Re: Issues with Quorum Commit

Tom Lane <tgl@sss.pgh.pa.us> writes:
> Well, actually, that's *considerably* more complicated than just a
> timeout.  How are you going to "mark the standby as degraded"?  The
> standby can't keep that information, because it's not even connected
> when the master makes the decision.  ISTM that this requires
>
> 1. a unique identifier for each standby (not just role names that
> multiple standbys might share);
>
> 2. state on the master associated with each possible standby -- not just
> the ones currently connected.
>
> Both of those are perhaps possible, but the sense I have of the
> discussion is that people want to avoid them.

What we'd like to avoid is for the users to have to cope with such
needs. Now, if that's internal to the code and automatic, that's not the
same thing at all.

What I'd have in mind is a "Database standby system identifier" that
would be part of the initial hand shake in the replication protocol. And
a system function to be able to "unregister" the standby.

> Actually, #2 seems rather difficult even if you want it.  Presumably
> you'd like to keep that state in reliable storage, so it survives master
> crashes.  But how you gonna commit a change to that state, if you just
> lost every standby (suppose master's ethernet cable got unplugged)?

I don't see that as a huge problem myself, because I'm already well sold
to the per-transaction replication-synchronous behaviour. So any change
done there by the master would be hard-coded as async. What I'm missing?

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support