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

[Roman Hershkovich]

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)

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

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
> >
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> --
> Ken Gaillot <kgaillot at redhat.com>
>
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