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Re: Simplifying replication

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Simplifying replication
Date: 2010-10-19 16:16:33
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Lists: pgsql-hackers
Dimitri, Greg,

> I want to say a big big +1 here. The way replication and PITR setup are
> implemented now are a very good prototype, it's time to consolidate and
> get to something usable by normal people, as opposed to PostgreSQL full
> time geeks.

Well, one thing to be addressed is separating the PITR functionality 
from replication.  PITR needs a lot of features -- timelines, recovery 
stop points, etc. -- which replication doesn't need or want.  I think 
that focussing on streaming replication functionality and ignoring the 
archive logs case is probably the best way to logically separate these 
two.  Presumably anyone who needs archive logs as well will be a 
professional DBA.

> I could prepare a patch given some advice on the replication protocol
> integration. For one, is streaming a base backup something that
> walsender should care about?

Yeah, I thought there was a prototype for this somewhere.   From a user 
perspective, using a 2nd pgport connection for the initial clone is 
fine.   I don't know if we want to worry about it otherwise from a 
resource management perspective; presumably the cloning process is going 
to be a pretty big performance hit on the master.

> BTW, do we have a clear idea of how to implement pg_ping, and should it
> reports current WAL location(s) of a standby?


> That needs a way to define a group of standby. There's nothing there
> that makes them know about each other.

Let me clarify.  I meant that if I try to make a *single* standby point 
to a new master, and that new master was behind the standby when it 
failed over, then the attempt to remaster should fail with an error.

I do *not* want to get into standby groups.  That way lies madness.  ;-)

> Now say we have pg_ping (or another tool) returning the current recv,
> applied and synced LSNs, it would be possible for any standby to figure
> out which other ones must be shot in case you failover here. The
> failover command could list those other standby in the group that you're
> behind of, and with a force command allow you to still failover to this
> one. Now you have to STONITH the one listed, but that's your problem
> after all.

The LSN isn't enough; as others have pointed out, we have a fairly 
serious failure case if a standby comes up as a master, accepts 
transactions, and then we try to remaster a 2nd standby which was 
actually ahead of the first standby at the time of master failure.  I 
haven't seen a solution posted to that yet; maybe I missed it?

 > Sorry, next time I'll make sure to bash Robert too. I don't have any
 > problems with the basic ideas you're proposing, just concerns about when
 > the right time to get into that whole giant subject is and who is going
 > to work on.

If not now, when?  The 2nd CommitFest is almost complete.   If we're 
going to make any substantial changes, we need to have patches for the 
3rd commitfest.  And I didn't see anyone discussing simplification until 
I brought it up.

I don't realistically think that we're going to get 100% simplification 
for 9.1.  But it would be nice to at least get some components, which 
means getting agreement on how things should work, at least roughly.

                                   -- Josh Berkus
                                      PostgreSQL Experts Inc.

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