Re: Standalone synchronous master

On 01/10/2014 06:27 PM, Bruce Momjian wrote:
> How would that work?  Would it be a tool in contrib?  There already is a
> timeout, so if a tool checked more frequently than the timeout, it
> should work.  The durable notification of the admin would happen in the
> tool, right?

Well, you know what tool *I'm* planning to use.

Thing is, when we talk about auto-degrade, we need to determine things
like "Is the replica down or is this just a network blip"? and take
action according to the user's desired configuration.  This is not
something, realistically, that we can do on a single request.  Whereas
it would be fairly simple for an external monitoring utility to do:

1. decide replica is offline for the duration (several poll attempts
have failed)

2. Send ALTER SYSTEM SET to the master and change/disable the
synch_replicas.

Such a tool would *also* be capable of detecting when the synchronous
replica was back up and operating, and switch back to sync mode,
something we simply can't do inside Postgres.  And it would be a lot
easier to configure an external tool with monitoring system integration
so that it can alert the DBA to degradation in a way which the DBA was
liable to actually see (which is NOT the Postgres log).

In other words, if we're going to have auto-degrade, the most
intelligent place for it is in
RepMgr/HandyRep/OmniPITR/pgPoolII/whatever.  It's also the *easiest*
place.  Anything we do *inside* Postgres is going to have a really,
really hard time determining when to degrade.


-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com