On Mon, 2022-04-25 at 19:51 +0530, Bharath Rupireddy wrote: > With synchronous replication typically all the transactions (txns) > first locally get committed, then streamed to the sync standbys and > the backend that generated the transaction will wait for ack from sync > standbys. While waiting for ack, it may happen that the query or the > txn gets canceled (QueryCancelPending is true) or the waiting backend > is asked to exit (ProcDiePending is true). In either of these cases, > the wait for ack gets canceled and leaves the txn in an inconsistent > state [...] > > Here's a proposal (mentioned previously by Satya [1]) to avoid the > above problems: > 1) Wait a configurable amount of time before canceling the sync > replication by the backends i.e. delay processing of > QueryCancelPending and ProcDiePending in Introduced a new timeout GUC > synchronous_replication_naptime_before_cancel, when set, it will let > the backends wait for the ack before canceling the synchronous > replication so that the transaction can be available in sync standbys > as well. > 2) Wait for sync standbys to catch up upon restart after the crash or > in the next txn after the old locally committed txn was canceled.
While this may mitigate the problem, I don't think it will deal with all the cases which could cause a transaction to end up committed locally, but not on the synchronous standby. I think that only using the full power of two-phase commit can make this bulletproof. Is it worth adding additional complexity that is not a complete solution? Yours, Laurenz Albe
