[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 16:08 +0200, Roman Hershkovich wrote:
> Great, thank you very much for explanation. Regarding returning error
> - i did not knew.
> So, basically i can have a service, that will probe for master DB, in
> case of its transfer - service will update /etc/hosts and return
> error, which will be caught by pcs and it will restart whole
> dependent set ? Sounds good.
> But how i can do 2 "main resources" ? I have webserver AND
> db_monitor. In case of failure of webserver - should all start on
> node b, but in case of DB change - only underlying resources ...
> Should i make webserver outside of set? 

If you want the webserver to move to another node after a single
failure (of the webserver itself), set its migration-threshold to 1. If
you want other resources to move with it, colocate them with the
webserver.

The db monitor won't affect that -- if the db monitor fails, anything
ordered after it will restart.

> On Wed, Mar 11, 2020 at 3:57 PM Ken Gaillot <kgaillot at redhat.com>
> wrote:
> > 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
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-- 
Ken Gaillot <kgaillot at redhat.com>