[Pdns-users] Multiple database servers

[bert hubert]

On Sun, Mar 06, 2005 at 01:34:58PM +0100, Lorens Kockum wrote:
> > I would like to have pdns try another slave if one of the slaves goes
> > down. Any insight would be appreciated.
> 
> I wouldn't try to set up something unusual inside PDNS.

Historically, I've not wanted to do this, and for good reason. What
constitutes 'down'? We typically have three situations:

	1) database is really down
		a) machine is off
		b) connection refused on the TCP port
	2) database does allow connection, but does not generate answers
		a) wrong password
		b) server out of memory etc etc
		c) database corrupted
	3) database works fine, but has no data, in other words, somebody
	   deleted our zones/records

These three situations are hard to detect. How often does a server fail
cleanly, which is only situation 1b and possibly 1a (the timeouts can hurt
in a big way).

Making PowerDNS do failover internally is one thing, but detecting when to
do so is way harder. 

Our current suggested solution is to monitor the availability of your
databases with available tools (nagios etc), and in case one goes down,
rewrite the pdns configuration, and restart it. Alternatively, rewrite
/etc/hosts and restart powerdns.

If we do it the other way, PowerDNS morphs into an uptime monitor, and those
are available already.

Good luck!

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