diff --git a/synapse/storage/state.py b/synapse/storage/state.py
new file mode 100644
index 0000000000..55ea567793
--- /dev/null
+++ b/synapse/storage/state.py
@@ -0,0 +1,127 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ._base import SQLBaseStore
+
+
+class StateStore(SQLBaseStore):
+ """ Keeps track of the state at a given event.
+
+ This is done by the concept of `state groups`. Every event is a assigned
+ a state group (identified by an arbitrary string), which references a
+ collection of state events. The current state of an event is then the
+ collection of state events referenced by the event's state group.
+
+ Hence, every change in the current state causes a new state group to be
+ generated. However, if no change happens (e.g., if we get a message event
+ with only one parent it inherits the state group from its parent.)
+
+ There are three tables:
+ * `state_groups`: Stores group name, first event with in the group and
+ room id.
+ * `event_to_state_groups`: Maps events to state groups.
+ * `state_groups_state`: Maps state group to state events.
+ """
+
+ def get_state_groups(self, event_ids):
+ """ Get the state groups for the given list of event_ids
+
+ The return value is a dict mapping group names to lists of events.
+ """
+
+ def f(txn):
+ groups = set()
+ for event_id in event_ids:
+ group = self._simple_select_one_onecol_txn(
+ txn,
+ table="event_to_state_groups",
+ keyvalues={"event_id": event_id},
+ retcol="state_group",
+ allow_none=True,
+ )
+ if group:
+ groups.add(group)
+
+ res = {}
+ for group in groups:
+ state_ids = self._simple_select_onecol_txn(
+ txn,
+ table="state_groups_state",
+ keyvalues={"state_group": group},
+ retcol="event_id",
+ )
+ state = []
+ for state_id in state_ids:
+ s = self._get_events_txn(
+ txn,
+ [state_id],
+ )
+ if s:
+ state.extend(s)
+
+ res[group] = state
+
+ return res
+
+ return self.runInteraction(
+ "get_state_groups",
+ f,
+ )
+
+ def store_state_groups(self, event):
+ return self.runInteraction(
+ "store_state_groups",
+ self._store_state_groups_txn, event
+ )
+
+ def _store_state_groups_txn(self, txn, event):
+ if not event.state_events:
+ return
+
+ state_group = event.state_group
+ if not state_group:
+ state_group = self._simple_insert_txn(
+ txn,
+ table="state_groups",
+ values={
+ "room_id": event.room_id,
+ "event_id": event.event_id,
+ },
+ or_ignore=True,
+ )
+
+ for state in event.state_events.values():
+ self._simple_insert_txn(
+ txn,
+ table="state_groups_state",
+ values={
+ "state_group": state_group,
+ "room_id": state.room_id,
+ "type": state.type,
+ "state_key": state.state_key,
+ "event_id": state.event_id,
+ },
+ or_ignore=True,
+ )
+
+ self._simple_insert_txn(
+ txn,
+ table="event_to_state_groups",
+ values={
+ "state_group": state_group,
+ "event_id": event.event_id,
+ },
+ or_replace=True,
+ )
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