[openstack-dev] [heat] convergence cancel messages

Clint Byrum clint at fewbar.com
Wed Feb 24 07:42:26 UTC 2016

Excerpts from Anant Patil's message of 2016-02-23 23:08:31 -0800:
> Hi,
> I would like the discuss various approaches towards fixing bug
> https://launchpad.net/bugs/1533176
> When convergence is on, and if the stack is stuck, there is no way to
> cancel the existing request. This feature was not implemented in
> convergence, as the user can again issue an update on an in-progress
> stack. But if a resource worker is stuck, the new update will wait
> for-ever on it and the update will not be effective.
> The solution is to implement cancel request. Since the work for a stack
> is distributed among heat engines, the cancel request will not work as
> it does in legacy way. Many or all of the heat engines might be running
> worker threads to provision a stack.
> I could think of two options which I would like to discuss:
> (a) When a user triggered cancel request is received, set the stack
> current traversal to None or something else other than current
> traversal. With this the new check-resources/workers will never be
> triggered. This is okay as long as the worker(s) is not stuck. The
> existing workers will finish running, and no new check-resource
> (workers) will be triggered, and it will be a graceful cancel.  But the
> workers that are stuck will be stuck for-ever till stack times-out.  To
> take care of such cases, we will have to implement logic of "polling"
> the DB at regular intervals (may be at each step() of scheduler task)
> and bail out if the current traversal is updated. Basically, each worker
> will "poll" the DB to see if the current traversal is still valid and if
> not, stop itself. The drawback of this approach is that all the workers
> will be hitting the DB and incur a significant overhead.  Besides, all
> the stack workers irrespective of whether they will be cancelled or not,
> will keep on hitting DB. The advantage is that it probably is easier to
> implement. Also, if the worker is stuck in particular "step", then this
> approach will not work.

I think this is the simplest option. And if the polling gets to be too
much, you can implement an observer pattern where one worker is just
assigned to poll the traversal and if it changes, RPC to the known
active workers that they should cancel any jobs using a now-cancelled
stack version.

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