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Re: Replication

From: Andreas Kostyrka <andreas(at)kostyrka(dot)org>
To: Craig James <craig_james(at)emolecules(dot)com>
Cc: Ben <bench(at)silentmedia(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Replication
Date: 2007-06-15 01:02:15
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Lists: pgsql-performance
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> Most of our data is replicated offline using custom tools tailored to
> our loading pattern, but we have a small amount of "global" information,
> such as user signups, system configuration, advertisements, and such,
> that go into a single small (~5-10 MB) "global database" used by all
> servers.

Slony provides near instantaneous failovers (in the single digit seconds
 range). You can script an automatic failover if the master server
becomes unreachable. That leaves you the problem of restarting your app
(or making it reconnect) to the new master.

5-10MB data implies such a fast initial replication, that making the
server rejoin the cluster by setting it up from scratch is not an issue.

> The problem is, there don't seem to be any "vote a new master" type of
> tools for Slony-I, and also, if the original master comes back online,
> it has no way to know that a new master has been elected.  So I'd have
> to write a bunch of SOAP services or something to do all of this.

You don't need SOAP services, and you do not need to elect a new master.
if dbX goes down, dbY takes over, you should be able to decide on a
static takeover pattern easily enough.

The point here is, that the servers need to react to a problem, but you
probably want to get the admin on duty to look at the situation as
quickly as possible anyway. With 5-10MB of data in the database, a
complete rejoin from scratch to the cluster is measured in minutes.

Furthermore, you need to checkout pgpool, I seem to remember that it has
some bad habits in routing queries. (E.g. it wants to apply write
queries to all nodes, but slony makes the other nodes readonly.
Furthermore, anything inside a BEGIN is sent to the master node, which
is bad with some ORMs, that by default wrap any access into a transaction)

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