Re: Logical decoding slots can go backwards when used from SQL, docs are wrong

Craig Ringer wrote:
> Hi all
>
> I think I found a couple of logical decoding issues while writing tests for
> failover slots.
>
> Despite the docs' claim that a logical slot will replay data "exactly
> once", a slot's confirmed_lsn can go backwards and the SQL functions can
> replay the same data more than once.We don't mark a slot as dirty if only
> its confirmed_lsn is advanced, so it isn't flushed to disk. For failover
> slots this means it also doesn't get replicated via WAL. After a master
> crash, or for failover slots after a promote event, the confirmed_lsn will
> go backwards. Users of the SQL interface must keep track of the safely
> locally flushed slot position themselves and throw the repeated data away.
> Unlike with the walsender protocol it has no way to ask the server to skip
> that data.
>
> Worse, because we don't dirty the slot even a *clean shutdown* causes slot
> confirmed_lsn to go backwards. That's a bug IMO. We should force a flush of
> all slots at the shutdown checkpoint, whether dirty or not, to address it.

Why don't we mark the slot dirty when confirmed_lsn advances? If we fix
that, doesn't it fix the other problems too?

--
Álvaro Herrera http://www.2ndQuadrant.com/
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