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Reduce serialization errors in MultiWriterIdGen (#8456)

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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.
This commit is contained in:
Erik Johnston 2020-10-07 15:15:57 +01:00 • committed by GitHub
parent 52a50e8686
commit ae5b2a72c0
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7 changed files with 112 additions and 8 deletions
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12
synapse/storage/util/id_generators.py
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@ -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__)
@ -548,7 +549,7 @@ 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.
"""
@ -598,10 +599,13 @@ 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
@ -632,10 +636,16 @@ 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