Re: Fetching timeline during recovery

Hi.

At Thu, 25 Jul 2019 19:38:08 +0200, Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com> wrote in <20190725193808(dot)1648ddc8(at)firost>
> On Wed, 24 Jul 2019 14:33:27 +0200
> Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com> wrote:
>
> > On Wed, 24 Jul 2019 09:49:05 +0900
> > Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> >
> > > On Tue, Jul 23, 2019 at 06:05:18PM +0200, Jehan-Guillaume de Rorthais
> > > wrote:
> [...]
> > > I think that there are arguments for being more flexible with it, and
> > > perhaps have a system-level view to be able to look at some of its fields.
> >
> > Great idea. I'll give it a try to keep the discussion on.
>
> After some thinking, I did not find enough data to expose to justify the
> creation a system-level view. As I just need the current timeline I
> wrote "pg_current_timeline()". Please, find the patch in attachment.
>
> The current behavior is quite simple:
> * if the cluster is in production, return ThisTimeLineID
> * else return walrcv->receivedTLI (using GetWalRcvWriteRecPtr)
>
> This is really naive implementation. We should probably add some code around
> the startup process to gather and share general recovery stats. This would
> allow to fetch eg. the current recovery method, latest xlog file name restored
> from archives or streaming, its timeline, etc.
>
> Any thoughts?

If replay is delayed behind timeline switch point, replay-LSN and
receive/write/flush LSNs are on different timelines. When
replica have not reached the new timeline to which alredy
received file belongs, the fucntion returns wrong file name,
specifically a name consisting of the latest segment number and
the older timeline where the segment doesn't belong to.

We have an LSN reporting function each for several objectives.

pg_current_wal_lsn
pg_current_wal_insert_lsn
pg_current_wal_flush_lsn
pg_last_wal_receive_lsn
pg_last_wal_replay_lsn

But, I'm not sure just adding further pg_last_*_timeline() to
this list is a good thing..

The function returns NULL for NULL input (STRICT behavior) but
returns (NULL, NULL) for undefined timeline. I don't think the
differene is meaningful.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center