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?
On Wed, Mar 11, 2020 at 3:57 PM Ken Gaillot <kgail...@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 <kgail...@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...@yahoo.com> wrote: > > > > > On March 10, 2020 7:31:27 PM GMT+02:00, Roman Hershkovich < > > > > > war...@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 <kgail...@redhat.com> > > _______________________________________________ > Manage your subscription: > https://lists.clusterlabs.org/mailman/listinfo/users > > ClusterLabs home: https://www.clusterlabs.org/ >
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