Re: [RFC: bug fix?] Connection attempt block forever when the synchronous standby is not running

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> I think we should rework RecordTransactionCommit() to only wait for the
> standby if `markXidCommitted' and not if `wrote_xlog'. There really
> isn't a reason to make a readonly transaction's commit wait just because
> it did some hot pruning.

Well, see the comment that explains why the logic is like this now:

* If we didn't create XLOG entries, we're done here; otherwise we
* should flush those entries the same as a commit record. (An
* example of a possible record that wouldn't cause an XID to be
* assigned is a sequence advance record due to nextval() --- we want
* to flush that to disk before reporting commit.)

I agree that HOT pruning isn't a reason to make a commit wait, but
nextval() is.

We could perhaps add more flags that would keep track of which sorts of
xlog entries justify a wait at commit, but TBH I'm skeptical of the entire
proposition. Having synchronous replication on with no live slave *will*
result in arbitrary hangs, and the argument that this particular case
should be exempt seems a bit thin to me. The sooner the user realizes
he's got a problem, the better. If read-only transactions don't show a
problem, the user might not realize he's got one until he starts to wonder
why autovac/autoanalyze aren't working.

I think a more useful line of thought would be to see if we can't complain
more loudly when we have no synchronous standby. Perhaps a "WARNING:
waiting forever for lack of a synchronous standby" could be emitted when
a transaction starts to wait.

regards, tom lane