[ClusterLabs] About the pacemaker

Jan Pokorný jpokorny at redhat.com
Thu Jan 10 08:53:42 EST 2019

On 08/01/19 10:14 -0600, Ken Gaillot wrote:
> On Tue, 2019-01-08 at 15:27 +0800, T. Ladd Omar wrote:
>> I have a question, if the Pacemaker has an event-notify interface
>> which is realized by push&pull. Recently I want to do something
>> extra using other process when the resources being started or
>> deleted. So I need a way to monitor the resources events.
>> ClusterMon and alerts both use external-scripts for extra actions,
>> but in my situation, the specific process might have not being
>> started. I hope pacemaker itself could store the old events and
>> flush them for updating until the specific process starts and
>> subscribe to Pacemaker, then pull all the old events. Also the
>> Pacemaker could push to it when new events come.
> I would use alerts with alert_file.sh (with custom modifications if
> desired) to record them to a file, then have your process look at that.
> (Tip: if you only care about events since the last boot, put the file
> in /run so you don't have to worry about cleaning it up.)

Based on what's been described, it sounds like asking for extended
functionality that might be served with an external "store-and-forward"
daemon.  Such a daemon would also alleviate the processing complexity
in case of plentiful alert subscribers when they are used for subsequent
forwarding and/or extraction to assist decisions, since it would
conceptually detach such postprocessing from the main executive flow
fully (e.g. no sharing of the same security boundaries, cgroup, etc.;
access control would be the sole responsibility of this daemon),
and allow for durability of the events with desired parameters.  

Such a daemon could then gradually overtake the responsibility to
keep event stream subscribers updated, itself making use of a more
suitable hooking directly into pacemaker.

That's how the future could evolve.  Contributions welcome.

>> Above is all what I thought, maybe it is not accurate. Anyway, I
>> need some advice.  By the way, there is no deletion notify in
>> ClusterMon and alerts, right ?
> Correct, configuration changes are not alerted. The only way I know of
> to get configuration changes is to use the C API for update/replace
> callbacks. It would also be possible to poll the configuration at
> intervals and use crm_diff to compare them, but that's probably not any
> easier.

The hypothetical daemon could keep up with whatever events that
get exposed internally.

Jan (Poki)
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