Re: Minimal logical decoding on standbys

On Sat, 13 Apr 2019 at 00:57, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2019-04-12 23:34:02 +0530, Amit Khandekar wrote:
> > I tried to see if I can quickly understand what's going on.
> >
> > Here, master wal_level is hot_standby, not logical, though slave
> > wal_level is logical.
>
> Oh, that's well diagnosed. Cool. Also nicely tested - this'd be ugly
> in production.

Tushar had made me aware of the fact that this reproduces only when
master wal_level is hot_standby.

>
> I assume the problem isn't present if you set the primary to wal_level =
> logical?

Right.

>
>
> > Not sure why this is happening. On slave, wal_level is logical, so
> > logical records should have tuple data. Not sure what does that have
> > to do with wal_level of master. Everything should be there on slave
> > after it replays the inserts; and also slave wal_level is logical.
>
> The standby doesn't write its own WAL, only primaries do. I thought we
> forbade running with wal_level=logical on a standby, when the primary is
> only set to replica. But that's not what we do, see
> CheckRequiredParameterValues().
>
> I've not yet thought this through, but I think we'll have to somehow
> error out in this case. I guess we could just check at the start of
> decoding what ControlFile->wal_level is set to,

By "start of decoding", I didn't get where exactly. Do you mean
CheckLogicalDecodingRequirements() ?

> and then raise an error
> in decode.c when we pass an XLOG_PARAMETER_CHANGE record that sets
> wal_level to something lower?

Didn't get where exactly we should error out. We don't do
XLOG_PARAMETER_CHANGE handling in decode.c , so obviously you meant
something else, which I didn't understand.

What I am thinking is :
In CheckLogicalDecodingRequirements(), besides checking wal_level,
also check ControlFile->wal_level when InHotStandby. I mean, when we
are InHotStandby, both wal_level and ControlFile->wal_level should be
>= WAL_LEVEL_LOGICAL. This will allow us to error out when using logical
slot when master has incompatible wal_level.

ControlFile is not accessible outside xlog.c so need to have an API to
extract this field.

>
> Could you try to implement that?
>
> Greetings,
>
> Andres Freund

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
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