Re: [proposal] recovery_target "latest"

At Fri, 8 Nov 2019 16:08:47 +0300, Grigory Smolkin <g(dot)smolkin(at)postgrespro(dot)ru> wrote in
>
> On 11/8/19 7:00 AM, Grigory Smolkin wrote:
> >
> > On 11/7/19 4:36 PM, Grigory Smolkin wrote:
> >> I gave it some thought and now think that prohibiting recovery_target
> >> 'latest' and standby was a bad idea.
> >> All recovery_targets follow the same pattern of usage, so
> >> recovery_target "latest" also must be capable of working in standby
> >> mode.
> >> All recovery_targets have a clear deterministic 'target' where
> >> recovery should end.
> >> In case of recovery_target "latest" this target is the end of
> >> available WAL, therefore the end of available WAL must be more clearly
> >> defined.
> >> I will work on it.
> >>
> >> Thank you for a feedback.
> >
> >
> > Attached new patch revision, now end of available WAL is defined as
> > the fact of missing required WAL.
> > In case of standby, the end of WAL is defined as 2 consecutive
> > switches of WAL source, that didn`t provided requested record.
> > In case of streaming standby, each switch of WAL source is forced
> > after 3 failed attempts to get new data from walreceiver.
> >
> > All constants are taken off the top of my head and serves as proof of
> > concept.
> > TAP test is updated.
> >
> Attached new revision, it contains some minor refactoring.

Thanks for the new patch. I found that it needs more than I thought,
but it seems a bit too complicated and less stable.

As the patch does, WaitForWALToBecomeAvailable needs to exit when
avaiable sources are exhausted. However, we don't need to count
failures to do that. It is enough that the function have two more exit
point. When streaming timeout fires, and when we found that streaming
is not set up when archive/wal failed.

In my opinion, it is better that we have less dependency to global
variables in such deep levels in a call hierachy. Such nformation can
be stored in XLogPageReadPrivate.

I think The doc needs to exiplain on the difference between default
and latest.

Please find the attached, which illustrates the first two points of
the aboves.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center