Re: Time delayed LR (WAS Re: logical replication restrictions)

On Thu, Feb 2, 2023 at 7:21 PM Takamichi Osumi (Fujitsu)
<osumi(dot)takamichi(at)fujitsu(dot)com> wrote:
>
...
>
>
> > Besides, I am not sure it's a stable test to check the log. Is it possible that there's
> > no such log on a slow machine? I modified the code to sleep 1s at the beginning
> > of apply_dispatch(), then the new added test failed because the server log
> > cannot match.
> To get the log by itself is necessary to ensure
> that the delay is conducted by the apply worker, because we emit the diffms
> only if it's bigger than 0 in maybe_apply_delay(). If we omit the step,
> we are not sure the delay is caused by other reasons or the time-delayed feature.
>
> As you mentioned, it's possible that no log is emitted on slow machine. Then,
> the idea to make the test safer for such machines should be to make the delayed time longer.
> But we shortened the delay time to 1 second to mitigate the long test execution time of this TAP test.
> So, I'm not sure if it's a good idea to make it longer again.

I think there are a couple of things that can be done about this problem:

1. If you need the code/test to remain as-is then at least the test
message could include some comforting text like "(this can fail on
slow machines when the delay time is already exceeded)" so then a test
failure will not cause undue alarm.

2. Try moving the DEBUG2 elog (in function maybe_apply_delay) so that
it will *always* log the remaining wait time even if that wait time
becomes negative. Then I think the test cases can be made
deterministic instead of relying on good luck. This seems like the
better option.

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Kind Regards,
Peter Smith.
Fujitsu Australia