Support pagination for tokens without chunk part

We also calculate a consistent topological ordering within a chunk, but
it isn't used yet.

CHANGES.rst

@@ -1,3 +1,11 @@
+Changes in <unreleased>
+=======================
+
+This release adds an index to the events table. This means that on first
+startup there will be an inceased amount of IO until the index is created, and
+an increase in disk usage.
+
+
 Changes in synapse v0.30.0 (2018-05-24)
 ==========================================
 
@@ -53,7 +61,6 @@ Bug Fixes:
 * Fix error in handling receipts (PR #3235)
 * Stop the transaction cache caching failures (PR #3255)
 
-
 Changes in synapse v0.29.1 (2018-05-17)
 ==========================================
 Changes:

synapse/handlers/federation.py

@@ -714,37 +714,15 @@ class FederationHandler(BaseHandler):
         defer.returnValue(events)
 
     @defer.inlineCallbacks
-    def maybe_backfill(self, room_id, current_depth):
+    def maybe_backfill(self, room_id, extremities):
         """Checks the database to see if we should backfill before paginating,
         and if so do.
+
+        Args:
+            room_id (str)
+            extremities (list[str]): List of event_ids to backfill from. These
+                should be event IDs that we don't yet have.
         """
-        extremities = yield self.store.get_oldest_events_with_depth_in_room(
-            room_id
-        )
-
-        if not extremities:
-            logger.debug("Not backfilling as no extremeties found.")
-            return
-
-        # Check if we reached a point where we should start backfilling.
-        sorted_extremeties_tuple = sorted(
-            extremities.items(),
-            key=lambda e: -int(e[1])
-        )
-        max_depth = sorted_extremeties_tuple[0][1]
-
-        # We don't want to specify too many extremities as it causes the backfill
-        # request URI to be too long.
-        extremities = dict(sorted_extremeties_tuple[:5])
-
-        if current_depth > max_depth:
-            logger.debug(
-                "Not backfilling as we don't need to. %d < %d",
-                max_depth, current_depth,
-            )
-            return
-
-        # Now we need to decide which hosts to hit first.
 
         # First we try hosts that are already in the room
         # TODO: HEURISTIC ALERT.
@@ -844,7 +822,7 @@ class FederationHandler(BaseHandler):
         tried_domains = set(likely_domains)
         tried_domains.add(self.server_name)
 
-        event_ids = list(extremities.iterkeys())
+        event_ids = list(extremities)
 
         logger.debug("calling resolve_state_groups in _maybe_backfill")
         resolve = logcontext.preserve_fn(
@@ -871,7 +849,7 @@ class FederationHandler(BaseHandler):
             } for key, state_dict in states.iteritems()
         }
 
-        for e_id, _ in sorted_extremeties_tuple:
+        for e_id in event_ids:
             likely_domains = get_domains_from_state(states[e_id])
 
             success = yield try_backfill([

synapse/handlers/message.py

@@ -211,31 +211,19 @@ class MessageHandler(BaseHandler):
             )
 
             if source_config.direction == 'b':
-                # if we're going backwards, we might need to backfill. This
-                # requires that we have a topo token.
-                if room_token.topological:
-                    max_topo = room_token.topological
-                else:
-                    max_topo = yield self.store.get_max_topological_token(
-                        room_id, room_token.stream
-                    )
-
                 if membership == Membership.LEAVE:
                     # If they have left the room then clamp the token to be before
                     # they left the room, to save the effort of loading from the
                     # database.
+
                     leave_token = yield self.store.get_topological_token_for_event(
-                        member_event_id
+                        member_event_id,
+                    )
+                    source_config.from_key = yield self.store.clamp_token_before(
+                        room_id, source_config.from_key, leave_token,
                     )
-                    leave_token = RoomStreamToken.parse(leave_token)
-                    if leave_token.topological < max_topo:
-                        source_config.from_key = str(leave_token)
 
-                yield self.hs.get_handlers().federation_handler.maybe_backfill(
+            events, next_key, extremities = yield self.store.paginate_room_events(
-                    room_id, max_topo
-                )
-
-            events, next_key = yield self.store.paginate_room_events(
                 room_id=room_id,
                 from_key=source_config.from_key,
                 to_key=source_config.to_key,
@@ -244,6 +232,20 @@ class MessageHandler(BaseHandler):
                 event_filter=event_filter,
             )
 
+            if source_config.direction == 'b' and extremities:
+                yield self.hs.get_handlers().federation_handler.maybe_backfill(
+                    room_id, extremities
+                )
+
+                events, next_key, extremities = yield self.store.paginate_room_events(
+                    room_id=room_id,
+                    from_key=source_config.from_key,
+                    to_key=source_config.to_key,
+                    direction=source_config.direction,
+                    limit=source_config.limit,
+                    event_filter=event_filter,
+                )
+
             next_token = pagin_config.from_token.copy_and_replace(
                 "room_key", next_key
             )

synapse/handlers/room.py

@@ -534,7 +534,7 @@ class RoomEventSource(object):
 
     @defer.inlineCallbacks
     def get_pagination_rows(self, user, config, key):
-        events, next_key = yield self.store.paginate_room_events(
+        events, next_key, _ = yield self.store.paginate_room_events(
             room_id=key,
             from_key=config.from_key,
             to_key=config.to_key,

synapse/storage/__init__.py

@@ -131,6 +131,7 @@ class DataStore(RoomMemberStore, RoomStore,
         self._group_updates_id_gen = StreamIdGenerator(
             db_conn, "local_group_updates", "stream_id",
         )
+        self._chunk_id_gen = IdGenerator(db_conn, "events", "chunk_id")
 
         if isinstance(self.database_engine, PostgresEngine):
             self._cache_id_gen = StreamIdGenerator(

synapse/storage/chunk_ordered_table.py

@@ -0,0 +1,319 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector 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.
+
+import math
+import logging
+
+from synapse.storage._base import SQLBaseStore
+from synapse.util.katriel_bodlaender import OrderedListStore
+from synapse.util.metrics import Measure
+
+import synapse.metrics
+
+metrics = synapse.metrics.get_metrics_for(__name__)
+rebalance_counter = metrics.register_counter("rebalances")
+
+
+logger = logging.getLogger(__name__)
+
+
+class ChunkDBOrderedListStore(OrderedListStore):
+    """Used as the list store for room chunks, efficiently maintaining them in
+    topological order on updates.
+
+    A room chunk is a connected portion of the room events DAG. Chunks are
+    constructed so that they have the additional property that for all events in
+    the chunk, either all of their prev_events are in that chunk or none of them
+    are. This ensures that no event that is subsequently received needs to be
+    inserted into the middle of a chunk, since it cannot both reference an event
+    in the chunk and be referenced by an event in the chunk (assuming no
+    cycles).
+
+    As such the set of chunks in a room inherits a DAG, i.e. if an event in one
+    chunk references an event in a second chunk, then we say that the first
+    chunk references the second, and thus forming a DAG. (This means that chunks
+    start off disconnected until an event is received that connects the two
+    chunks.)
+
+    We can therefore end up with multiple chunks in a room when the server
+    misses some events, e.g. due to the server being offline for a time.
+
+    The server may only have a subset of all events in a room, in which case
+    its possible for the server to have chunks that are unconnected from each
+    other. The ordering between unconnected chunks is arbitrary.
+
+    The class is designed for use inside transactions and so takes a
+    transaction object in the constructor. This means that it needs to be
+    re-instantiated in each transaction, so all state needs to be stored
+    in the database.
+
+    Internally the ordering is implemented using floats, and the average is
+    taken when a node is inserted between other nodes. To avoid precision
+    errors a minimum difference between sucessive orderings is attempted to be
+    kept; whenever the difference is too small we attempt to rebalance. See
+    the `_rebalance` function for implementation details.
+
+    Note that OrderedListStore orders nodes such that source of an edge
+    comes before the target. This is counter intuitive when edges represent
+    causality, so for the purposes of ordering algorithm we invert the edge
+    directions, i.e. if chunk A has a prev chunk of B then we say that the
+    edge is from B to A. This ensures that newer chunks get inserted at the
+    end (rather than the start).
+
+    Note: Calls to `add_node` and `add_edge` cannot overlap for the same room,
+    and so callers should perform some form of per-room locking when using
+    this class.
+
+    Args:
+        txn
+        room_id (str)
+        clock
+        rebalance_digits (int): When a rebalance is triggered we rebalance
+            in a range around the node, where the bounds are rounded to this
+            number of digits.
+        min_difference (int): A rebalance is triggered when the difference
+            between two successive orderings is less than the reciprocal of
+            this.
+    """
+    def __init__(self,
+                 txn, room_id, clock,
+                 rebalance_digits=3,
+                 min_difference=1000000):
+        self.txn = txn
+        self.room_id = room_id
+        self.clock = clock
+
+        self.rebalance_digits = rebalance_digits
+        self.min_difference = 1. / min_difference
+
+    def is_before(self, a, b):
+        """Implements OrderedListStore"""
+        return self._get_order(a) < self._get_order(b)
+
+    def get_prev(self, node_id):
+        """Implements OrderedListStore"""
+        order = self._get_order(node_id)
+
+        sql = """
+            SELECT chunk_id FROM chunk_linearized
+            WHERE ordering < ? AND room_id = ?
+            ORDER BY ordering DESC
+            LIMIT 1
+        """
+
+        self.txn.execute(sql, (order, self.room_id,))
+
+        row = self.txn.fetchone()
+        if row:
+            return row[0]
+        return None
+
+    def get_next(self, node_id):
+        """Implements OrderedListStore"""
+        order = self._get_order(node_id)
+
+        sql = """
+            SELECT chunk_id FROM chunk_linearized
+            WHERE ordering > ? AND room_id = ?
+            ORDER BY ordering ASC
+            LIMIT 1
+        """
+
+        self.txn.execute(sql, (order, self.room_id,))
+
+        row = self.txn.fetchone()
+        if row:
+            return row[0]
+        return None
+
+    def _insert_before(self, node_id, target_id):
+        """Implements OrderedListStore"""
+
+        rebalance = False  # Set to true if we need to trigger a rebalance
+
+        if target_id:
+            target_order = self._get_order(target_id)
+            before_id = self.get_prev(target_id)
+
+            if before_id:
+                before_order = self._get_order(before_id)
+                new_order = (target_order + before_order) / 2.
+
+                rebalance = math.fabs(target_order - before_order) < self.min_difference
+            else:
+                new_order = math.floor(target_order) - 1
+        else:
+            # If target_id is None then we insert at the end.
+            self.txn.execute("""
+                SELECT COALESCE(MAX(ordering), 0) + 1
+                FROM chunk_linearized
+                WHERE room_id = ?
+            """, (self.room_id,))
+
+            new_order, = self.txn.fetchone()
+
+        self._insert(node_id, new_order)
+
+        if rebalance:
+            self._rebalance(node_id)
+
+    def _insert_after(self, node_id, target_id):
+        """Implements OrderedListStore"""
+
+        rebalance = False  # Set to true if we need to trigger a rebalance
+
+        if target_id:
+            target_order = self._get_order(target_id)
+            after_id = self.get_next(target_id)
+            if after_id:
+                after_order = self._get_order(after_id)
+                new_order = (target_order + after_order) / 2.
+
+                rebalance = math.fabs(target_order - after_order) < self.min_difference
+            else:
+                new_order = math.ceil(target_order) + 1
+        else:
+            # If target_id is None then we insert at the start.
+            self.txn.execute("""
+                SELECT COALESCE(MIN(ordering), 0) - 1
+                FROM chunk_linearized
+                WHERE room_id = ?
+            """, (self.room_id,))
+
+            new_order, = self.txn.fetchone()
+
+        self._insert(node_id, new_order)
+
+        if rebalance:
+            self._rebalance(node_id)
+
+    def get_nodes_with_edges_to(self, node_id):
+        """Implements OrderedListStore"""
+
+        # Note that we use the inverse relation here
+        sql = """
+            SELECT l.ordering, l.chunk_id FROM chunk_graph AS g
+            INNER JOIN chunk_linearized AS l ON g.prev_id = l.chunk_id
+            WHERE g.chunk_id = ?
+        """
+        self.txn.execute(sql, (node_id,))
+        return self.txn.fetchall()
+
+    def get_nodes_with_edges_from(self, node_id):
+        """Implements OrderedListStore"""
+
+        # Note that we use the inverse relation here
+        sql = """
+            SELECT l.ordering, l.chunk_id FROM chunk_graph AS g
+            INNER JOIN chunk_linearized AS l ON g.chunk_id = l.chunk_id
+            WHERE g.prev_id = ?
+        """
+        self.txn.execute(sql, (node_id,))
+        return self.txn.fetchall()
+
+    def _delete_ordering(self, node_id):
+        """Implements OrderedListStore"""
+
+        SQLBaseStore._simple_delete_txn(
+            self.txn,
+            table="chunk_linearized",
+            keyvalues={"chunk_id": node_id},
+        )
+
+    def _add_edge_to_graph(self, source_id, target_id):
+        """Implements OrderedListStore"""
+
+        # Note that we use the inverse relation
+        SQLBaseStore._simple_insert_txn(
+            self.txn,
+            table="chunk_graph",
+            values={"chunk_id": target_id, "prev_id": source_id}
+        )
+
+    def _insert(self, node_id, order):
+        """Inserts the node with the given ordering.
+        """
+        SQLBaseStore._simple_insert_txn(
+            self.txn,
+            table="chunk_linearized",
+            values={
+                "chunk_id": node_id,
+                "room_id": self.room_id,
+                "ordering": order,
+            }
+        )
+
+    def _get_order(self, node_id):
+        """Get the ordering of the given node.
+        """
+
+        return SQLBaseStore._simple_select_one_onecol_txn(
+            self.txn,
+            table="chunk_linearized",
+            keyvalues={"chunk_id": node_id},
+            retcol="ordering"
+        )
+
+    def _rebalance(self, node_id):
+        """Rebalances the list around the given node to ensure that the
+        ordering floats don't get too small.
+
+        This works by finding a range that includes the given node, and
+        recalculating the ordering floats such that they're equidistant in
+        that range.
+        """
+
+        logger.info("Rebalancing room %s, chunk %s", self.room_id, node_id)
+
+        with Measure(self.clock, "chunk_rebalance"):
+            # We pick the interval to try and minimise the number of decimal
+            # places, i.e. we round to nearest float with `rebalance_digits` and
+            # use that as one side of the interval
+            order = self._get_order(node_id)
+            a = round(order, self.rebalance_digits)
+            min_order = a - 10 ** -self.rebalance_digits
+            max_order = a + 10 ** -self.rebalance_digits
+
+            # Now we get all the nodes in the range. We add the minimum difference
+            # to the bounds to ensure that we don't accidentally move a node to be
+            # within the minimum difference of a node outside the range.
+            sql = """
+                SELECT chunk_id FROM chunk_linearized
+                WHERE ordering >= ? AND ordering <= ? AND room_id = ?
+            """
+            self.txn.execute(sql, (
+                min_order - self.min_difference,
+                max_order + self.min_difference,
+                self.room_id,
+            ))
+
+            chunk_ids = [c for c, in self.txn]
+
+            sql = """
+                UPDATE chunk_linearized
+                SET ordering = ?
+                WHERE chunk_id = ?
+            """
+
+            step = (max_order - min_order) / len(chunk_ids)
+            self.txn.executemany(
+                sql,
+                (
+                    ((idx * step + min_order), chunk_id)
+                    for idx, chunk_id in enumerate(chunk_ids)
+                )
+            )
+
+            rebalance_counter.inc()

synapse/storage/events.py

@@ -23,6 +23,7 @@ import simplejson as json
 from twisted.internet import defer
 
 from synapse.storage.events_worker import EventsWorkerStore
+from synapse.storage.chunk_ordered_table import ChunkDBOrderedListStore
 from synapse.util.async import ObservableDeferred
 from synapse.util.frozenutils import frozendict_json_encoder
 from synapse.util.logcontext import (
@@ -232,6 +233,15 @@ class EventsStore(EventsWorkerStore):
             psql_only=True,
         )
 
+        self.register_background_index_update(
+            "events_chunk_index",
+            index_name="events_chunk_index",
+            table="events",
+            columns=["room_id", "chunk_id", "topological_ordering", "stream_ordering"],
+            unique=True,
+            psql_only=True,
+        )
+
         self._event_persist_queue = _EventPeristenceQueue()
 
         self._state_resolution_handler = hs.get_state_resolution_handler()
@@ -1010,13 +1020,20 @@ class EventsStore(EventsWorkerStore):
                     }
                 )
 
-                sql = (
+                chunk_id, topo = self._insert_into_chunk_txn(
-                    "UPDATE events SET outlier = ?"
+                    txn, event.room_id, event.event_id,
-                    " WHERE event_id = ?"
+                    [eid for eid, _ in event.prev_events],
                 )
-                txn.execute(
+
-                    sql,
+                self._simple_update_txn(
-                    (False, event.event_id,)
+                    txn,
+                    table="events",
+                    keyvalues={"event_id": event.event_id},
+                    updatevalues={
+                        "outlier": False,
+                        "chunk_id": chunk_id,
+                        "topological_ordering": topo,
+                    },
                 )
 
                 # Update the event_backward_extremities table now that this
@@ -1099,13 +1116,22 @@ class EventsStore(EventsWorkerStore):
             ],
         )
 
-        self._simple_insert_many_txn(
+        for event, _ in events_and_contexts:
-            txn,
+            if event.internal_metadata.is_outlier():
-            table="events",
+                chunk_id, topo = None, 0
-            values=[
+            else:
-                {
+                chunk_id, topo = self._insert_into_chunk_txn(
+                    txn, event.room_id, event.event_id,
+                    [eid for eid, _ in event.prev_events],
+                )
+
+            self._simple_insert_txn(
+                txn,
+                table="events",
+                values={
                     "stream_ordering": event.internal_metadata.stream_ordering,
-                    "topological_ordering": event.depth,
+                    "chunk_id": chunk_id,
+                    "topological_ordering": topo,
                     "depth": event.depth,
                     "event_id": event.event_id,
                     "room_id": event.room_id,
@@ -1120,10 +1146,8 @@ class EventsStore(EventsWorkerStore):
                         "url" in event.content
                         and isinstance(event.content["url"], basestring)
                     ),
-                }
+                },
-                for event, _ in events_and_contexts
+            )
-            ],
-        )
 
     def _store_rejected_events_txn(self, txn, events_and_contexts):
         """Add rows to the 'rejections' table for received events which were
@@ -1335,6 +1359,177 @@ class EventsStore(EventsWorkerStore):
             (event.event_id, event.redacts)
         )
 
+    def _insert_into_chunk_txn(self, txn, room_id, event_id, prev_event_ids):
+        """Computes the chunk ID and topological ordering for an event and
+        handles updating chunk_graph table.
+
+        Args:
+            txn,
+            room_id (str)
+            event_id (str)
+            prev_event_ids (list[str])
+
+        Returns:
+            tuple[int, int]: Returns the chunk_id, topological_ordering for
+            the event
+        """
+
+        # We calculate the chunk for an event using the following rules:
+        #
+        # 1. If all prev events have the same chunk ID then use that chunk ID
+        # 2. If we have none of the prev events but do have events pointing to
+        #    the event, then we use their chunk ID if:
+        #     - They're all in the same chunk, and
+        #     - All their prev events match the events being inserted
+        # 3. Otherwise, create a new chunk and use that
+
+        # Set of chunks that the event refers to. Includes None if there were
+        # prev events that we don't have (or don't have a chunk for)
+        prev_chunk_ids = set()
+
+        for eid in prev_event_ids:
+            chunk_id = self._simple_select_one_onecol_txn(
+                txn,
+                table="events",
+                keyvalues={"event_id": eid},
+                retcol="chunk_id",
+                allow_none=True,
+            )
+
+            prev_chunk_ids.add(chunk_id)
+
+        forward_events = self._simple_select_onecol_txn(
+            txn,
+            table="event_edges",
+            keyvalues={
+                "prev_event_id": event_id,
+                "is_state": False,
+            },
+            retcol="event_id",
+        )
+
+        # Set of chunks that refer to this event.
+        forward_chunk_ids = set()
+
+        # All the prev_events of events in `forward_events`.
+        # Note that this will include the current event_id.
+        sibling_events = set()
+        for eid in forward_events:
+            chunk_id = self._simple_select_one_onecol_txn(
+                txn,
+                table="events",
+                keyvalues={"event_id": eid},
+                retcol="chunk_id",
+                allow_none=True,
+            )
+
+            if chunk_id is not None:
+                # chunk_id can be None if it's an outlier
+                forward_chunk_ids.add(chunk_id)
+
+            pes = self._simple_select_onecol_txn(
+                txn,
+                table="event_edges",
+                keyvalues={
+                    "event_id": eid,
+                    "is_state": False,
+                },
+                retcol="prev_event_id",
+            )
+
+            sibling_events.update(pes)
+
+        table = ChunkDBOrderedListStore(
+            txn, room_id, self.clock,
+        )
+
+        # If there is only one previous chunk (and that isn't None), then this
+        # satisfies condition one.
+        if len(prev_chunk_ids) == 1 and None not in prev_chunk_ids:
+            chunk_id = list(prev_chunk_ids)[0]
+
+            # This event is being inserted at the end of the chunk
+            new_topo = self._simple_select_one_onecol_txn(
+                txn,
+                table="events",
+                keyvalues={
+                    "room_id": room_id,
+                    "chunk_id": chunk_id,
+                },
+                retcol="MAX(topological_ordering)",
+            )
+            new_topo += 1
+
+        # If there is only one forward chunk and only one sibling event (which
+        # would be the given event), then this satisfies condition two.
+        elif len(forward_chunk_ids) == 1 and len(sibling_events) == 1:
+            chunk_id = list(forward_chunk_ids)[0]
+
+            # This event is being inserted at the start of the chunk
+            new_topo = self._simple_select_one_onecol_txn(
+                txn,
+                table="events",
+                keyvalues={
+                    "room_id": room_id,
+                    "chunk_id": chunk_id,
+                },
+                retcol="MIN(topological_ordering)",
+            )
+            new_topo -= 1
+        else:
+            chunk_id = self._chunk_id_gen.get_next()
+            new_topo = 0
+
+            # We've generated a new chunk, so we have to tell the
+            # ChunkDBOrderedListStore about that.
+            table.add_node(chunk_id)
+
+        # We need to now update the database with any new edges between chunks
+        current_prev_ids = self._simple_select_onecol_txn(
+            txn,
+            table="chunk_graph",
+            keyvalues={
+                "chunk_id": chunk_id,
+            },
+            retcol="prev_id",
+        )
+
+        current_forward_ids = self._simple_select_onecol_txn(
+            txn,
+            table="chunk_graph",
+            keyvalues={
+                "prev_id": chunk_id,
+            },
+            retcol="chunk_id",
+        )
+
+        for pid in prev_chunk_ids:
+            if pid is not None and pid not in current_prev_ids and pid != chunk_id:
+                # Note that the edge direction is reversed than what you might
+                # expect. See ChunkDBOrderedListStore for more details.
+                table.add_edge(pid, chunk_id)
+
+        for fid in forward_chunk_ids:
+            # Note that the edge direction is reversed than what you might
+            # expect. See ChunkDBOrderedListStore for more details.
+            if fid not in current_forward_ids and fid != chunk_id:
+                table.add_edge(chunk_id, fid)
+
+        # We now need to update the backwards extremities for the chunks.
+
+        txn.executemany("""
+            INSERT INTO chunk_backwards_extremities (chunk_id, event_id)
+            SELECT ?, ? WHERE ? NOT IN (SELECT event_id FROM events)
+        """, [(chunk_id, eid, eid) for eid in prev_event_ids])
+
+        self._simple_delete_txn(
+            txn,
+            table="chunk_backwards_extremities",
+            keyvalues={"event_id": event_id},
+        )
+
+        return chunk_id, new_topo
+
     @defer.inlineCallbacks
     def have_events_in_timeline(self, event_ids):
         """Given a list of event ids, check if we have already processed and
@@ -1908,17 +2103,42 @@ class EventsStore(EventsWorkerStore):
             should_delete_expr += " AND event_id NOT LIKE ?"
             should_delete_params += ("%:" + self.hs.hostname, )
 
-        should_delete_params += (room_id, token.topological)
+        next_token = RoomStreamToken(
-
+            token.chunk,
-        txn.execute(
+            token.topological - 1,
-            "INSERT INTO events_to_purge"
+            token.stream,
-            " SELECT event_id, %s"
-            " FROM events AS e LEFT JOIN state_events USING (event_id)"
-            " WHERE e.room_id = ? AND topological_ordering < ?" % (
-                should_delete_expr,
-            ),
-            should_delete_params,
         )
+        while True:
+            rows, next_token, _ = self._paginate_room_events_txn(
+                txn, room_id, next_token, direction='b', limit=1000,
+            )
+            next_token = RoomStreamToken.parse(next_token)
+
+            if len(rows) == 0:
+                break
+
+            txn.executemany(
+                """INSERT INTO events_to_purge
+                SELECT event_id, %s
+                FROM events
+                LEFT JOIN state_events USING (event_id)
+                WHERE event_id = ?
+                """ % (
+                    should_delete_expr,
+                ),
+                (
+                    should_delete_params + (row.event_id,)
+                    for row in rows
+                ),
+            )
+
+        txn.execute("""
+            DELETE FROM events_to_purge
+            WHERE event_id IN (
+                SELECT event_id FROM event_forward_extremities
+            )
+        """)
+
         txn.execute(
             "SELECT event_id, should_delete FROM events_to_purge"
         )
@@ -1957,6 +2177,27 @@ class EventsStore(EventsWorkerStore):
             ]
         )
 
+        txn.execute(
+            """DELETE FROM chunk_backwards_extremities
+            WHERE event_id IN (
+                SELECT event_id FROM events WHERE room_id = ?
+            )
+            """,
+            (room_id,)
+        )
+
+        txn.execute(
+            """
+            INSERT INTO chunk_backwards_extremities
+            SELECT DISTINCT ee.chunk_id, e.event_id
+            FROM events_to_purge AS e
+            INNER JOIN event_edges AS ed ON e.event_id = ed.prev_event_id
+            INNER JOIN events AS ee ON ee.event_id = ed.event_id
+            LEFT JOIN events_to_purge AS ep2 ON ed.event_id = ep2.event_id
+            WHERE ep2.event_id IS NULL
+            """,
+        )
+
         logger.info("[purge] finding redundant state groups")
 
         # Get all state groups that are only referenced by events that are
@@ -2109,7 +2350,7 @@ class EventsStore(EventsWorkerStore):
         # Mark all state and own events as outliers
         logger.info("[purge] marking remaining events as outliers")
         txn.execute(
-            "UPDATE events SET outlier = ?"
+            "UPDATE events SET outlier = ?, chunk_id = NULL"
             " WHERE event_id IN ("
             "    SELECT event_id FROM events_to_purge "
             "    WHERE NOT should_delete"

synapse/storage/schema/delta/49/event_chunks.sql

@@ -0,0 +1,49 @@
+/* Copyright 2018 New Vector 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.
+ */
+
+ALTER TABLE events ADD COLUMN chunk_id BIGINT;
+
+INSERT INTO background_updates (update_name, progress_json) VALUES
+    ('events_chunk_index', '{}');
+
+-- Stores how chunks of graph relate to each other
+CREATE TABLE chunk_graph (
+    chunk_id BIGINT NOT NULL,
+    prev_id BIGINT NOT NULL
+);
+
+CREATE UNIQUE INDEX chunk_graph_id ON chunk_graph (chunk_id, prev_id);
+CREATE INDEX chunk_graph_prev_id ON chunk_graph (prev_id);
+
+-- The extremities in each chunk. Note that these are pointing to events that
+-- we don't have, rather than boundary between chunks.
+CREATE TABLE chunk_backwards_extremities (
+    chunk_id BIGINT NOT NULL,
+    event_id TEXT NOT NULL
+);
+
+CREATE INDEX chunk_backwards_extremities_id ON chunk_backwards_extremities(chunk_id, event_id);
+CREATE INDEX chunk_backwards_extremities_event_id ON chunk_backwards_extremities(event_id);
+
+-- Maintains an absolute ordering of chunks. Gets updated when we see new
+-- edges between chunks.
+CREATE TABLE chunk_linearized (
+    chunk_id BIGINT NOT NULL,
+    room_id TEXT NOT NULL,
+    ordering DOUBLE PRECISION NOT NULL
+);
+
+CREATE UNIQUE INDEX chunk_linearized_id ON chunk_linearized (chunk_id);
+CREATE INDEX chunk_linearized_ordering ON chunk_linearized (room_id, ordering);

synapse/storage/stream.py

@@ -41,6 +41,7 @@ from synapse.storage.events import EventsWorkerStore
 from synapse.types import RoomStreamToken
 from synapse.util.caches.stream_change_cache import StreamChangeCache
 from synapse.util.logcontext import make_deferred_yieldable, run_in_background
+from synapse.storage.chunk_ordered_table import ChunkDBOrderedListStore
 from synapse.storage.engines import PostgresEngine
 
 import abc
@@ -62,24 +63,25 @@ _TOPOLOGICAL_TOKEN = "topological"
 
 # Used as return values for pagination APIs
 _EventDictReturn = namedtuple("_EventDictReturn", (
-    "event_id", "topological_ordering", "stream_ordering",
+    "event_id", "chunk_id", "topological_ordering", "stream_ordering",
 ))
 
 
 def lower_bound(token, engine, inclusive=False):
     inclusive = "=" if inclusive else ""
-    if token.topological is None:
+    if token.chunk is None:
         return "(%d <%s %s)" % (token.stream, inclusive, "stream_ordering")
     else:
         if isinstance(engine, PostgresEngine):
             # Postgres doesn't optimise ``(x < a) OR (x=a AND y<b)`` as well
             # as it optimises ``(x,y) < (a,b)`` on multicolumn indexes. So we
             # use the later form when running against postgres.
-            return "((%d,%d) <%s (%s,%s))" % (
+            return "(chunk_id = %d AND (%d,%d) <%s (%s,%s))" % (
-                token.topological, token.stream, inclusive,
+                token.chunk, token.topological, token.stream, inclusive,
                 "topological_ordering", "stream_ordering",
             )
-        return "(%d < %s OR (%d = %s AND %d <%s %s))" % (
+        return "(chunk_id = %d AND (%d < %s OR (%d = %s AND %d <%s %s)))" % (
+            token.chunk,
             token.topological, "topological_ordering",
             token.topological, "topological_ordering",
             token.stream, inclusive, "stream_ordering",
@@ -88,18 +90,19 @@ def lower_bound(token, engine, inclusive=False):
 
 def upper_bound(token, engine, inclusive=True):
     inclusive = "=" if inclusive else ""
-    if token.topological is None:
+    if token.chunk is None:
         return "(%d >%s %s)" % (token.stream, inclusive, "stream_ordering")
     else:
         if isinstance(engine, PostgresEngine):
             # Postgres doesn't optimise ``(x > a) OR (x=a AND y>b)`` as well
             # as it optimises ``(x,y) > (a,b)`` on multicolumn indexes. So we
             # use the later form when running against postgres.
-            return "((%d,%d) >%s (%s,%s))" % (
+            return "(chunk_id = %d AND (%d,%d) >%s (%s,%s))" % (
-                token.topological, token.stream, inclusive,
+                token.chunk, token.topological, token.stream, inclusive,
                 "topological_ordering", "stream_ordering",
             )
-        return "(%d > %s OR (%d = %s AND %d >%s %s))" % (
+        return "(chunk_id = %d AND (%d > %s OR (%d = %s AND %d >%s %s)))" % (
+            token.chunk,
             token.topological, "topological_ordering",
             token.topological, "topological_ordering",
             token.stream, inclusive, "stream_ordering",
@@ -275,7 +278,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             ) % (order,)
             txn.execute(sql, (room_id, from_id, to_id, limit))
 
-            rows = [_EventDictReturn(row[0], None, row[1]) for row in txn]
+            rows = [_EventDictReturn(row[0], None, None, row[1]) for row in txn]
             return rows
 
         rows = yield self.runInteraction("get_room_events_stream_for_room", f)
@@ -325,7 +328,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             )
             txn.execute(sql, (user_id, from_id, to_id,))
 
-            rows = [_EventDictReturn(row[0], None, row[1]) for row in txn]
+            rows = [_EventDictReturn(row[0], None, None, row[1]) for row in txn]
 
             return rows
 
@@ -392,7 +395,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
 
         end_token = RoomStreamToken.parse(end_token)
 
-        rows, token = yield self.runInteraction(
+        rows, token, _ = yield self.runInteraction(
             "get_recent_event_ids_for_room", self._paginate_room_events_txn,
             room_id, from_token=end_token, limit=limit,
         )
@@ -437,15 +440,17 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         `room_id` causes it to return the current room specific topological
         token.
         """
-        token = yield self.get_room_max_stream_ordering()
         if room_id is None:
-            defer.returnValue("s%d" % (token,))
+            token = yield self.get_room_max_stream_ordering()
+            defer.returnValue(str(RoomStreamToken(None, None, token)))
         else:
-            topo = yield self.runInteraction(
+            token = yield self.runInteraction(
-                "_get_max_topological_txn", self._get_max_topological_txn,
+                "get_room_events_max_id", self._get_topological_token_for_room_txn,
                 room_id,
             )
-            defer.returnValue("t%d-%d" % (topo, token))
+            if not token:
+                raise Exception("Server not in room")
+            defer.returnValue(str(token))
 
     def get_stream_token_for_event(self, event_id):
         """The stream token for an event
@@ -460,7 +465,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             table="events",
             keyvalues={"event_id": event_id},
             retcol="stream_ordering",
-        ).addCallback(lambda row: "s%d" % (row,))
+        ).addCallback(lambda row: str(RoomStreamToken(None, None, row)))
 
     def get_topological_token_for_event(self, event_id):
         """The stream token for an event
@@ -469,16 +474,34 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         Raises:
             StoreError if the event wasn't in the database.
         Returns:
-            A deferred "t%d-%d" topological token.
+            A deferred topological token.
         """
         return self._simple_select_one(
             table="events",
             keyvalues={"event_id": event_id},
-            retcols=("stream_ordering", "topological_ordering"),
+            retcols=("stream_ordering", "topological_ordering", "chunk_id"),
             desc="get_topological_token_for_event",
-        ).addCallback(lambda row: "t%d-%d" % (
+        ).addCallback(lambda row: str(RoomStreamToken(
-            row["topological_ordering"], row["stream_ordering"],)
+            row["chunk_id"],
-        )
+            row["topological_ordering"],
+            row["stream_ordering"],
+        )))
+
+    def _get_topological_token_for_room_txn(self, txn, room_id):
+        sql = """
+            SELECT chunk_id, topological_ordering, stream_ordering
+            FROM events
+            NATURAL JOIN event_forward_extremities
+            WHERE room_id = ?
+            ORDER BY stream_ordering DESC
+            LIMIT 1
+        """
+        txn.execute(sql, (room_id,))
+        row = txn.fetchone()
+        if row:
+            c, t, s = row
+            return RoomStreamToken(c, t, s)
+        return None
 
     def get_max_topological_token(self, room_id, stream_key):
         sql = (
@@ -515,18 +538,25 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 null topological_ordering.
         """
         for event, row in zip(events, rows):
+            chunk = row.chunk_id
+            topo = row.topological_ordering
             stream = row.stream_ordering
-            if topo_order and row.topological_ordering:
+
-                topo = row.topological_ordering
-            else:
-                topo = None
             internal = event.internal_metadata
-            internal.before = str(RoomStreamToken(topo, stream - 1))
+            if topo_order and chunk:
-            internal.after = str(RoomStreamToken(topo, stream))
+                internal.before = str(RoomStreamToken(chunk, topo, stream - 1))
-            internal.order = (
+                internal.after = str(RoomStreamToken(chunk, topo, stream))
-                int(topo) if topo else 0,
+                internal.order = (
-                int(stream),
+                    int(chunk) if chunk else 0,
-            )
+                    int(topo) if topo else 0,
+                    int(stream),
+                )
+            else:
+                internal.before = str(RoomStreamToken(None, None, stream - 1))
+                internal.after = str(RoomStreamToken(None, None, stream))
+                internal.order = (
+                    0, 0, int(stream),
+                )
 
     @defer.inlineCallbacks
     def get_events_around(self, room_id, event_id, before_limit, after_limit):
@@ -586,27 +616,29 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 "event_id": event_id,
                 "room_id": room_id,
             },
-            retcols=["stream_ordering", "topological_ordering"],
+            retcols=["stream_ordering", "topological_ordering", "chunk_id"],
         )
 
         # Paginating backwards includes the event at the token, but paginating
         # forward doesn't.
         before_token = RoomStreamToken(
-            results["topological_ordering"] - 1,
+            results["chunk_id"],
-            results["stream_ordering"],
+            results["topological_ordering"],
+            results["stream_ordering"] - 1,
         )
 
         after_token = RoomStreamToken(
+            results["chunk_id"],
             results["topological_ordering"],
             results["stream_ordering"],
         )
 
-        rows, start_token = self._paginate_room_events_txn(
+        rows, start_token, _ = self._paginate_room_events_txn(
             txn, room_id, before_token, direction='b', limit=before_limit,
         )
         events_before = [r.event_id for r in rows]
 
-        rows, end_token = self._paginate_room_events_txn(
+        rows, end_token, _ = self._paginate_room_events_txn(
             txn, room_id, after_token, direction='f', limit=after_limit,
         )
         events_after = [r.event_id for r in rows]
@@ -689,13 +721,37 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 those that match the filter.
 
         Returns:
-            Deferred[tuple[list[_EventDictReturn], str]]: Returns the results
+            Deferred[tuple[list[_EventDictReturn], str, list[int]]: Returns
-            as a list of _EventDictReturn and a token that points to the end
+            the results as a list of _EventDictReturn, a token that points to
-            of the result set.
+            the end of the result set, and a list of chunks iterated over.
         """
 
         assert int(limit) >= 0
 
+        # For backwards compatibility we need to check if the token has a
+        # topological part but no chunk part. If that's the case we can use the
+        # stream part to generate an appropriate topological token.
+        if from_token.chunk is None and from_token.topological is not None:
+            res = self._simple_select_one_txn(
+                txn,
+                table="events",
+                keyvalues={
+                    "stream_ordering": from_token.stream,
+                },
+                retcols=(
+                    "chunk_id",
+                    "topological_ordering",
+                    "stream_ordering",
+                ),
+                allow_none=True,
+            )
+            if res and res["chunk_id"] is not None:
+                from_token = RoomStreamToken(
+                    res["chunk_id"],
+                    res["topological_ordering"],
+                    res["stream_ordering"],
+                )
+
         # Tokens really represent positions between elements, but we use
         # the convention of pointing to the event before the gap. Hence
         # we have a bit of asymmetry when it comes to equalities.
@@ -725,10 +781,11 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             bounds += " AND " + filter_clause
             args.extend(filter_args)
 
-        args.append(int(limit))
+        limit = int(limit)
+        args.append(limit)
 
         sql = (
-            "SELECT event_id, topological_ordering, stream_ordering"
+            "SELECT event_id, chunk_id, topological_ordering, stream_ordering"
             " FROM events"
             " WHERE outlier = ? AND room_id = ? AND %(bounds)s"
             " ORDER BY topological_ordering %(order)s,"
@@ -740,9 +797,59 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
 
         txn.execute(sql, args)
 
-        rows = [_EventDictReturn(row[0], row[1], row[2]) for row in txn]
+        rows = [_EventDictReturn(row[0], row[1], row[2], row[3]) for row in txn]
+
+        iterated_chunks = []
+
+        chunk_id = None
+        if rows:
+            chunk_id = rows[-1].chunk_id
+            iterated_chunks = [r.chunk_id for r in rows]
+        elif from_token.chunk:
+            chunk_id = from_token.chunk
+            iterated_chunks = [chunk_id]
+
+        table = ChunkDBOrderedListStore(
+            txn, room_id, self.clock,
+        )
+
+        while chunk_id and (limit <= 0 or len(rows) < limit):
+            if chunk_id not in iterated_chunks:
+                iterated_chunks.append(chunk_id)
+
+            if direction == 'b':
+                # FIXME: There may be multiple things here
+                chunk_id = table.get_prev(chunk_id)
+            else:
+                chunk_id = table.get_next(chunk_id)
+
+            if chunk_id is None:
+                break
+
+            sql = (
+                "SELECT event_id, chunk_id, topological_ordering, stream_ordering"
+                " FROM events"
+                " WHERE outlier = ? AND room_id = ? AND chunk_id = %(chunk_id)d"
+                " ORDER BY topological_ordering %(order)s,"
+                " stream_ordering %(order)s LIMIT ?"
+            ) % {
+                "chunk_id": chunk_id,
+                "order": order,
+            }
+
+            txn.execute(sql, args)
+            new_rows = [_EventDictReturn(row[0], row[1], row[2], row[3]) for row in txn]
+
+            if not new_rows:
+                break
+
+            rows.extend(new_rows)
+
+        if limit > 0:
+            rows = rows[:limit]
 
         if rows:
+            chunk = rows[-1].chunk_id
             topo = rows[-1].topological_ordering
             toke = rows[-1].stream_ordering
             if direction == 'b':
@@ -752,12 +859,12 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 # when we are going backwards so we subtract one from the
                 # stream part.
                 toke -= 1
-            next_token = RoomStreamToken(topo, toke)
+            next_token = RoomStreamToken(chunk, topo, toke)
         else:
             # TODO (erikj): We should work out what to do here instead.
             next_token = to_token if to_token else from_token
 
-        return rows, str(next_token),
+        return rows, str(next_token), iterated_chunks,
 
     @defer.inlineCallbacks
     def paginate_room_events(self, room_id, from_key, to_key=None,
@@ -779,16 +886,38 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         Returns:
             tuple[list[dict], str]: Returns the results as a list of dicts and
             a token that points to the end of the result set. The dicts have
-            the keys "event_id", "topological_ordering" and "stream_orderign".
+            the keys "event_id", "topological_ordering" and "stream_ordering".
         """
 
         from_key = RoomStreamToken.parse(from_key)
         if to_key:
             to_key = RoomStreamToken.parse(to_key)
 
-        rows, token = yield self.runInteraction(
+        def _do_paginate_room_events(txn):
-            "paginate_room_events", self._paginate_room_events_txn,
+            rows, token, chunks = self._paginate_room_events_txn(
-            room_id, from_key, to_key, direction, limit, event_filter,
+                txn, room_id, from_key, to_key, direction, limit, event_filter,
+            )
+
+            extremities = []
+            seen = set()
+            for chunk_id in chunks:
+                if chunk_id in seen:
+                    continue
+                seen.add(chunk_id)
+
+                event_ids = self._simple_select_onecol_txn(
+                    txn,
+                    table="chunk_backwards_extremities",
+                    keyvalues={"chunk_id": chunk_id},
+                    retcol="event_id"
+                )
+
+                extremities.extend(e for e in event_ids if e not in extremities)
+
+            return rows, token, extremities
+
+        rows, token, extremities = yield self.runInteraction(
+            "paginate_room_events", _do_paginate_room_events,
         )
 
         events = yield self._get_events(
@@ -798,7 +927,49 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
 
         self._set_before_and_after(events, rows)
 
-        defer.returnValue((events, token))
+        defer.returnValue((events, token, extremities))
+
+    def clamp_token_before(self, room_id, token, clamp_to):
+        token = RoomStreamToken.parse(token)
+        clamp_to = RoomStreamToken.parse(clamp_to)
+
+        def clamp_token_before_txn(txn, token):
+            if not token.topological:
+                sql = """
+                    SELECT chunk_id, topological_ordering FROM events
+                    WHERE room_id = ? AND stream_ordering <= ?
+                    ORDER BY stream_ordering DESC
+                """
+                txn.execute(sql, (room_id, token.stream,))
+                row = txn.fetchone()
+                if not row:
+                    return str(token)
+
+                chunk_id, topo = row
+                token = RoomStreamToken(chunk_id, topo, token.stream)
+
+            if token.chunk == clamp_to.chunk:
+                if token.topological < clamp_to.topological:
+                    return str(token)
+                else:
+                    return str(clamp_to)
+
+            sql = "SELECT rationale FROM chunk_linearized WHERE chunk_id = ?"
+
+            txn.execute(sql, (token.chunk,))
+            token_order, = txn.fetchone()
+
+            txn.execute(sql, (clamp_to.chunk,))
+            clamp_order, = txn.fetchone()
+
+            if token_order < clamp_order:
+                return str(token)
+            else:
+                return str(clamp_to)
+
+        return self.runInteraction(
+            "clamp_token_before", clamp_token_before_txn, token
+        )
 
 
 class StreamStore(StreamWorkerStore):

synapse/types.py

@@ -306,7 +306,7 @@ StreamToken.START = StreamToken(
 )
 
 
-class RoomStreamToken(namedtuple("_StreamToken", "topological stream")):
+class RoomStreamToken(namedtuple("_StreamToken", "chunk topological stream")):
     """Tokens are positions between events. The token "s1" comes after event 1.
 
             s0    s1
@@ -334,10 +334,17 @@ class RoomStreamToken(namedtuple("_StreamToken", "topological stream")):
     def parse(cls, string):
         try:
             if string[0] == 's':
-                return cls(topological=None, stream=int(string[1:]))
+                return cls(chunk=None, topological=None, stream=int(string[1:]))
-            if string[0] == 't':
+            if string[0] == 't':  # For backwards compat with older tokens.
                 parts = string[1:].split('-', 1)
-                return cls(topological=int(parts[0]), stream=int(parts[1]))
+                return cls(chunk=None, topological=int(parts[0]), stream=int(parts[1]))
+            if string[0] == 'c':
+                parts = string[1:].split('~', 2)
+                return cls(
+                    chunk=int(parts[0]),
+                    topological=int(parts[1]),
+                    stream=int(parts[2]),
+                )
         except Exception:
             pass
         raise SynapseError(400, "Invalid token %r" % (string,))
@@ -346,12 +353,14 @@ class RoomStreamToken(namedtuple("_StreamToken", "topological stream")):
     def parse_stream_token(cls, string):
         try:
             if string[0] == 's':
-                return cls(topological=None, stream=int(string[1:]))
+                return cls(chunk=None, topological=None, stream=int(string[1:]))
         except Exception:
             pass
         raise SynapseError(400, "Invalid token %r" % (string,))
 
     def __str__(self):
+        if self.chunk is not None:
+            return "c%d~%d~%d" % (self.chunk, self.topological, self.stream)
         if self.topological is not None:
             return "t%d-%d" % (self.topological, self.stream)
         else:

synapse/util/katriel_bodlaender.py

@@ -0,0 +1,337 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector 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.
+
+"""This module contains an implementation of the Katriel-Bodlaender algorithm,
+which is used to do online topological ordering of graphs.
+
+Note that the ordering derived from the graph is such that the source node of
+an edge comes before the target node of the edge, i.e. a graph of A -> B -> C
+would produce the ordering [A, B, C].
+
+This ordering is therefore opposite to what one might expect when considering
+the room DAG, as newer messages would be added to the start rather than the
+end.
+
+***The ChunkDBOrderedListStore therefore inverts the direction of edges***
+
+See:
+    A tight analysis of the Katriel–Bodlaender algorithm for online topological
+    ordering
+    Hsiao-Fei Liua and Kun-Mao Chao
+    https://www.sciencedirect.com/science/article/pii/S0304397507006573
+and:
+    Online Topological Ordering
+    Irit Katriel and Hans L. Bodlaender
+    http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.78.7933 )
+"""
+
+from abc import ABCMeta, abstractmethod
+
+
+class OrderedListStore(object):
+    """An abstract base class that is used to store a graph and maintain a
+    topological consistent, total ordering.
+
+    Internally this uses the Katriel-Bodlaender algorithm, which requires the
+    store expose an interface for the total ordering that supports:
+
+        - Insertion of the node into the ordering either immediately before or
+          after another node.
+        - Deletion of the node from the ordering
+        - Comparing the relative ordering of two arbitary nodes
+        - Get the node immediately before or after a given node in the ordering
+
+    It also needs to be able to interact with the graph in the following ways:
+
+        - Query the number of edges from a node in the graph
+        - Query the number of edges into a node in the graph
+        - Add an edge to the graph
+
+
+    Users of subclasses should call `add_node` and `add_edge` whenever editing
+    the graph. The total ordering exposed will remain constant until the next
+    call to one of these methods.
+
+    Note: Calls to `add_node` and `add_edge` cannot overlap, and so callers
+    should perform some form of locking.
+    """
+
+    __metaclass__ = ABCMeta
+
+    def add_node(self, node_id):
+        """Adds a node to the graph.
+
+        Args:
+            node_id (str)
+        """
+        self._insert_before(node_id, None)
+
+    def add_edge(self, source, target):
+        """Adds a new edge to the graph and updates the ordering.
+
+        See module level docs.
+
+        Note that both the source and target nodes must have been inserted into
+        the store (at an arbitrary position) already.
+
+        Args:
+            source (str): The source node of the new edge
+            target (str): The target node of the new edge
+        """
+
+        # The following is the Katriel-Bodlaender algorithm.
+
+        to_s = []
+        from_t = []
+        to_s_neighbours = []
+        from_t_neighbours = []
+        to_s_indegree = 0
+        from_t_outdegree = 0
+        s = source
+        t = target
+
+        while s and t and not self.is_before(s, t):
+            m_s = to_s_indegree
+            m_t = from_t_outdegree
+
+            # These functions return a tuple where the first term is a float
+            # that can be used to order the the list of neighbours.
+            # These are valid until the next write
+            pe_s = self.get_nodes_with_edges_to(s)
+            fe_t = self.get_nodes_with_edges_from(t)
+
+            l_s = len(pe_s)
+            l_t = len(fe_t)
+
+            if m_s + l_s <= m_t + l_t:
+                to_s.append(s)
+                to_s_neighbours.extend(pe_s)
+                to_s_indegree += l_s
+
+                if to_s_neighbours:
+                    to_s_neighbours.sort()
+                    _, s = to_s_neighbours.pop()
+                else:
+                    s = None
+
+            if m_s + l_s >= m_t + l_t:
+                from_t.append(t)
+                from_t_neighbours.extend(fe_t)
+                from_t_outdegree += l_t
+
+                if from_t_neighbours:
+                    from_t_neighbours.sort(reverse=True)
+                    _, t = from_t_neighbours.pop()
+                else:
+                    t = None
+
+        if s is None:
+            s = self.get_prev(target)
+
+        if t is None:
+            t = self.get_next(source)
+
+        while to_s:
+            s1 = to_s.pop()
+            self._delete_ordering(s1)
+            self._insert_after(s1, s)
+            s = s1
+
+        while from_t:
+            t1 = from_t.pop()
+            self._delete_ordering(t1)
+            self._insert_before(t1, t)
+            t = t1
+
+        self._add_edge_to_graph(source, target)
+
+    @abstractmethod
+    def is_before(self, first_node, second_node):
+        """Returns whether the first node is before the second node.
+
+        Args:
+            first_node (str)
+            second_node (str)
+
+        Returns:
+            bool: True if first_node is before second_node
+        """
+        pass
+
+    @abstractmethod
+    def get_prev(self, node_id):
+        """Gets the node immediately before the given node in the topological
+        ordering.
+
+        Args:
+            node_id (str)
+
+        Returns:
+            str|None: A node ID or None if no preceding node exists
+        """
+        pass
+
+    @abstractmethod
+    def get_next(self, node_id):
+        """Gets the node immediately after the given node in the topological
+        ordering.
+
+        Args:
+            node_id (str)
+
+        Returns:
+            str|None: A node ID or None if no proceding node exists
+        """
+        pass
+
+    @abstractmethod
+    def get_nodes_with_edges_to(self, node_id):
+        """Get all nodes with edges to the given node
+
+        Args:
+            node_id (str)
+
+        Returns:
+            list[tuple[float, str]]: Returns a list of tuple of an ordering
+            term and the node ID. The ordering term can be used to sort the
+            returned list.
+            The ordering is valid until subsequent calls to `add_edge`
+            functions
+        """
+        pass
+
+    @abstractmethod
+    def get_nodes_with_edges_from(self, node_id):
+        """Get all nodes with edges from the given node
+
+        Args:
+            node_id (str)
+
+        Returns:
+            list[tuple[float, str]]: Returns a list of tuple of an ordering
+            term and the node ID. The ordering term can be used to sort the
+            returned list.
+            The ordering is valid until subsequent calls to `add_edge`
+            functions
+        """
+        pass
+
+    @abstractmethod
+    def _insert_before(self, node_id, target_id):
+        """Inserts node immediately before target node.
+
+        If target_id is None then the node is inserted at the end of the list
+
+        Args:
+            node_id (str)
+            target_id (str|None)
+        """
+        pass
+
+    @abstractmethod
+    def _insert_after(self, node_id, target_id):
+        """Inserts node immediately after target node.
+
+        If target_id is None then the node is inserted at the start of the list
+
+        Args:
+            node_id (str)
+            target_id (str|None)
+        """
+        pass
+
+    @abstractmethod
+    def _delete_ordering(self, node_id):
+        """Deletes the given node from the ordered list (but not the graph).
+
+        Used when we want to reinsert it into a different position
+
+        Args:
+            node_id (str)
+        """
+        pass
+
+    @abstractmethod
+    def _add_edge_to_graph(self, source_id, target_id):
+        """Adds an edge to the graph from source to target.
+
+        Does not update ordering.
+
+        Args:
+            source_id (str)
+            target_id (str)
+        """
+        pass
+
+
+class InMemoryOrderedListStore(OrderedListStore):
+    """An in memory OrderedListStore
+    """
+
+    def __init__(self):
+        # The ordered list of nodes
+        self.list = []
+
+        # Map from node to set of nodes that it references
+        self.edges_from = {}
+
+        # Map from node to set of nodes that it is referenced by
+        self.edges_to = {}
+
+    def is_before(self, first_node, second_node):
+        return self.list.index(first_node) < self.list.index(second_node)
+
+    def get_prev(self, node_id):
+        idx = self.list.index(node_id) - 1
+        if idx >= 0:
+            return self.list[idx]
+        else:
+            return None
+
+    def get_next(self, node_id):
+        idx = self.list.index(node_id) + 1
+        if idx < len(self.list):
+            return self.list[idx]
+        else:
+            return None
+
+    def _insert_before(self, node_id, target_id):
+        if target_id is not None:
+            idx = self.list.index(target_id)
+            self.list.insert(idx, node_id)
+        else:
+            self.list.append(node_id)
+
+    def _insert_after(self, node_id, target_id):
+        if target_id is not None:
+            idx = self.list.index(target_id) + 1
+            self.list.insert(idx, node_id)
+        else:
+            self.list.insert(0, node_id)
+
+    def _delete_ordering(self, node_id):
+        self.list.remove(node_id)
+
+    def get_nodes_with_edges_to(self, node_id):
+        to_nodes = self.edges_to.get(node_id, [])
+        return [(self.list.index(nid), nid) for nid in to_nodes]
+
+    def get_nodes_with_edges_from(self, node_id):
+        from_nodes = self.edges_from.get(node_id, [])
+        return [(self.list.index(nid), nid) for nid in from_nodes]
+
+    def _add_edge_to_graph(self, source_id, target_id):
+        self.edges_from.setdefault(source_id, set()).add(target_id)
+        self.edges_to.setdefault(target_id, set()).add(source_id)

tests/storage/test_chunk_linearizer_table.py

@@ -0,0 +1,185 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector 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 twisted.internet import defer
+
+import random
+import tests.unittest
+import tests.utils
+
+from synapse.storage.chunk_ordered_table import ChunkDBOrderedListStore
+
+
+class ChunkLinearizerStoreTestCase(tests.unittest.TestCase):
+    """Tests to ensure that the ordering and rebalancing functions of
+    ChunkDBOrderedListStore work as expected.
+    """
+
+    def __init__(self, *args, **kwargs):
+        super(ChunkLinearizerStoreTestCase, self).__init__(*args, **kwargs)
+
+    @defer.inlineCallbacks
+    def setUp(self):
+        hs = yield tests.utils.setup_test_homeserver()
+        self.store = hs.get_datastore()
+        self.clock = hs.get_clock()
+
+    @defer.inlineCallbacks
+    def test_simple_insert_fetch(self):
+        room_id = "foo_room1"
+
+        def test_txn(txn):
+            table = ChunkDBOrderedListStore(
+                txn, room_id, self.clock, 1, 100,
+            )
+
+            table.add_node("A")
+            table._insert_after("B", "A")
+            table._insert_before("C", "A")
+
+            sql = """
+                SELECT chunk_id FROM chunk_linearized
+                WHERE room_id = ?
+                ORDER BY ordering ASC
+            """
+            txn.execute(sql, (room_id,))
+
+            ordered = [r for r, in txn]
+
+            self.assertEqual(["C", "A", "B"], ordered)
+
+        yield self.store.runInteraction("test", test_txn)
+
+    @defer.inlineCallbacks
+    def test_many_insert_fetch(self):
+        room_id = "foo_room2"
+
+        def test_txn(txn):
+            table = ChunkDBOrderedListStore(
+                txn, room_id, self.clock, 1, 20,
+            )
+
+            nodes = [(i, "node_%d" % (i,)) for i in xrange(1, 1000)]
+            expected = [n for _, n in nodes]
+
+            already_inserted = []
+
+            random.shuffle(nodes)
+            while nodes:
+                i, node_id = nodes.pop()
+                if not already_inserted:
+                    table.add_node(node_id)
+                else:
+                    for j, target_id in already_inserted:
+                        if j > i:
+                            break
+
+                    if j < i:
+                        table._insert_after(node_id, target_id)
+                    else:
+                        table._insert_before(node_id, target_id)
+
+                already_inserted.append((i, node_id))
+                already_inserted.sort()
+
+            sql = """
+                SELECT chunk_id FROM chunk_linearized
+                WHERE room_id = ?
+                ORDER BY ordering ASC
+            """
+            txn.execute(sql, (room_id,))
+
+            ordered = [r for r, in txn]
+
+            self.assertEqual(expected, ordered)
+
+        yield self.store.runInteraction("test", test_txn)
+
+    @defer.inlineCallbacks
+    def test_prepend_and_append(self):
+        room_id = "foo_room3"
+
+        def test_txn(txn):
+            table = ChunkDBOrderedListStore(
+                txn, room_id, self.clock, 1, 20,
+            )
+
+            table.add_node("a")
+
+            expected = ["a"]
+
+            for i in xrange(1, 1000):
+                node_id = "node_id_before_%d" % i
+                table._insert_before(node_id, expected[0])
+                expected.insert(0, node_id)
+
+            for i in xrange(1, 1000):
+                node_id = "node_id_after_%d" % i
+                table._insert_after(node_id, expected[-1])
+                expected.append(node_id)
+
+            sql = """
+                SELECT chunk_id FROM chunk_linearized
+                WHERE room_id = ?
+                ORDER BY ordering ASC
+            """
+            txn.execute(sql, (room_id,))
+
+            ordered = [r for r, in txn]
+
+            self.assertEqual(expected, ordered)
+
+        yield self.store.runInteraction("test", test_txn)
+
+    @defer.inlineCallbacks
+    def test_worst_case(self):
+        room_id = "foo_room3"
+
+        def test_txn(txn):
+            table = ChunkDBOrderedListStore(
+                txn, room_id, self.clock, 1, 100,
+            )
+
+            table.add_node("a")
+
+            prev_node = "a"
+
+            expected_prefix = ["a"]
+            expected_suffix = []
+
+            for i in xrange(1, 100):
+                node_id = "node_id_%d" % i
+                if i % 2 == 0:
+                    table._insert_before(node_id, prev_node)
+                    expected_prefix.append(node_id)
+                else:
+                    table._insert_after(node_id, prev_node)
+                    expected_suffix.append(node_id)
+                prev_node = node_id
+
+            sql = """
+                SELECT chunk_id FROM chunk_linearized
+                WHERE room_id = ?
+                ORDER BY ordering ASC
+            """
+            txn.execute(sql, (room_id,))
+
+            ordered = [r for r, in txn]
+
+            expected = expected_prefix + list(reversed(expected_suffix))
+
+            self.assertEqual(expected, ordered)
+
+        yield self.store.runInteraction("test", test_txn)

tests/util/test_katriel_bodlaender.py

@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector 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 synapse.util.katriel_bodlaender import InMemoryOrderedListStore
+
+from tests import unittest
+
+
+class KatrielBodlaenderTests(unittest.TestCase):
+    def test_simple_graph(self):
+        store = InMemoryOrderedListStore()
+
+        nodes = [
+            "node_1",
+            "node_2",
+            "node_3",
+            "node_4",
+        ]
+
+        for node in nodes:
+            store.add_node(node)
+
+        store.add_edge("node_2", "node_3")
+        store.add_edge("node_1", "node_2")
+        store.add_edge("node_3", "node_4")
+
+        self.assertEqual(nodes, store.list)
+
+    def test_reverse_graph(self):
+        store = InMemoryOrderedListStore()
+
+        nodes = [
+            "node_1",
+            "node_2",
+            "node_3",
+            "node_4",
+        ]
+
+        for node in nodes:
+            store.add_node(node)
+
+        store.add_edge("node_3", "node_2")
+        store.add_edge("node_2", "node_1")
+        store.add_edge("node_4", "node_3")
+
+        self.assertEqual(list(reversed(nodes)), store.list)