[Pacemaker] Failover with multiple services on one node

[Matthew Palmer]

On Mon, Mar 08, 2010 at 01:34:01PM +0800, Martin Aspeli wrote:
> This question was sort of implied in my thread last week, but I'm going  
> to re-ask it properly, to reduce my own confusion if nothing else.
>
> We have two servers, master and slave. In the cluster, we have:

[bunchteen services, some HA, some not, one service IP]

> Each Zope instance is configured with a database connection string for  
> Postgres (e.g. postgres://192.168.245.10:5432) and a similar connection  
> string for memcached (e.g. 192.168.245.10:11211).
>
> My question is this: Do we need to group all the clustered resources  
> (the IP address, HAProxy, Postgres, memcached) so that if any one of  
> them fails, they all fail over to slave?
>
> If we don't do this, how can we manage the connection strings in Zope?  
> Since Zope needs a domain name or IP address as part of the connection  
> string, it'd be no good if, e.g. memcached failed over to slave, but the  
> IP address stayed with master, because Zope would still be looking for  
> it on master.
>
> What is the normal way to handle this? Do people have one floating IP  
> address per service?

This is how I prefer to do it.  RFC1918 IP addresses are cheap, IPv6 address
quintuply so.  Having everything tied to one address causes some mayhem when
something falls over (failover is quick, but it ain't instantaneous), so
it's far better to give everything it's own address and let them drift about
independently.  Also, it makes load-spreading (run PgSQL on one machine,
memcached on the other, for instance) much easier.

> Use groups to consider all those services together  
> at all times?

I wouldn't recommend it, given that there's no logical reason that they all
have to be together.

> Use some kind of hosts file trickery?

You *know* you're doing something wrong when hacking the hosts file is the
answer.

- Matt