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Only try to backfill event if we haven't tried before recently (#13635)
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Only try to backfill event if we haven't tried before recently (exponential backoff). No need to keep trying the same backfill point that fails over and over.

Fix #13622
Fix #8451

Follow-up to #13589

Part of #13356
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@MadLittleMods
MadLittleMods committed Sep 23, 2022
1 parent f49f73c commit ac1a317
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1 change: 1 addition & 0 deletions changelog.d/13635.feature
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Exponentially backoff from backfilling the same event over and over.
4 changes: 1 addition & 3 deletions synapse/handlers/federation.py
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Expand Up @@ -226,9 +226,7 @@ async def _maybe_backfill_inner(
"""
backwards_extremities = [
_BackfillPoint(event_id, depth, _BackfillPointType.BACKWARDS_EXTREMITY)
for event_id, depth in await self.store.get_oldest_event_ids_with_depth_in_room(
room_id
)
for event_id, depth in await self.store.get_backfill_points_in_room(room_id)
]

insertion_events_to_be_backfilled: List[_BackfillPoint] = []
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188 changes: 147 additions & 41 deletions synapse/storage/databases/main/event_federation.py
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Expand Up @@ -11,6 +11,7 @@
# 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 datetime
import itertools
import logging
from queue import Empty, PriorityQueue
Expand Down Expand Up @@ -43,7 +44,7 @@
)
from synapse.storage.databases.main.events_worker import EventsWorkerStore
from synapse.storage.databases.main.signatures import SignatureWorkerStore
from synapse.storage.engines import PostgresEngine
from synapse.storage.engines import PostgresEngine, Sqlite3Engine
from synapse.types import JsonDict
from synapse.util import json_encoder
from synapse.util.caches.descriptors import cached
Expand Down Expand Up @@ -72,6 +73,13 @@

logger = logging.getLogger(__name__)

BACKFILL_EVENT_BACKOFF_UPPER_BOUND_SECONDS: int = int(
datetime.timedelta(days=7).total_seconds()
)
BACKFILL_EVENT_EXPONENTIAL_BACKOFF_STEP_SECONDS: int = int(
datetime.timedelta(hours=1).total_seconds()
)


# All the info we need while iterating the DAG while backfilling
@attr.s(frozen=True, slots=True, auto_attribs=True)
Expand Down Expand Up @@ -715,96 +723,189 @@ def _get_auth_chain_difference_txn(

@trace
@tag_args
async def get_oldest_event_ids_with_depth_in_room(
self, room_id: str
async def get_backfill_points_in_room(
self,
room_id: str,
) -> List[Tuple[str, int]]:
"""Gets the oldest events(backwards extremities) in the room along with the
aproximate depth.
We use this function so that we can compare and see if someones current
depth at their current scrollback is within pagination range of the
event extremeties. If the current depth is close to the depth of given
oldest event, we can trigger a backfill.
"""
Gets the oldest events(backwards extremities) in the room along with the
approximate depth. Sorted by depth, highest to lowest (descending).
Args:
room_id: Room where we want to find the oldest events
Returns:
List of (event_id, depth) tuples
List of (event_id, depth) tuples. Sorted by depth, highest to lowest
(descending)
"""

def get_oldest_event_ids_with_depth_in_room_txn(
def get_backfill_points_in_room_txn(
txn: LoggingTransaction, room_id: str
) -> List[Tuple[str, int]]:
# Assemble a dictionary with event_id -> depth for the oldest events
# Assemble a tuple lookup of event_id -> depth for the oldest events
# we know of in the room. Backwards extremeties are the oldest
# events we know of in the room but we only know of them because
# some other event referenced them by prev_event and aren't peristed
# in our database yet (meaning we don't know their depth
# specifically). So we need to look for the aproximate depth from
# some other event referenced them by prev_event and aren't
# persisted in our database yet (meaning we don't know their depth
# specifically). So we need to look for the approximate depth from
# the events connected to the current backwards extremeties.
sql = """
SELECT b.event_id, MAX(e.depth) FROM events as e
SELECT backward_extrem.event_id, event.depth FROM events AS event
/**
* Get the edge connections from the event_edges table
* so we can see whether this event's prev_events points
* to a backward extremity in the next join.
*/
INNER JOIN event_edges as g
ON g.event_id = e.event_id
INNER JOIN event_edges AS edge
ON edge.event_id = event.event_id
/**
* We find the "oldest" events in the room by looking for
* events connected to backwards extremeties (oldest events
* in the room that we know of so far).
*/
INNER JOIN event_backward_extremities as b
ON g.prev_event_id = b.event_id
WHERE b.room_id = ? AND g.is_state is ?
GROUP BY b.event_id
INNER JOIN event_backward_extremities AS backward_extrem
ON edge.prev_event_id = backward_extrem.event_id
/**
* We use this info to make sure we don't retry to use a backfill point
* if we've already attempted to backfill from it recently.
*/
LEFT JOIN event_failed_pull_attempts AS failed_backfill_attempt_info
ON
failed_backfill_attempt_info.room_id = backward_extrem.room_id
AND failed_backfill_attempt_info.event_id = backward_extrem.event_id
WHERE
backward_extrem.room_id = ?
/* We only care about non-state edges because we used to use
* `event_edges` for two different sorts of "edges" (the current
* event DAG, but also a link to the previous state, for state
* events). These legacy state event edges can be distinguished by
* `is_state` and are removed from the codebase and schema but
* because the schema change is in a background update, it's not
* necessarily safe to assume that it will have been completed.
*/
AND edge.is_state is ? /* False */
/**
* Exponential back-off (up to the upper bound) so we don't retry the
* same backfill point over and over. ex. 2hr, 4hr, 8hr, 16hr, etc.
*
* We use `1 << n` as a power of 2 equivalent for compatibility
* with older SQLites. The left shift equivalent only works with
* powers of 2 because left shift is a binary operation (base-2).
* Otherwise, we would use `power(2, n)` or the power operator, `2^n`.
*/
AND (
failed_backfill_attempt_info.event_id IS NULL
OR ? /* current_time */ >= failed_backfill_attempt_info.last_attempt_ts + /*least*/%s((1 << failed_backfill_attempt_info.num_attempts) * ? /* step */, ? /* upper bound */)
)
/**
* Sort from highest to the lowest depth. Then tie-break on
* alphabetical order of the event_ids so we get a consistent
* ordering which is nice when asserting things in tests.
*/
ORDER BY event.depth DESC, backward_extrem.event_id DESC
"""

txn.execute(sql, (room_id, False))
if isinstance(self.database_engine, PostgresEngine):
least_function = "least"
elif isinstance(self.database_engine, Sqlite3Engine):
least_function = "min"
else:
raise RuntimeError("Unknown database engine")

txn.execute(
sql % (least_function,),
(
room_id,
False,
self._clock.time_msec(),
1000 * BACKFILL_EVENT_EXPONENTIAL_BACKOFF_STEP_SECONDS,
1000 * BACKFILL_EVENT_BACKOFF_UPPER_BOUND_SECONDS,
),
)

return cast(List[Tuple[str, int]], txn.fetchall())

return await self.db_pool.runInteraction(
"get_oldest_event_ids_with_depth_in_room",
get_oldest_event_ids_with_depth_in_room_txn,
"get_backfill_points_in_room",
get_backfill_points_in_room_txn,
room_id,
)

@trace
async def get_insertion_event_backward_extremities_in_room(
self, room_id: str
self,
room_id: str,
) -> List[Tuple[str, int]]:
"""Get the insertion events we know about that we haven't backfilled yet.
We use this function so that we can compare and see if someones current
depth at their current scrollback is within pagination range of the
insertion event. If the current depth is close to the depth of given
insertion event, we can trigger a backfill.
"""
Get the insertion events we know about that we haven't backfilled yet
along with the approximate depth. Sorted by depth, highest to lowest
(descending).
Args:
room_id: Room where we want to find the oldest events
Returns:
List of (event_id, depth) tuples
List of (event_id, depth) tuples. Sorted by depth, highest to lowest
(descending)
"""

def get_insertion_event_backward_extremities_in_room_txn(
txn: LoggingTransaction, room_id: str
) -> List[Tuple[str, int]]:
sql = """
SELECT b.event_id, MAX(e.depth) FROM insertion_events as i
SELECT
insertion_event_extremity.event_id, event.depth
/* We only want insertion events that are also marked as backwards extremities */
INNER JOIN insertion_event_extremities as b USING (event_id)
FROM insertion_event_extremities AS insertion_event_extremity
/* Get the depth of the insertion event from the events table */
INNER JOIN events AS e USING (event_id)
WHERE b.room_id = ?
GROUP BY b.event_id
INNER JOIN events AS event USING (event_id)
/**
* We use this info to make sure we don't retry to use a backfill point
* if we've already attempted to backfill from it recently.
*/
LEFT JOIN event_failed_pull_attempts AS failed_backfill_attempt_info
ON
failed_backfill_attempt_info.room_id = insertion_event_extremity.room_id
AND failed_backfill_attempt_info.event_id = insertion_event_extremity.event_id
WHERE
insertion_event_extremity.room_id = ?
/**
* Exponential back-off (up to the upper bound) so we don't retry the
* same backfill point over and over. ex. 2hr, 4hr, 8hr, 16hr, etc
*
* We use `1 << n` as a power of 2 equivalent for compatibility
* with older SQLites. The left shift equivalent only works with
* powers of 2 because left shift is a binary operation (base-2).
* Otherwise, we would use `power(2, n)` or the power operator, `2^n`.
*/
AND (
failed_backfill_attempt_info.event_id IS NULL
OR ? /* current_time */ >= failed_backfill_attempt_info.last_attempt_ts + /*least*/%s((1 << failed_backfill_attempt_info.num_attempts) * ? /* step */, ? /* upper bound */)
)
/**
* Sort from highest to the lowest depth. Then tie-break on
* alphabetical order of the event_ids so we get a consistent
* ordering which is nice when asserting things in tests.
*/
ORDER BY event.depth DESC, insertion_event_extremity.event_id DESC
"""

txn.execute(sql, (room_id,))
if isinstance(self.database_engine, PostgresEngine):
least_function = "least"
elif isinstance(self.database_engine, Sqlite3Engine):
least_function = "min"
else:
raise RuntimeError("Unknown database engine")

txn.execute(
sql % (least_function,),
(
room_id,
self._clock.time_msec(),
1000 * BACKFILL_EVENT_EXPONENTIAL_BACKOFF_STEP_SECONDS,
1000 * BACKFILL_EVENT_BACKOFF_UPPER_BOUND_SECONDS,
),
)
return cast(List[Tuple[str, int]], txn.fetchall())

return await self.db_pool.runInteraction(
Expand Down Expand Up @@ -1539,7 +1640,12 @@ async def get_next_staged_event_id_for_room(
self,
room_id: str,
) -> Optional[Tuple[str, str]]:
"""Get the next event ID in the staging area for the given room."""
"""
Get the next event ID in the staging area for the given room.
Returns:
Tuple of the `origin` and `event_id`
"""

def _get_next_staged_event_id_for_room_txn(
txn: LoggingTransaction,
Expand Down
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