[ClusterLabs] Antw: Doing reload right

[Ulrich Windl]

>>> Ken Gaillot <kgaillot at redhat.com> schrieb am 30.06.2016 um 18:58 in Nachricht
<57754F9F.8070908 at redhat.com>:
> Hello all,
> 
> I've been meaning to address the implementation of "reload" in Pacemaker
> for a while now, and I think the next release will be a good time, as it
> seems to be coming up more frequently.
> 
> In the current implementation, Pacemaker considers a resource parameter
> "reloadable" if the resource agent supports the "reload" action, and the
> agent's metadata marks the parameter with "unique=0". If (only) such
> parameters get changed in the resource's pacemaker configuration,
> pacemaker will call the agent's reload action rather than the
> stop-then-start it usually does for parameter changes.
> 
> This is completely broken for two reasons:

I agree ;-)

> 
> 1. It relies on "unique=0" to determine reloadability. "unique" was
> originally intended (and is widely used by existing resource agents) as
> a hint to UIs to indicate which parameters uniquely determine a resource
> instance. That is, two resource instances should never have the same
> value of a "unique" parameter. For this purpose, it makes perfect sense
> that (for example) the path to a binary command would have unique=0 --
> multiple resource instances could (and likely would) use the same
> binary. However, such a parameter could never be reloadable.

I tought unique=0 were reloadable (unique=1 were not)...


> 
> 2. Every known resource agent that implements a reload action does so
> incorrectly. Pacemaker uses reload for changes in the resource's
> *pacemaker* configuration, while all known RAs use reload for a
> service's native reload capability of its own configuration file. As an
> example, the ocf:heartbeat:named RA calls "rndc reload" for its reload
> action, which will have zero effect on any pacemaker-configured
> parameters -- and on top of that, the RA uses "unique=0" in its correct
> UI sense, and none of those parameters are actually reloadable.

Maybe LSB confusion...

> 
> My proposed solution is:
> 
> * Add a new "reloadable" attribute for resource agent metadata, to
> indicate reloadable parameters. Pacemaker would use this instead of
> "unique".

No objections if you change the XML metadata version number this time ;-)

> 
> * Add a new "reload-options" RA action for the ability to reload
> Pacemaker-configured options. Pacemaker would call this instead if "reload".

Why not "reload-parameters"?

> 
> * Formalize that "reload" means reload the service's own configuration,
> legitimizing the most common existing RA implementations. (Pacemaker
> itself will not use this, but tools such as crm_resource might.)

Maybe be precise what your "reload-options" is expected to do, compared to the "reload" action.
I'm still a bit confused. Maybe a working example...


> 
> * Review all ocf:pacemaker and ocf:heartbeat agents to make sure they
> use unique, reloadable, reload, and reload-options properly.
> 
> The downside is that this breaks backward compatibility. Any RA that
> actually implements unique and reload so that reload works will lose
> reload capability until it is updated to the new style.

Maybe there's a solution that is even simpler: Keep the action name, but don't use "relaod" unless the RA indicated at least one "reloadable" parameter. Naturally old RAs don't do it. And once an author touches the RA to fix thing, he/she should do so (not just adding "relaodable" to the metadata).

> 
> While we usually go to great lengths to preserve backward compatibility,
> I think it is OK to break it in this case, because most RAs that
> implement reload do so wrongly: some implement it as a service reload, a
> few advertise reload but don't actually implement it, and others map
> reload to start, which might theoretically work in some cases (I'm not
> familiar enough with iSCSILogicalUnit and iSCSITarget to be sure), but
> typically won't, as the previous service options are not reverted (for
> example, I think Route would incorrectly leave the old route in the old
> table).
> 
> So, I think breaking backward compatibility is actually a good thing
> here, since the most reload can do with existing RAs is trigger bad
> behavior.

See my comment above.

> 
> The opposing view would be that we shouldn't punish any RA writer who
> implemented this correctly. However, there's no solution that preserves

The software could give a warning if the RA provides "reload", but does not define any reloadble parameter. So to notify users and developers that some change is needed.

> backward compatibility with both UI usage of unique and reload usage of
> unique. Plus, the worst that would happen is that the RA would stop
> being reloadable -- not as bad as the current possibilities from
> mis-implemented reload.

Agree on that.

> 
> My questions are:
> 
> Does anyone know of an RA that uses reload correctly? Dummy doesn't
> count ;-)

Revalidation by the RA authors (and adding "reloadable" attribute to metadata parameters) ;-)

> 
> Does anyone object to the (backward-incompatible) solution proposed here?

I commented inline.

Regards,
Ulrich

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