From fa8934b175467d589dd34fae18639cac0d738fc9 Mon Sep 17 00:00:00 2001 From: Erik Johnston Date: Wed, 7 Oct 2020 15:15:57 +0100 Subject: Reduce serialization errors in MultiWriterIdGen (#8456) We call `_update_stream_positions_table_txn` a lot, which is an UPSERT that can conflict in `REPEATABLE READ` isolation level. Instead of doing a transaction consisting of a single query we may as well run it outside of a transaction. --- synapse/storage/util/id_generators.py | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) (limited to 'synapse/storage/util/id_generators.py') diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py index 48efbb5067..ad017207aa 100644 --- a/synapse/storage/util/id_generators.py +++ b/synapse/storage/util/id_generators.py @@ -24,6 +24,7 @@ from typing_extensions import Deque from synapse.metrics.background_process_metrics import run_as_background_process from synapse.storage.database import DatabasePool, LoggingTransaction +from synapse.storage.types import Cursor from synapse.storage.util.sequence import PostgresSequenceGenerator logger = logging.getLogger(__name__) @@ -552,7 +553,7 @@ class MultiWriterIdGenerator: # do. break - def _update_stream_positions_table_txn(self, txn): + def _update_stream_positions_table_txn(self, txn: Cursor): """Update the `stream_positions` table with newly persisted position. """ @@ -602,10 +603,13 @@ class _MultiWriterCtxManager: stream_ids = attr.ib(type=List[int], factory=list) async def __aenter__(self) -> Union[int, List[int]]: + # It's safe to run this in autocommit mode as fetching values from a + # sequence ignores transaction semantics anyway. self.stream_ids = await self.id_gen._db.runInteraction( "_load_next_mult_id", self.id_gen._load_next_mult_id_txn, self.multiple_ids or 1, + db_autocommit=True, ) # Assert the fetched ID is actually greater than any ID we've already @@ -636,10 +640,16 @@ class _MultiWriterCtxManager: # # We only do this on the success path so that the persisted current # position points to a persisted row with the correct instance name. + # + # We do this in autocommit mode as a) the upsert works correctly outside + # transactions and b) reduces the amount of time the rows are locked + # for. If we don't do this then we'll often hit serialization errors due + # to the fact we default to REPEATABLE READ isolation levels. if self.id_gen._writers: await self.id_gen._db.runInteraction( "MultiWriterIdGenerator._update_table", self.id_gen._update_stream_positions_table_txn, + db_autocommit=True, ) return False -- cgit 1.5.1