and try again).
+
+        Number 1 only happens when more than one user device is syncing at the
+        same time. Number 2 depends on not-so-frequent coincidence of
+        code execution.
+
+        Also, in the race between threads for a generation number there's
+        always one thread that wins. so if there are N threads in the race, the
+        expected number of repetitions of the loop for each thread would be
+        N/2. If N is equal to the number of devices that the user has, the
+        number of possible repetitions of the loop should always be low.
+        """
+        while True:
+            try:
+                # add the gen document
+                gen, _ = self.get_generation_info()
+                new_gen = gen + 1
+                gen_doc = {
+                    '_id': _get_gen_doc_id(new_gen),
+                    GENERATION_KEY: new_gen,
+                    DOC_ID_KEY: doc_id,
+                    TRANSACTION_ID_KEY: transaction_id,
+                }
+                self._database.save(gen_doc)
+                break  # succeeded allocating a new generation, proceed
+            except ResourceConflict:
+                pass  # try again!
 
     def save_document(self, old_doc, doc, transaction_id):
         """
@@ -701,19 +723,6 @@ class CouchDatabase(object):
 
         :raise RevisionConflict: Raised when trying to update a document but
                                  couch revisions mismatch.
-        :raise MissingDesignDocError: Raised when tried to access a missing
-                                      design document.
-        :raise MissingDesignDocListFunctionError: Raised when trying to access
-                                                  a missing list function on a
-                                                  design document.
-        :raise MissingDesignDocNamedViewError: Raised when trying to access a
-                                               missing named view on a design
-                                               document.
-        :raise MissingDesignDocDeletedError: Raised when trying to access a
-                                             deleted design document.
-        :raise MissingDesignDocUnknownError: Raised when failed to access a
-                                             design document for an yet
-                                             unknown reason.
         """
         attachments = {}  # we save content and conflicts as attachments
         parts = []  # and we put it using couch's multipart PUT
@@ -726,6 +735,7 @@ class CouchDatabase(object):
                 'length': len(content),
             }
             parts.append(content)
+
         # save conflicts as attachment
         if doc.has_conflicts is True:
             conflicts = json.dumps(
@@ -737,21 +747,11 @@ class CouchDatabase(object):
                 'length': len(conflicts),
             }
             parts.append(conflicts)
-        # store old transactions, if any
-        transactions = old_doc.transactions[:] if old_doc is not None else []
-        # create a new transaction id and timestamp it so the transaction log
-        # is consistent when querying the database.
-        transactions.append(
-            # here we store milliseconds to keep consistent with javascript
-            # Date.prototype.getTime() which was used before inside a couchdb
-            # update handler.
-            (int(time.time() * 1000),
-             transaction_id))
+
         # build the couch document
         couch_doc = {
             '_id': doc.doc_id,
             'u1db_rev': doc.rev,
-            'u1db_transactions': transactions,
             '_attachments': attachments,
         }
         # if we are updating a doc we have to add the couch doc revision
@@ -761,7 +761,19 @@ class CouchDatabase(object):
         if not self.batching:
             buf = StringIO()
             envelope = MultipartWriter(buf)
-            envelope.add('application/json', json.dumps(couch_doc))
+            # the order in which attachments are described inside the
+            # serialization of the couch document must match the order in
+            # which they are actually written in the multipart structure.
+            # Because of that, we use `sorted_keys=True` in the json
+            # serialization (so "u1db_conflicts" comes before
+            # "u1db_content" on the couch document attachments
+            # description), and also reverse the order of the parts before
+            # writing them, so the "conflict" part is written before the
+            # "content" part.
+            envelope.add(
+                'application/json',
+                json.dumps(couch_doc, sort_keys=True))
+            parts.reverse()
             for part in parts:
                 envelope.add('application/octet-stream', part)
             envelope.close()
@@ -778,12 +790,14 @@ class CouchDatabase(object):
                 del attachment['follows']
                 del attachment['length']
                 index = 0 if name is 'u1db_content' else 1
-                attachment['data'] = binascii.b2a_base64(parts[index]).strip()
+                attachment['data'] = binascii.b2a_base64(
+                    parts[index]).strip()
             couch_doc['_attachments'] = attachments
             self.batch_docs[doc.doc_id] = couch_doc
             last_gen, last_trans_id = self.batch_generation
             self.batch_generation = (last_gen + 1, transaction_id)
-        return transactions[-1][1]
+
+        self._allocate_new_generation(doc.doc_id, transaction_id)
 
     def _new_resource(self, *path):
         """
diff --git a/common/src/leap/soledad/common/couch/errors.py b/common/src/leap/soledad/common/couch/errors.py
deleted file mode 100644
index 9b287c76..00000000
--- a/common/src/leap/soledad/common/couch/errors.py
+++ /dev/null
@@ -1,144 +0,0 @@
-# -*- coding: utf-8 -*-
-# errors.py
-# Copyright (C) 2015 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/>.
-from leap.soledad.common.errors import SoledadError, BackendNotReadyError
-from leap.soledad.common.errors import register_exception
-
-"""
-Specific errors that can be raised by CouchDatabase.
-"""
-
-
-@register_exception
-class MissingDesignDocError(BackendNotReadyError):
-
-    """
-    Raised when trying to access a missing couch design document.
-    """
-
-    wire_description = "missing design document"
-    status = 500
-
-
-@register_exception
-class MissingDesignDocNamedViewError(SoledadError):
-
-    """
-    Raised when trying to access a missing named view on a couch design
-    document.
-    """
-
-    wire_description = "missing design document named function"
-    status = 500
-
-
-@register_exception
-class MissingDesignDocListFunctionError(SoledadError):
-
-    """
-    Raised when trying to access a missing list function on a couch design
-    document.
-    """
-
-    wire_description = "missing design document list function"
-    status = 500
-
-
-@register_exception
-class MissingDesignDocDeletedError(SoledadError):
-
-    """
-    Raised when trying to access a deleted couch design document.
-    """
-
-    wire_description = "design document was deleted"
-    status = 500
-
-
-@register_exception
-class DesignDocUnknownError(SoledadError):
-
-    """
-    Raised when trying to access a couch design document and getting an
-    unknown error.
-    """
-
-    wire_description = "missing design document unknown error"
-    status = 500
-
-
-def raise_missing_design_doc_error(exc, ddoc_path):
-    """
-    Raise an appropriate exception when catching a ResourceNotFound when
-    accessing a design document.
-
-    :param exc: The exception cought.
-    :type exc: ResourceNotFound
-    :param ddoc_path: A list representing the requested path.
-    :type ddoc_path: list
-
-    :raise MissingDesignDocError: Raised when tried to access a missing design
-                                  document.
-    :raise MissingDesignDocListFunctionError: Raised when trying to access a
-                                              missing list function on a
-                                              design document.
-    :raise MissingDesignDocNamedViewError: Raised when trying to access a
-                                           missing named view on a design
-                                           document.
-    :raise MissingDesignDocDeletedError: Raised when trying to access a
-                                         deleted design document.
-    :raise MissingDesignDocUnknownError: Raised when failed to access a design
-                                         document for an yet unknown reason.
-    """
-    path = "".join(ddoc_path)
-    if exc.message[1] == 'missing':
-        raise MissingDesignDocError(path)
-    elif exc.message[1] == 'missing function' or \
-            exc.message[1].startswith('missing lists function'):
-        raise MissingDesignDocListFunctionError(path)
-    elif exc.message[1] == 'missing_named_view':
-        raise MissingDesignDocNamedViewError(path)
-    elif exc.message[1] == 'deleted':
-        raise MissingDesignDocDeletedError(path)
-    # other errors are unknown for now
-    raise DesignDocUnknownError("%s: %s" % (path, str(exc.message)))
-
-
-def raise_server_error(exc, ddoc_path):
-    """
-    Raise an appropriate exception when catching a ServerError when
-    accessing a design document.
-
-    :param exc: The exception cought.
-    :type exc: ResourceNotFound
-    :param ddoc_path: A list representing the requested path.
-    :type ddoc_path: list
-
-    :raise MissingDesignDocListFunctionError: Raised when trying to access a
-                                              missing list function on a
-                                              design document.
-    :raise MissingDesignDocUnknownError: Raised when failed to access a design
-                                         document for an yet unknown reason.
-    """
-    path = "".join(ddoc_path)
-    msg = exc.message[1][0]
-    if msg == 'unnamed_error':
-        raise MissingDesignDocListFunctionError(path)
-    elif msg == 'TypeError':
-        if 'point is undefined' in exc.message[1][1]:
-            raise MissingDesignDocListFunctionError
-    # other errors are unknown for now
-    raise DesignDocUnknownError("%s: %s" % (path, str(exc.message)))
diff --git a/common/src/leap/soledad/common/couch/state.py b/common/src/leap/soledad/common/couch/state.py
index 9b40a264..523ac0b0 100644
--- a/common/src/leap/soledad/common/couch/state.py
+++ b/common/src/leap/soledad/common/couch/state.py
@@ -17,20 +17,26 @@
 """
 Server state using CouchDatabase as backend.
 """
-import logging
+import couchdb
 import re
 import time
 from urlparse import urljoin
 from hashlib import sha512
 
+from leap.soledad.common.log import getLogger
 from leap.soledad.common.couch import CouchDatabase
 from leap.soledad.common.couch import couch_server
+from leap.soledad.common.couch import CONFIG_DOC_ID
+from leap.soledad.common.couch import SCHEMA_VERSION
+from leap.soledad.common.couch import SCHEMA_VERSION_KEY
 from leap.soledad.common.command import exec_validated_cmd
 from leap.soledad.common.l2db.remote.server_state import ServerState
 from leap.soledad.common.l2db.errors import Unauthorized
+from leap.soledad.common.errors import WrongCouchSchemaVersionError
+from leap.soledad.common.errors import MissingCouchConfigDocumentError
 
 
-logger = logging.getLogger(__name__)
+logger = getLogger(__name__)
 
 
 def is_db_name_valid(name):
@@ -59,15 +65,47 @@ class CouchServerState(ServerState):
     TOKENS_TYPE_DEF = "Token"
     TOKENS_USER_ID_KEY = "user_id"
 
-    def __init__(self, couch_url, create_cmd=None):
+    def __init__(self, couch_url, create_cmd=None,
+                 check_schema_versions=False):
         """
         Initialize the couch server state.
 
         :param couch_url: The URL for the couch database.
         :type couch_url: str
+        :param create_cmd: Command to be executed for user db creation. It will
+                           receive a properly sanitized parameter with user db
+                           name and should access CouchDB with necessary
+                           privileges, which server lacks for security reasons.
+        :type create_cmd: str
+        :param check_schema_versions: Whether to check couch schema version of
+                                      user dbs. Set to False as this is only
+                                      intended to run once during start-up.
+        :type check_schema_versions: bool
         """
         self.couch_url = couch_url
         self.create_cmd = create_cmd
+        if check_schema_versions:
+            self._check_schema_versions()
+
+    def _check_schema_versions(self):
+        """
+        Check that all user databases use the correct couch schema.
+        """
+        server = couchdb.client.Server(self.couch_url)
+        for dbname in server:
+            if not dbname.startswith('user-'):
+                continue
+            db = server[dbname]
+
+            # if there are documents, ensure that a config doc exists
+            config_doc = db.get(CONFIG_DOC_ID)
+            if config_doc:
+                if config_doc[SCHEMA_VERSION_KEY] != SCHEMA_VERSION:
+                    raise WrongCouchSchemaVersionError(dbname)
+            else:
+                result = db.view('_all_docs', limit=1)
+                if result.total_rows != 0:
+                    raise MissingCouchConfigDocumentError(dbname)
 
     def open_database(self, dbname):
         """
@@ -80,7 +118,7 @@ class CouchServerState(ServerState):
         :rtype: SoledadBackend
         """
         url = urljoin(self.couch_url, dbname)
-        db = CouchDatabase.open_database(url, create=False, ensure_ddocs=False)
+        db = CouchDatabase.open_database(url, create=False)
         return db
 
     def ensure_database(self, dbname):
diff --git a/common/src/leap/soledad/common/ddocs/README.txt b/common/src/leap/soledad/common/ddocs/README.txt
deleted file mode 100644
index 5569d929..00000000
--- a/common/src/leap/soledad/common/ddocs/README.txt
+++ /dev/null
@@ -1,34 +0,0 @@
-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/views/get/map.js b/common/src/leap/soledad/common/ddocs/docs/views/get/map.js
deleted file mode 100644
index ae08d9e9..00000000
--- a/common/src/leap/soledad/common/ddocs/docs/views/get/map.js
+++ /dev/null
@@ -1,20 +0,0 @@
-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/state.js b/common/src/leap/soledad/common/ddocs/syncs/updates/state.js
deleted file mode 100644
index d62aeb40..00000000
--- a/common/src/leap/soledad/common/ddocs/syncs/updates/state.js
+++ /dev/null
@@ -1,105 +0,0 @@
-/**
- * This update handler stores information about ongoing synchronization
- * attempts from distinct source replicas.
- *
- * Normally, u1db synchronization occurs during one POST request. In order to
- * split that into many serial POST requests, we store the state of each sync
- * in the server, using a document with id 'u1db_sync_state'.  To identify
- * each sync attempt, we use a sync_id sent by the client. If we ever receive
- * a new sync_id, we trash current data for that source replica and start
- * over.
- *
- * We expect the following in the document body:
- *
- * {
- *     'source_replica_uid': '<source_replica_uid>',
- *     'sync_id': '<sync_id>',
- *     'seen_ids': [['<doc_id>', <at_gen>], ...],     // optional
- *     'changes_to_return': [                         // optional
- *         'gen': <gen>,
- *         'trans_id': '<trans_id>',
- *         'changes_to_return': [[<doc_id>', <gen>, '<trans_id>'], ...]
- *     ],
- * }
- *
- * The format of the final document stored on server is:
- *
- * {
- *     '_id': '<str>',
- *     '_rev' '<str>',
- *     'ongoing_syncs': {
- *         '<source_replica_uid>': {
- *             'sync_id': '<sync_id>',
- *             'seen_ids': [['<doc_id>', <at_gen>[, ...],
- *             'changes_to_return': {
- *                  'gen': <gen>,
- *                  'trans_id': '<trans_id>',
- *                  'changes_to_return': [
- *                          ['<doc_id>', <gen>, '<trans_id>'],
- *                          ...,
- *                  ],
- *             },
- *         },
- *         ... // info about other source replicas here
- *     }
- * }
- */
-function(doc, req) {
-
-    // prevent updates to alien documents
-    if (doc != null && doc['_id'] != 'u1db_sync_state')
-        return [null, 'invalid data'];
-
-    // create the document if it doesn't exist
-    if (!doc)
-        doc = {
-            '_id': 'u1db_sync_state',
-            'ongoing_syncs': {},
-        };
-
-    // parse and validate incoming data
-    var body = JSON.parse(req.body);
-    if (body['source_replica_uid'] == null)
-        return [null, 'invalid data'];
-    var source_replica_uid = body['source_replica_uid'];
-
-    if (body['sync_id'] == null)
-        return [null, 'invalid data'];
-    var sync_id = body['sync_id'];
-
-    // trash outdated sync data for that replica if that exists
-    if (doc['ongoing_syncs'][source_replica_uid] != null &&
-            doc['ongoing_syncs'][source_replica_uid]['sync_id'] != sync_id)
-        delete doc['ongoing_syncs'][source_replica_uid];
-
-    // create an entry for that source replica
-    if (doc['ongoing_syncs'][source_replica_uid] == null)
-        doc['ongoing_syncs'][source_replica_uid] = {
-            'sync_id': sync_id,
-            'seen_ids': {},
-            'changes_to_return': null,
-        };
-
-    // incoming meta-data values should be exclusive, so we count how many
-    // arrived and deny to accomplish the transaction if the count is high.
-    var incoming_values = 0;
-    var info = doc['ongoing_syncs'][source_replica_uid]
-
-    // add incoming seen id
-    if ('seen_id' in body) {
-        info['seen_ids'][body['seen_id'][0]] = body['seen_id'][1];
-        incoming_values += 1;
-    }
-
-    // add incoming changes_to_return
-    if ('changes_to_return' in body) {
-        info['changes_to_return'] = body['changes_to_return'];
-        incoming_values += 1;
-    }
-
-    if (incoming_values != 1)
-        return [null, 'invalid data'];
-
-    return [doc, 'ok'];
-}
-
diff --git a/common/src/leap/soledad/common/ddocs/syncs/views/changes_to_return/map.js b/common/src/leap/soledad/common/ddocs/syncs/views/changes_to_return/map.js
deleted file mode 100644
index 94b7e767..00000000
--- a/common/src/leap/soledad/common/ddocs/syncs/views/changes_to_return/map.js
+++ /dev/null
@@ -1,20 +0,0 @@
-function(doc) {
-  if (doc['_id'] == 'u1db_sync_state' && doc['ongoing_syncs'] != null)
-    for (var source_replica_uid in doc['ongoing_syncs']) {
-      var changes = doc['ongoing_syncs'][source_replica_uid]['changes_to_return'];
-      var sync_id = doc['ongoing_syncs'][source_replica_uid]['sync_id'];
-      if (changes == null)
-        emit([source_replica_uid, sync_id, 0], null);
-      else if (changes.length == 0)
-        emit([source_replica_uid, sync_id, 0], []);
-      else
-        for (var i = 0; i < changes['changes_to_return'].length; i++)
-          emit(
-            [source_replica_uid, sync_id, i],
-            {
-              'gen': changes['gen'],
-              'trans_id': changes['trans_id'],
-              'next_change_to_return': changes['changes_to_return'][i],
-            });
-    }
-}
diff --git a/common/src/leap/soledad/common/ddocs/syncs/views/seen_ids/map.js b/common/src/leap/soledad/common/ddocs/syncs/views/seen_ids/map.js
deleted file mode 100644
index 16118e88..00000000
--- a/common/src/leap/soledad/common/ddocs/syncs/views/seen_ids/map.js
+++ /dev/null
@@ -1,11 +0,0 @@
-function(doc) {
-  if (doc['_id'] == 'u1db_sync_state' && doc['ongoing_syncs'] != null)
-    for (var source_replica_uid in doc['ongoing_syncs']) {
-      var sync_id = doc['ongoing_syncs'][source_replica_uid]['sync_id'];
-      emit(
-        [source_replica_uid, sync_id],
-        {
-          'seen_ids': doc['ongoing_syncs'][source_replica_uid]['seen_ids'],
-        });
-    }
-}
diff --git a/common/src/leap/soledad/common/ddocs/syncs/views/state/map.js b/common/src/leap/soledad/common/ddocs/syncs/views/state/map.js
deleted file mode 100644
index e88c6ebb..00000000
--- a/common/src/leap/soledad/common/ddocs/syncs/views/state/map.js
+++ /dev/null
@@ -1,17 +0,0 @@
-function(doc) {
-  if (doc['_id'] == 'u1db_sync_state' && doc['ongoing_syncs'] != null)
-    for (var source_replica_uid in doc['ongoing_syncs']) {
-      var changes = doc['ongoing_syncs'][source_replica_uid]['changes_to_return'];
-      var sync_id = doc['ongoing_syncs'][source_replica_uid]['sync_id'];
-      if (changes == null)
-        emit([source_replica_uid, sync_id], null);
-      else
-        emit(
-          [source_replica_uid, sync_id],
-          {
-            'gen': changes['gen'],
-            'trans_id': changes['trans_id'],
-            'number_of_changes': changes['changes_to_return'].length
-          });
-    }
-}
diff --git a/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js b/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js
deleted file mode 100644
index dbdfff0d..00000000
--- a/common/src/leap/soledad/common/ddocs/transactions/lists/generation.js
+++ /dev/null
@@ -1,20 +0,0 @@
-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
deleted file mode 100644
index 2ec91794..00000000
--- a/common/src/leap/soledad/common/ddocs/transactions/lists/trans_id_for_gen.js
+++ /dev/null
@@ -1,19 +0,0 @@
-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
deleted file mode 100644
index b35cdf51..00000000
--- a/common/src/leap/soledad/common/ddocs/transactions/lists/whats_changed.js
+++ /dev/null
@@ -1,22 +0,0 @@
-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
deleted file mode 100644
index 94ef63ca..00000000
--- a/common/src/leap/soledad/common/ddocs/transactions/views/log/map.js
+++ /dev/null
@@ -1,7 +0,0 @@
-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/errors.py b/common/src/leap/soledad/common/errors.py
index dec871c9..d543a3de 100644
--- a/common/src/leap/soledad/common/errors.py
+++ b/common/src/leap/soledad/common/errors.py
@@ -77,7 +77,6 @@ http_errors.ERROR_STATUSES = set(
 
 
 class InvalidURLError(Exception):
-
     """
     Exception raised when Soledad encounters a malformed URL.
     """
@@ -90,3 +89,15 @@ class BackendNotReadyError(SoledadError):
     """
     wire_description = "backend not ready"
     status = 500
+
+
+class WrongCouchSchemaVersionError(SoledadError):
+    """
+    Raised in case there is a user database with wrong couch schema version.
+    """
+
+
+class MissingCouchConfigDocumentError(SoledadError):
+    """
+    Raised if a database has documents but lacks the couch config document.
+    """
diff --git a/common/src/leap/soledad/common/l2db/__init__.py b/common/src/leap/soledad/common/l2db/__init__.py
index c0bd15fe..568897c4 100644
--- a/common/src/leap/soledad/common/l2db/__init__.py
+++ b/common/src/leap/soledad/common/l2db/__init__.py
@@ -16,10 +16,7 @@
 
 """L2DB"""
 
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 
 from leap.soledad.common.l2db.errors import InvalidJSON, InvalidContent
 
diff --git a/common/src/leap/soledad/common/l2db/backends/__init__.py b/common/src/leap/soledad/common/l2db/backends/__init__.py
index 922daafd..c731c3d3 100644
--- a/common/src/leap/soledad/common/l2db/backends/__init__.py
+++ b/common/src/leap/soledad/common/l2db/backends/__init__.py
@@ -17,10 +17,7 @@
 """Abstract classes and common implementations for the backends."""
 
 import re
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 import uuid
 
 from leap.soledad.common import l2db
diff --git a/common/src/leap/soledad/common/l2db/backends/inmemory.py b/common/src/leap/soledad/common/l2db/backends/inmemory.py
index 06a934a6..6fd251af 100644
--- a/common/src/leap/soledad/common/l2db/backends/inmemory.py
+++ b/common/src/leap/soledad/common/l2db/backends/inmemory.py
@@ -16,10 +16,7 @@
 
 """The in-memory Database class for U1DB."""
 
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 
 from leap.soledad.common.l2db import (
     Document, errors,
diff --git a/common/src/leap/soledad/common/l2db/backends/sqlite_backend.py b/common/src/leap/soledad/common/l2db/backends/sqlite_backend.py
index ba273039..d73c0d16 100644
--- a/common/src/leap/soledad/common/l2db/backends/sqlite_backend.py
+++ b/common/src/leap/soledad/common/l2db/backends/sqlite_backend.py
@@ -21,17 +21,14 @@ A L2DB implementation that uses SQLite as its persistence layer.
 
 import errno
 import os
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
-from sqlite3 import dbapi2
+import json
 import sys
 import time
 import uuid
-
 import pkg_resources
 
+from sqlite3 import dbapi2
+
 from leap.soledad.common.l2db.backends import CommonBackend, CommonSyncTarget
 from leap.soledad.common.l2db import (
     Document, errors,
diff --git a/common/src/leap/soledad/common/l2db/remote/basic_auth_middleware.py b/common/src/leap/soledad/common/l2db/remote/basic_auth_middleware.py
index a2cbff62..96d0d872 100644
--- a/common/src/leap/soledad/common/l2db/remote/basic_auth_middleware.py
+++ b/common/src/leap/soledad/common/l2db/remote/basic_auth_middleware.py
@@ -15,10 +15,8 @@
 # along with u1db.  If not, see <http://www.gnu.org/licenses/>.
 """U1DB Basic Auth authorisation WSGI middleware."""
 import httplib
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
+
 from wsgiref.util import shift_path_info
 
 
diff --git a/common/src/leap/soledad/common/l2db/remote/http_app.py b/common/src/leap/soledad/common/l2db/remote/http_app.py
index 65277bd1..5cf6645e 100644
--- a/common/src/leap/soledad/common/l2db/remote/http_app.py
+++ b/common/src/leap/soledad/common/l2db/remote/http_app.py
@@ -23,10 +23,7 @@ HTTP Application exposing U1DB.
 import functools
 import httplib
 import inspect
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 import sys
 import urlparse
 
diff --git a/common/src/leap/soledad/common/l2db/remote/http_client.py b/common/src/leap/soledad/common/l2db/remote/http_client.py
index a65264b6..53363c0a 100644
--- a/common/src/leap/soledad/common/l2db/remote/http_client.py
+++ b/common/src/leap/soledad/common/l2db/remote/http_client.py
@@ -17,10 +17,7 @@
 """Base class to make requests to a remote HTTP server."""
 
 import httplib
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 import socket
 import ssl
 import sys
diff --git a/common/src/leap/soledad/common/l2db/remote/http_database.py b/common/src/leap/soledad/common/l2db/remote/http_database.py
index b2b48dee..7512379f 100644
--- a/common/src/leap/soledad/common/l2db/remote/http_database.py
+++ b/common/src/leap/soledad/common/l2db/remote/http_database.py
@@ -16,10 +16,7 @@
 
 """HTTPDatabase to access a remote db over the HTTP API."""
 
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 import uuid
 
 from leap.soledad.common.l2db import (
diff --git a/common/src/leap/soledad/common/l2db/remote/http_target.py b/common/src/leap/soledad/common/l2db/remote/http_target.py
index 7e7f366f..38804f01 100644
--- a/common/src/leap/soledad/common/l2db/remote/http_target.py
+++ b/common/src/leap/soledad/common/l2db/remote/http_target.py
@@ -16,10 +16,7 @@
 
 """SyncTarget API implementation to a remote HTTP server."""
 
-try:
-    import simplejson as json
-except ImportError:
-    import json  # noqa
+import json
 
 from leap.soledad.common.l2db import Document, SyncTarget
 from leap.soledad.common.l2db.errors import BrokenSyncStream
diff --git a/common/src/leap/soledad/common/l2db/remote/server_state.py b/common/src/leap/soledad/common/l2db/remote/server_state.py
index f131e09e..e20b4679 100644
--- a/common/src/leap/soledad/common/l2db/remote/server_state.py
+++ b/common/src/leap/soledad/common/l2db/remote/server_state.py
@@ -15,8 +15,6 @@
 # along with u1db.  If not, see <http://www.gnu.org/licenses/>.
 
 """State for servers exposing a set of U1DB databases."""
-import os
-import errno
 
 
 class ServerState(object):
diff --git a/common/src/leap/soledad/common/l2db/sync.py b/common/src/leap/soledad/common/l2db/sync.py
index c612629f..5e9b22f4 100644
--- a/common/src/leap/soledad/common/l2db/sync.py
+++ b/common/src/leap/soledad/common/l2db/sync.py
@@ -126,8 +126,8 @@ class Synchronizer(object):
             target_last_known_gen, target_last_known_trans_id = 0, ''
         else:
             target_last_known_gen, target_last_known_trans_id = (
-            self.source._get_replica_gen_and_trans_id(  # nopep8
-                self.target_replica_uid))
+                self.source._get_replica_gen_and_trans_id(  # nopep8
+                    self.target_replica_uid))
         if not changes and target_last_known_gen == target_gen:
             if target_trans_id != target_last_known_trans_id:
                 raise errors.InvalidTransactionId
diff --git a/common/src/leap/soledad/common/log.py b/common/src/leap/soledad/common/log.py
new file mode 100644
index 00000000..3f026045
--- /dev/null
+++ b/common/src/leap/soledad/common/log.py
@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+# log.py
+# Copyright (C) 2016 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/>.
+
+
+"""
+This module centralizes logging facilities and allows for different behaviours,
+as using the python logging module instead of twisted logger, and to print logs
+to stdout, mainly for development purposes.
+"""
+
+
+import os
+import sys
+
+from twisted.logger import Logger
+from twisted.logger import textFileLogObserver
+
+
+def getLogger(*args, **kwargs):
+
+    if os.environ.get('SOLEDAD_USE_PYTHON_LOGGING'):
+        import logging
+        return logging.getLogger(__name__)
+
+    if os.environ.get('SOLEDAD_LOG_TO_STDOUT'):
+        kwargs({'observer': textFileLogObserver(sys.stdout)})
+
+    return Logger(*args, **kwargs)
+
+
+__all__ = ['getLogger']