[ClusterLabs] RFC: allowing soft recovery attempts before ignore/block/etc.

[Ken Gaillot]

On 09/22/2016 05:58 PM, Andrew Beekhof wrote:
> 
> 
> On Fri, Sep 23, 2016 at 1:58 AM, Ken Gaillot <kgaillot at redhat.com
> <mailto:kgaillot at redhat.com>> wrote:
> 
>     On 09/22/2016 09:53 AM, Jan Pokorný wrote:
>     > On 22/09/16 08:42 +0200, Kristoffer Grönlund wrote:
>     >> Ken Gaillot <kgaillot at redhat.com <mailto:kgaillot at redhat.com>> writes:
>     >>
>     >>> I'm not saying it's a bad idea, just that it's more complicated than it
>     >>> first sounds, so it's worth thinking through the implications.
>     >>
>     >> Thinking about it and looking at how complicated it gets, maybe what
>     >> you'd really want, to make it clearer for the user, is the ability to
>     >> explicitly configure the behavior, either globally or per-resource. So
>     >> instead of having to tweak a set of variables that interact in complex
>     >> ways, you'd configure something like rule expressions,
>     >>
>     >> <on_fail>
>     >>   <restart repeat="3" />
>     >>   <migrate timeout="60s" />
>     >>   <fence/>
>     >> </on_fail>
>     >>
>     >> So, try to restart the service 3 times, if that fails migrate the
>     >> service, if it still fails, fence the node.
>     >>
>     >> (obviously the details and XML syntax are just an example)
>     >>
>     >> This would then replace on-fail, migration-threshold, etc.
>     >
>     > I must admit that in previous emails in this thread, I wasn't able to
>     > follow during the first pass, which is not the case with this procedural
>     > (sequence-ordered) approach.  Though someone can argue it doesn't take
>     > type of operation into account, which might again open the door for
>     > non-obvious interactions.
> 
>     "restart" is the only on-fail value that it makes sense to escalate.
> 
>     block/stop/fence/standby are final. Block means "don't touch the
>     resource again", so there can't be any further response to failures.
>     Stop/fence/standby move the resource off the local node, so failure
>     handling is reset (there are 0 failures on the new node to begin with).
> 
>     "Ignore" is theoretically possible to escalate, e.g. "ignore 3 failures
>     then migrate", but I can't think of a real-world situation where that
>     makes sense, 
> 
> 
> really?
> 
> it is not uncommon to hear "i know its failed, but i dont want the
> cluster to do anything until its _really_ failed"  

Hmm, I guess that would be similar to how monitoring systems such as
nagios can be configured to send an alert only if N checks in a row
fail. That's useful where transient outages (e.g. a webserver hitting
its request limit) are acceptable for a short time.

I'm not sure that's translatable to Pacemaker. Pacemaker's error count
is not "in a row" but "since the count was last cleared".

"Ignore up to three monitor failures if they occur in a row [or, within
10 minutes?], then try soft recovery for the next two monitor failures,
then ban this node for the next monitor failure." Not sure being able to
say that is worth the complexity.

> 
>     and it would be a significant re-implementation of "ignore"
>     (which currently ignores the state of having failed, as opposed to a
>     particular instance of failure).
> 
> 
> agreed
>  
> 
> 
>     What the interface needs to express is: "If this operation fails,
>     optionally try a soft recovery [always stop+start], but if <N> failures
>     occur on the same node, proceed to a [configurable] hard recovery".
> 
>     And of course the interface will need to be different depending on how
>     certain details are decided, e.g. whether any failures count toward <N>
>     or just failures of one particular operation type, and whether the hard
>     recovery type can vary depending on what operation failed.