Re: Outdated replication protocol error?

Hi,

On 2021-01-14 16:40:26 +0900, Fujii Masao wrote:
> On 2021/01/12 9:06, Jeff Davis wrote:
> > Commit 5ee2197767 (about 4 years old) introduced the error:
> >
> > "IDENTIFY_SYSTEM has not been run before START_REPLICATION"
> >
> > But it seems like running START_REPLICATION first works (at least in
> > the simple case).
> >
> > We should either:
> >
> > 1. Document that IDENTIFY_SYSTEM must always be run before
> > START_REPLICATION, and always issue a WARNING if that's not done (an
> > ERROR might break existing applications); or
> >
> > 2. If the commit is out of date and no longer needed, or if it's easy
> > enough to fix, just remove the error (and Assert a valid
> > ThisTimeLineID).
>
> +1 to remove the error if START_REPLICATION can always work fine without
> IDENTIFY_SYSTEM. I found that the error happens when we connect to the standby
> and just run START_REPLICATION without IDENTIFY_SYSTEM. But I'm not sure
> if IDENTIFY_SYSTEM is actually necessary even in that case.

The current approach seems quite bad to me too. By that point the value
could be just about arbitrarily out of date - but that doesn't really
matter because GetStandbyFlushRecPtr() updates it. And for the
!am_cascading_walsender it's trivial to compute.

Has anybody dug out the thread leading to the commit?

Greetings,

Andres Freund