Re: Switching timeline over streaming replication

On Tuesday, September 25, 2012 12:39 PM Heikki Linnakangas wrote:
> On 24.09.2012 16:33, Amit Kapila wrote:
> > On Tuesday, September 11, 2012 10:53 PM Heikki Linnakangas wrote:
> >> I've been working on the often-requested feature to handle timeline
> >> changes over streaming replication. At the moment, if you kill the
> >> master and promote a standby server, and you have another standby
> >> server that you'd like to keep following the new master server, you
> >> need a WAL archive in addition to streaming replication to make it
> >> cross the timeline change. Streaming replication will just error
> out.
> >> Having a WAL archive is usually a good idea in complex replication
> >> scenarios anyway, but it would be good to not require it.
> >
> > Confirm my understanding of this feature:
> >
> > This feature is for case when standby-1 who is going to be promoted
> to
> > master has archive mode 'on'.
>
> No. This is for the case where there is no WAL archive.
> archive_mode='off' on all servers.
>
> Or to be precise, you can also have a WAL archive, but this patch
> doesn't affect that in any way. This is strictly about streaming
> replication.
>
> > As in that case only its timeline will change.
>
> The timeline changes whenever you promote a standby. It's not related
> to
> whether you have a WAL archive or not.

Yes that is correct. I thought timeline change happens only when somebody
does PITR.
Can you please tell me why we change timeline after promotion, because the
original
Timeline concept was for PITR and I am not able to trace from code the
reason
why on promotion it is required?

>
> > If above is right, then there can be other similar scenario's where
> it can
> > be used:
> >
> > Scenario-1 (1 Master, 1 Stand-by)
> > 1. Master (archive_mode=on) goes down.
> > 2. Master again comes up
> > 3. Stand-by tries to follow it
> >
> > Now in above scenario also due to timeline mismatch it gives error,
> but your
> > patch should fix it.
>
> If the master simply crashes or is shut down, and then restarted, the
> timeline doesn't change. The standby will reconnect / poll the archive,
> and sync up just fine, even without this patch.

How about when Master does PITR when it comes again?

With Regards,
Amit Kapila.