[ClusterLabs] I want to have some resource monitored and based on that make an acton. Is it possible?

[Ken Gaillot]

On Tue, 2020-03-10 at 21:03 +0200, Roman Hershkovich wrote:
> DB servers are not in PCS cluster. Basically you say that i need to
> add them to PCS cluster and then start them? but in case if DB1 fails
> - DB2 autopromoted and not required start of service again>
> 
> Regarding colocation rule - i'm kind of missing logic how it works -
> how i can "colocate" 1 of 2 APP servers to be around a master DB ? 

If I understand correctly, what you want is that both apps are
restarted if the master changes?

I'm thinking you'll need a custom OCF agent for the app servers. The
monitor action, in addition to checking the app's status, could also
check which db is master, and return an error if it's changed since the
last monitor. (The start action would have to record the initial
master.) Pacemaker will restart the app to recover from the error.

That is a little hacky because you'll have errors in the status every
time the master moves, but maybe that's worth knowing in your situation
anyway.

> On Tue, Mar 10, 2020 at 8:42 PM Strahil Nikolov <
> hunter86_bg at yahoo.com> wrote:
> > On March 10, 2020 7:31:27 PM GMT+02:00, Roman Hershkovich <
> > warpik at gmail.com> wrote:
> > >I have 2 DB servers (master/slave with replica) and 2 APP servers.
> > >2 APP servers managed by pacemaker  (active/passive) , but i want
> > also
> > >to
> > >monitor "which DB is master".  I can't use VIP (which could be
> > sticked
> > >on
> > >master DB) - it is very limited virtual environment.
> > >
> > >Is it possible to create a rule or some other scenario, so in case
> > if
> > >master moved - pacemaker will restart APP (app does not support
> > >failover) ?
> > 
> > Hi Roman,
> > 
> > If you set an order rule that  starts  first the master  and then
> > the app, during a failover  the app will be stoped  and once the
> > master  is switched  (slave is promoted) the  app will be started
> > again.
> > 
> > Also you can consider  a  colocation rule that all  apps are 
> > started  where  the master  DB is running  -  so the lattency will
> > be minimal.
> > 
> > Best Regards,
> > Strahil Nikolov
> 
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-- 
Ken Gaillot <kgaillot at redhat.com>