[Pacemaker] Validate strategy for RA on DRBD standby node

[Dejan Muhamedagic]

On Thu, Feb 24, 2011 at 10:49:27AM -0500, David McCurley wrote:
> 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.

migrate_to/from are for something else. It's just promote
(slave->master) and demote (master->slave).

> 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.

No problem with python. Actually, there was an idea to provide a
python class (RA or so) which would make implementing resource
agents easier.

> > > 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.

IIRC, the old configuration style is quite easy to parse.

> > > 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.

OK, hope that it's clear now.

Thanks,

Dejan

> 
> Very helpful stuff, thanks!
> 
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