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

[Ken Gaillot]

On Wed, 2020-03-11 at 02:27 +0200, Roman Hershkovich wrote:
> Yes.
> I have only 1 APP active at same time, and so I want this app to be
> restarted whenever DB changes. Another one is a "standby" APP, where
> all resources are shut.
> So i thought about adding some "service" script, which will probe a
> DB , and in case if it finds a CHANGE - will trigger pcs to reload a
> set of resources, where one of resource would be a systemctl file,
> which will continue to run a script, so in case of next change of DB
> - it will restart APP set again. Is it sounds reasonable? (i don't
> care of errors. I mean - i do, i want to log, but i'm ok to see them)

That sounds fine, but I'd trigger the restart by returning an error
code from the db-monitoring script, rather than directly attempt to
restart the resources via pcs. If you order the other resources after
the db-monitoring script, pacemaker will automatically restart them
when the db-monitoring script returns an error.

> In addition - i thought maybe bringing PAF here could be useful - but
> this is even more complex ... 

If bringing the db into the cluster is a possibility, that would
probably be more reliable, with a quicker response too.

In that case you would simply order the dependent resources after the
database master promotion. pcs example: pcs constraint order promote
DB-RSC then start DEPENDENT-RSC

> On Tue, Mar 10, 2020 at 10:28 PM Ken Gaillot <kgaillot at redhat.com>
> wrote:
> > 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
-- 
Ken Gaillot <kgaillot at redhat.com>