[Pacemaker] Validate strategy for RA on DRBD standby node

[David McCurley]

Thanks for the quick reply and especially the link.  It was much better and more thorough in testing than the other shell ra ldap link I found.

> That would be the first python RA. BTW, there was recently posted
> slapd RA (implemented in shell), which I should review, but
> haven't done that yet. /At any rate, that RA does not support
> multi-state resources which I think would be essential. Did you
> plan to do that? At any rate, I'd suggest that you check that
> code too and then see if you need to do your own implementation.
> The thread starts here:
> 
> http://marc.info/?l=linux-ha-dev&m=129666245428850&w=2

Great stuff and very thorough.   But it doesn't look like it will work in our config because the files won't exist on the DRBD slave.  I will use this as my new example.

I'm not sure what you are referring to with "multi-state" resources.  Is this in relation to "promote,demote, migrate_to, migrate_from" in the guide, i.e. master vs slave or is there more to it?  I plan to support a master and slave later on down the road -- under pressure to get this rolled out now.  Is there a good discussion / resource other than the dev guide?  I had planned to try to wade through the DRBD RA scripts to figure it out.

Why python?  Because shell is harder for me to read and I have to make a good clear verbose example for some others who will also be doing some RA's (in python) for our custom apps.

> 
> > The config file for OpenLDAP is stored in
> > /etc/ldap/slapd.d/cn=config.ldif.  This is on a DRBD
> > active-passive system and the /etc/ldap directory is actually a
> > symlink to the DRBD controlled share /vcoreshare/etc/ldap.  The
> > real config file is at
> > /vcoreshare/etc/ldap/slapd.d/cn=config.ldif.
> 
> What about the old style configuration? I assume that there are
> still quite a few installations/distributions using those.

Yes, I have some code for that, using the example slapd init script and the other examples I found, but no test environment for that.

> > So I'm trying to be very judicious with every function and
> > validation, checking file permissions, etc.  But the problem is
> > that /etc/ldap/slapd.d/cn=config.ldif is only present on the
> > active DRBD node.  My validate function checks that the file is
> > readable by the user/group that slapd is to run as.  Now, as soon
> > as I start ldap in the cluster, it starts fine, but validate fails
> > on the standby node (because the DRBD volume isn't mounted) and
> > crm_mon shows a failed action:
> 
> On probes (monitor with interval 0), some parts of validation
> which concern the local node and not the configuration should say
> OCF_NOT_RUNNING instead of error. This is exactly that case. No
> worries, because if the next action is start validation is
> invoked again. Probes are issued by pacemaker to establish if the
> resource is running and normally it is expected to be not running
> (for instance on node startup).

Ah!  I have a function that checks for probes but wasn't using it because I didn't quite understand the semantics.

Very helpful stuff, thanks!