Re: Listen / Notify - what to do when the queue is full

Tom Lane wrote:
> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> A better approach is to do something similar to what we do now: at
>> prepare, just store the notifications in the state file like we do
>> already. In notify_twophase_postcommit(), copy the messages to the
>> shared queue. Although it's the same approach we have now, it
>> becomes a lot cleaner with the patch, because we're not
>> piggybacking the messages on the backend-private queue of the
>> current transaction, but sending the messages directly on behalf of
>> the prepared transaction being committed.
>
> This is still ignoring the complaint: you are creating a clear risk
> that COMMIT PREPARED will fail.
>
> I'm not sure that it's really worth it, but one way this could be
> made safe would be for PREPARE to "reserve" the required amount of
> queue space, such that nobody else could use it during the window
> from PREPARE to COMMIT PREPARED.

I'd see no problem with "COMMIT PREPARED" failing, as long as it was
possible to retry the COMMIT PREPARED at a later time. There surely are
other failure cases for COMMIT PREPARED too, like an IO error that
prevents the clog bit from being set, or a server crash half-way through
COMMIT PREPARED.

best regards,
Florian Pflug