Replace client sync state by a sync_id.

8 files changed, 84 insertions, 246 deletions

diff --git a/client/src/leap/soledad/client/sqlcipher.py b/client/src/leap/soledad/client/sqlcipher.py
index 74351116..26238af6 100644
--- a/client/src/leap/soledad/client/sqlcipher.py
+++ b/client/src/leap/soledad/client/sqlcipher.py
@@ -57,7 +57,7 @@ from pysqlcipher import dbapi2
 from u1db.backends import sqlite_backend
 from u1db import errors as u1db_errors
 
-from leap.soledad.client.sync import Synchronizer, ClientSyncState
+from leap.soledad.client.sync import Synchronizer
 from leap.soledad.client.target import SoledadSyncTarget
 from leap.soledad.common.document import SoledadDocument
 
@@ -889,45 +889,5 @@ class SQLCipherDatabase(sqlite_backend.SQLitePartialExpandDatabase):
         if self._db_handle is not None:
             self._db_handle.close()
 
-    def _get_stored_sync_state(self):
-        """
-        Retrieve the currently stored sync state.
-
-        :return: The current stored sync state or None if there's no stored
-                 state.
-        :rtype: dict or None
-        """
-        c = self._db_handle.cursor()
-        c.execute("SELECT value FROM u1db_config"
-                  " WHERE name = 'sync_state'")
-        val = c.fetchone()
-        if val is None:
-            return None
-        return json.loads(val[0])
-
-    def _set_stored_sync_state(self, state):
-        """
-        Stored the sync state.
-
-        :param state: The sync state to be stored or None to delete any stored
-                      state.
-        :type state: dict or None
-        """
-        c = self._db_handle.cursor()
-        if state is None:
-            c.execute("DELETE FROM u1db_config"
-                      " WHERE name = 'sync_state'")
-        else:
-            c.execute("INSERT OR REPLACE INTO u1db_config"
-                      " VALUES ('sync_state', ?)",
-                      (json.dumps(state),))
-
-    stored_sync_state = property(
-        _get_stored_sync_state, _set_stored_sync_state,
-        doc="The current sync state dict.")
-
-    @property
-    def sync_state(self):
-        return ClientSyncState(self)
 
 sqlite_backend.SQLiteDatabase.register_implementation(SQLCipherDatabase)
diff --git a/client/src/leap/soledad/client/sync.py b/client/src/leap/soledad/client/sync.py
index 5285d540..56e63416 100644
--- a/client/src/leap/soledad/client/sync.py
+++ b/client/src/leap/soledad/client/sync.py
@@ -27,103 +27,6 @@ from u1db import errors
 from u1db.sync import Synchronizer as U1DBSynchronizer
 
 
-class ClientSyncState(object):
-    """
-    The state of the current sync session, as stored on the client.
-    """
-
-    _private_attrs = [
-        '_db',
-    ]
-
-    _public_attrs = {
-        'target_replica_uid': None,
-        'target_gen': None,
-        'target_trans_id': None,
-        'target_my_gen': None,
-        'target_my_trans_id': None,
-        'target_last_known_gen': None,
-        'target_last_known_trans_id': None,
-        'my_gen': None,
-        'changes': None,
-        'sent': 0,
-        'received': 0,
-    }
-
-    @property
-    def _public_attr_keys(self):
-        return self._public_attrs.keys()
-
-    def __init__(self, db=None):
-        """
-        Initialize the client sync state.
-
-        :param db: The database where to fetch/store the sync state.
-        :type db: SQLCipherDatabase
-        """
-        self._db = db
-        self._init_state()
-
-    def __setattr__(self, attr, val):
-        """
-        Prevent setting arbitrary attributes.
-
-        :param attr: The attribute name.
-        :type attr: str
-        :param val: The value to be set.
-        :type val: anything
-        """
-        if attr not in self._public_attr_keys + self._private_attrs:
-            raise Exception
-        object.__setattr__(self, attr, val)
-
-    def _init_state(self):
-        """
-        Initialize current sync state, potentially fetching sync info stored
-        in database.
-        """
-        # set local default attributes
-        for attr in self._public_attr_keys:
-            setattr(self, attr, self._public_attrs[attr])
-        # fetch info from stored sync state
-        sync_state_dict = None
-        if self._db is not None:
-            sync_state_dict = self._db.stored_sync_state
-        if sync_state_dict is not None:
-            for attr in self._public_attr_keys:
-                setattr(self, attr, sync_state_dict[attr])
-
-    def save(self):
-        """
-        Save the current sync state in the database.
-        """
-        sync_state_dict = {}
-        for attr in self._public_attr_keys:
-            sync_state_dict[attr] = getattr(self, attr)
-        if self._db is not None:
-            self._db.stored_sync_state = sync_state_dict
-
-    def clear(self):
-        """
-        Clear the sync state info data.
-        """
-        if self._db is not None:
-            self._db.stored_sync_state = None
-        self._init_state()
-
-    def has_stored_info(self):
-        """
-        Return whether there is any sync state info stored on the database.
-
-        :return: Whether there's any sync state info store on db.
-        :rtype: bool
-        """
-        return self._db is not None and self._db.stored_sync_state is not None
-
-    def __str__(self):
-        return 'ClientSyncState: %s' % ', '.join(
-            ['%s: %s' % (k, getattr(self, k)) for k in self._public_attr_keys])
-
 class Synchronizer(U1DBSynchronizer):
     """
     Collect the state around synchronizing 2 U1DB replicas.
@@ -146,36 +49,20 @@ class Synchronizer(U1DBSynchronizer):
         """
         sync_target = self.sync_target
 
-        # recover current sync state from source database
-        sync_state = self.source.sync_state
-        self.target_replica_uid = sync_state.target_replica_uid
-        target_gen = sync_state.target_gen
-        target_trans_id = sync_state.target_trans_id
-        target_my_gen = sync_state.target_my_gen
-        target_my_trans_id = sync_state.target_my_trans_id
-        target_last_known_gen = sync_state.target_last_known_gen
-        target_last_known_trans_id = \
-            sync_state.target_last_known_trans_id
-        my_gen = sync_state.my_gen
-        changes = sync_state.changes
-        sent = sync_state.sent
-        received = sync_state.received
-
         # get target identifier, its current generation,
         # and its last-seen database generation for this source
         ensure_callback = None
-        if not sync_state.has_stored_info():
-            try:
-                (self.target_replica_uid, target_gen, target_trans_id,
-                 target_my_gen, target_my_trans_id) = \
-                    sync_target.get_sync_info(self.source._replica_uid)
-            except errors.DatabaseDoesNotExist:
-                if not autocreate:
-                    raise
-                # will try to ask sync_exchange() to create the db
-                self.target_replica_uid = None
-                target_gen, target_trans_id = (0, '')
-                target_my_gen, target_my_trans_id = (0, '')
+        try:
+            (self.target_replica_uid, target_gen, target_trans_id,
+             target_my_gen, target_my_trans_id) = \
+                sync_target.get_sync_info(self.source._replica_uid)
+        except errors.DatabaseDoesNotExist:
+            if not autocreate:
+                raise
+            # will try to ask sync_exchange() to create the db
+            self.target_replica_uid = None
+            target_gen, target_trans_id = (0, '')
+            target_my_gen, target_my_trans_id = (0, '')
 
         # make sure we'll have access to target replica uid once it exists
         if self.target_replica_uid is None:
@@ -192,17 +79,15 @@ class Synchronizer(U1DBSynchronizer):
             target_my_gen, target_my_trans_id)
 
         # what's changed since that generation and this current gen
-        if not sync_state.has_stored_info():
-            my_gen, _, changes = self.source.whats_changed(target_my_gen)
+        my_gen, _, changes = self.source.whats_changed(target_my_gen)
 
         # get source last-seen database generation for the target
-        if not sync_state.has_stored_info():
-            if self.target_replica_uid is None:
-                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(
-                        self.target_replica_uid)
+        if self.target_replica_uid is None:
+            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(
+                    self.target_replica_uid)
 
         # validate transaction ids
         if not changes and target_last_known_gen == target_gen:
@@ -220,20 +105,6 @@ class Synchronizer(U1DBSynchronizer):
             _, gen, trans = changes[idx]
             docs_by_generation.append((doc, gen, trans))
             idx += 1
-        # store current sync state info
-        if not sync_state.has_stored_info():
-            sync_state.target_replica_uid = self.target_replica_uid
-            sync_state.target_gen = target_gen
-            sync_state.target_trans_id = target_trans_id
-            sync_state.target_my_gen = target_my_gen
-            sync_state.target_my_trans_id = target_my_trans_id
-            sync_state.my_gen = my_gen
-            sync_state.changes = changes
-            sync_state.target_last_known_trans_id = \
-                target_last_known_trans_id
-            sync_state.target_last_known_gen = target_last_known_gen
-            sync_state.sent = sent = 0
-            sync_state.received = received = 0
 
         # exchange documents and try to insert the returned ones with
         # the target, return target synced-up-to gen.
@@ -243,16 +114,7 @@ class Synchronizer(U1DBSynchronizer):
         new_gen, new_trans_id = sync_target.sync_exchange(
             docs_by_generation, self.source._replica_uid,
             target_last_known_gen, target_last_known_trans_id,
-            self._insert_doc_from_target, ensure_callback=ensure_callback,
-            sync_state=sync_state)
-
-        # save sync state info if the sync was interrupted
-        if new_gen is None and new_trans_id is None:
-            sync_state.save()
-            return my_gen
-
-        # sync exchange was succesfull, remove sync state info from source
-        sync_state.clear()
+            self._insert_doc_from_target, ensure_callback=ensure_callback)
 
         # record target synced-up-to generation including applying what we sent
         self.source._set_replica_gen_and_trans_id(
diff --git a/client/src/leap/soledad/client/target.py b/client/src/leap/soledad/client/target.py
index 93de98d3..8f753f74 100644
--- a/client/src/leap/soledad/client/target.py
+++ b/client/src/leap/soledad/client/target.py
@@ -63,7 +63,6 @@ from leap.soledad.client.events import (
     SOLEDAD_SYNC_RECEIVE_STATUS,
     signal,
 )
-from leap.soledad.client.sync import ClientSyncState
 
 logger = logging.getLogger(__name__)
 
@@ -321,7 +320,6 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         HTTPSyncTarget.__init__(self, url, creds)
         self._crypto = crypto
         self._stopped = True
-        self._sync_state = None
         self._stop_lock = threading.Lock()
 
     def _init_post_request(self, url, action, headers, content_length):
@@ -347,7 +345,7 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         self._conn.endheaders()
 
     def _get_remote_docs(self, url, last_known_generation, last_known_trans_id,
-                         headers, return_doc_cb, ensure_callback=None):
+                         headers, return_doc_cb, ensure_callback, sync_id):
         """
         Fetch sync documents from the remote database and insert them in the
         local database.
@@ -377,7 +375,7 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         :rtype: list of str
         """
 
-        def _post_get_doc():
+        def _post_get_doc(received):
             """
             Get a sync document from server by means of a POST request.
             """
@@ -388,10 +386,11 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
                 '', entries,
                 last_known_generation=last_known_generation,
                 last_known_trans_id=last_known_trans_id,
+                sync_id=sync_id,
                 ensure=ensure_callback is not None)
             # inform server of how many documents have already been received
             size += self._prepare(
-                ',', entries, received=self._sync_state.received)
+                ',', entries, received=received)
             entries.append('\r\n]')
             size += len(entries[-1])
             # send headers
@@ -402,14 +401,16 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
             return self._response()
 
         number_of_changes = None
+        received = 0
 
-        while number_of_changes is None or \
-                self._sync_state.received < number_of_changes:
+        new_generation = last_known_generation
+        new_transaction_id = last_known_trans_id
+        while number_of_changes is None or received < number_of_changes:
             # bail out if sync process was interrupted
             if self.stopped is True:
-                return None, None
+                return last_known_generation, last_known_trans_id
             # try to fetch one document from target
-            data, _ = _post_get_doc()
+            data, _ = _post_get_doc(received)
             # decode incoming stream
             parts = data.splitlines()
             if not parts or parts[0] != '[' or parts[-1] != ']':
@@ -424,6 +425,8 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
                 soledad_assert('new_generation' in metadata)
                 soledad_assert('new_transaction_id' in metadata)
                 number_of_changes = metadata['number_of_changes']
+                new_generation = metadata['new_generation']
+                new_transaction_id = metadata['new_transaction_id']
             except json.JSONDecodeError, AssertionError:
                 raise BrokenSyncStream
             # make sure we have replica_uid from fresh new dbs
@@ -453,12 +456,12 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
             # end of symmetric decryption
             # -------------------------------------------------------------
             return_doc_cb(doc, entry['gen'], entry['trans_id'])
-            self._sync_state.received += 1
+            received += 1
             signal(
                 SOLEDAD_SYNC_RECEIVE_STATUS,
                 "%d/%d" %
-                (self._sync_state.received, number_of_changes))
-        return metadata['new_generation'], metadata['new_transaction_id']
+                (received, number_of_changes))
+        return new_generation, new_transaction_id
 
     def _request(self, method, url_parts, params=None, body=None,
                  content_type=None):
@@ -566,8 +569,7 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
 
     def sync_exchange(self, docs_by_generations, source_replica_uid,
                       last_known_generation, last_known_trans_id,
-                      return_doc_cb, ensure_callback=None,
-                      sync_state=None):
+                      return_doc_cb, ensure_callback=None):
         """
         Find out which documents the remote database does not know about,
         encrypt and send them.
@@ -596,12 +598,8 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         :return: The new generation and transaction id of the target replica.
         :rtype: tuple
         """
-        # get the sync state information from client
-        self._sync_state = sync_state
-        if self._sync_state is None:
-            self._sync_state = ClientSyncState()
-
         self.start()
+        sync_id = str(uuid4())
 
         self._ensure_connection()
         if self._trace_hook:  # for tests
@@ -610,7 +608,7 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         headers = self._sign_request('POST', url, {})
 
         def _post_put_doc(headers, last_known_generation, last_known_trans_id,
-                          id, rev, content, gen, trans_id):
+                          id, rev, content, gen, trans_id, sync_id):
             """
             Put a sync document on server by means of a POST request.
 
@@ -626,6 +624,7 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
                 '', entries,
                 last_known_generation=last_known_generation,
                 last_known_trans_id=last_known_trans_id,
+                sync_id=sync_id,
                 ensure=ensure_callback is not None)
             # add the document to the request
             size += self._prepare(
@@ -645,11 +644,11 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
         cur_target_gen = last_known_generation
         cur_target_trans_id = last_known_trans_id
 
-        # skip docs that were already sent
-        if self._sync_state.sent > 0:
-            docs_by_generations = docs_by_generations[self._sync_state.sent:]
-
         # send docs
+        sent = 0
+        signal(
+            SOLEDAD_SYNC_SEND_STATUS,
+            "%d/%d" % (0, len(docs_by_generations)))
         for doc, gen, trans_id in docs_by_generations:
             # allow for interrupting the sync process
             if self.stopped is True:
@@ -668,17 +667,18 @@ class SoledadSyncTarget(HTTPSyncTarget, TokenBasedAuth):
             # -------------------------------------------------------------
             cur_target_gen, cur_target_trans_id = _post_put_doc(
                 headers, cur_target_gen, cur_target_trans_id, id=doc.doc_id,
-                rev=doc.rev, content=doc_json, gen=gen, trans_id=trans_id)
-            self._sync_state.sent += 1
+                rev=doc.rev, content=doc_json, gen=gen, trans_id=trans_id,
+                sync_id=sync_id)
+            sent += 1
             signal(
                 SOLEDAD_SYNC_SEND_STATUS,
-                "%d/%d" % (self._sync_state.sent, len(docs_by_generations)))
+                "%d/%d" % (sent, len(docs_by_generations)))
 
         # get docs from target
         cur_target_gen, cur_target_trans_id = self._get_remote_docs(
             url,
             last_known_generation, last_known_trans_id, headers,
-            return_doc_cb, ensure_callback)
+            return_doc_cb, ensure_callback, sync_id)
         self.stop()
         return cur_target_gen, cur_target_trans_id
 
diff --git a/common/src/leap/soledad/common/ddocs/syncs/updates/state.js b/common/src/leap/soledad/common/ddocs/syncs/updates/state.js
index cb2b6b7b..d62aeb40 100644
--- a/common/src/leap/soledad/common/ddocs/syncs/updates/state.js
+++ b/common/src/leap/soledad/common/ddocs/syncs/updates/state.js
@@ -29,6 +29,7 @@
  *     '_rev' '<str>',
  *     'ongoing_syncs': {
  *         '<source_replica_uid>': {
+ *             'sync_id': '<sync_id>',
  *             'seen_ids': [['<doc_id>', <at_gen>[, ...],
  *             'changes_to_return': {
  *                  'gen': <gen>,
@@ -59,17 +60,22 @@ function(doc, req) {
     // parse and validate incoming data
     var body = JSON.parse(req.body);
     if (body['source_replica_uid'] == null)
-        return [null, 'invalid data']
+        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] == 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,
         };
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
index 04ceb2ec..94b7e767 100644
--- 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
@@ -2,14 +2,15 @@ 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, 0], null);
+        emit([source_replica_uid, sync_id, 0], null);
       else if (changes.length == 0)
-        emit([source_replica_uid, 0], []);
+        emit([source_replica_uid, sync_id, 0], []);
       else
         for (var i = 0; i < changes['changes_to_return'].length; i++)
           emit(
-            [source_replica_uid, i],
+            [source_replica_uid, sync_id, i],
             {
               'gen': changes['gen'],
               'trans_id': changes['trans_id'],
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
index 34c65b3f..16118e88 100644
--- 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
@@ -1,9 +1,11 @@
 function(doc) {
   if (doc['_id'] == 'u1db_sync_state' && doc['ongoing_syncs'] != null)
-    for (var source_replica_uid in doc['ongoing_syncs'])
+    for (var source_replica_uid in doc['ongoing_syncs']) {
+      var sync_id = doc['ongoing_syncs'][source_replica_uid]['sync_id'];
       emit(
-        source_replica_uid,
+        [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
index 1d8f8e84..e88c6ebb 100644
--- a/common/src/leap/soledad/common/ddocs/syncs/views/state/map.js
+++ b/common/src/leap/soledad/common/ddocs/syncs/views/state/map.js
@@ -2,11 +2,12 @@ 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, null);
+        emit([source_replica_uid, sync_id], null);
       else
         emit(
-          source_replica_uid,
+          [source_replica_uid, sync_id],
           {
             'gen': changes['gen'],
             'trans_id': changes['trans_id'],
diff --git a/server/src/leap/soledad/server/sync.py b/server/src/leap/soledad/server/sync.py
index 16926f14..c6928aaa 100644
--- a/server/src/leap/soledad/server/sync.py
+++ b/server/src/leap/soledad/server/sync.py
@@ -48,7 +48,7 @@ class ServerSyncState(object):
     called 'u1db_sync_state'.
     """
 
-    def __init__(self, db, source_replica_uid):
+    def __init__(self, db, source_replica_uid, sync_id):
         """
         Initialize the sync state object.
 
@@ -59,6 +59,7 @@ class ServerSyncState(object):
         """
         self._db = db
         self._source_replica_uid = source_replica_uid
+        self._sync_id = sync_id
 
     def _key(self, key):
         """
@@ -91,6 +92,7 @@ class ServerSyncState(object):
         with CouchDatabase.sync_info_lock[self._db.replica_uid]:
             res.put_json(
                 body={
+                    'sync_id': self._sync_id,
                     'source_replica_uid': self._source_replica_uid,
                     key: value,
                 },
@@ -118,7 +120,8 @@ class ServerSyncState(object):
         """
         ddoc_path = ['_design', 'syncs', '_view', 'seen_ids']
         resource = self._db._database.resource(*ddoc_path)
-        response = resource.get_json(key=self._key(self._source_replica_uid))
+        response = resource.get_json(
+            key=self._key([self._source_replica_uid, self._sync_id]))
         data = response[2]
         if data['rows']:
             entry = data['rows'].pop()
@@ -160,7 +163,8 @@ class ServerSyncState(object):
         """
         ddoc_path = ['_design', 'syncs', '_view', 'state']
         resource = self._db._database.resource(*ddoc_path)
-        response = resource.get_json(key=self._key(self._source_replica_uid))
+        response = resource.get_json(
+            key=self._key([self._source_replica_uid, self._sync_id]))
         data = response[2]
         gen = None
         trans_id = None
@@ -184,7 +188,7 @@ class ServerSyncState(object):
         resource = self._db._database.resource(*ddoc_path)
         response = resource.get_json(
             key=self._key(
-                [self._source_replica_uid, received]))
+                [self._source_replica_uid, self._sync_id, received]))
         data = response[2]
         if not data['rows']:
             return None, None, None
@@ -197,7 +201,7 @@ class ServerSyncState(object):
 
 class SyncExchange(sync.SyncExchange):
 
-    def __init__(self, db, source_replica_uid, last_known_generation):
+    def __init__(self, db, source_replica_uid, last_known_generation, sync_id):
         """
         :param db: The target syncing database.
         :type db: CouchDatabase
@@ -210,11 +214,13 @@ class SyncExchange(sync.SyncExchange):
         self._db = db
         self.source_replica_uid = source_replica_uid
         self.source_last_known_generation = last_known_generation
+        self.sync_id = sync_id
         self.new_gen = None
         self.new_trans_id = None
         self._trace_hook = None
         # recover sync state
-        self._sync_state = ServerSyncState(self._db, self.source_replica_uid)
+        self._sync_state = ServerSyncState(
+            self._db, self.source_replica_uid, sync_id)
 
 
     def find_changes_to_return(self, received):
@@ -322,9 +328,9 @@ class SyncResource(http_app.SyncResource):
 
     @http_app.http_method(
         last_known_generation=int, last_known_trans_id=http_app.none_or_str,
-        content_as_args=True)
+        sync_id=http_app.none_or_str, content_as_args=True)
     def post_args(self, last_known_generation, last_known_trans_id=None,
-                  ensure=False):
+                  sync_id=None, ensure=False):
         """
         Handle the initial arguments for the sync POST request from client.
 
@@ -348,7 +354,7 @@ class SyncResource(http_app.SyncResource):
             last_known_generation, last_known_trans_id)
         # get a sync exchange object
         self.sync_exch = self.sync_exchange_class(
-            db, self.source_replica_uid, last_known_generation)
+            db, self.source_replica_uid, last_known_generation, sync_id)
 
     @http_app.http_method(content_as_args=True)
     def post_put(self, id, rev, content, gen, trans_id):
@@ -405,8 +411,8 @@ class SyncResource(http_app.SyncResource):
 
     def post_end(self):
         """
-        Return the current generation and transaction_id after inserting a
-        series of incoming documents.
+        Return the current generation and transaction_id after inserting one
+        incoming document.
         """
         self.responder.content_type = 'application/x-soledad-sync-response'
         self.responder.start_response(200)