Open Stack

Excerpts from Anant Patil's message of 2015-09-30 00:10:52 -0700:
> Hi,
> 
> One of remaining items in convergence is detecting and handling engine
> (the engine worker) failures, and here are my thoughts.
> 
> Background: Since the work is distributed among heat engines, by some
> means heat needs to detect the failure and pick up the tasks from failed
> engine and re-distribute or run the task again.
> 
> One of the simple way is to poll the DB to detect the liveliness by
> checking the table populated by heat-manage. Each engine records its
> presence periodically by updating current timestamp. All the engines
> will have a periodic task for checking the DB for liveliness of other
> engines. Each engine will check for timestamp updated by other engines
> and if it finds one which is older than the periodicity of timestamp
> updates, then it detects a failure. When this happens, the remaining
> engines, as and when they detect the failures, will try to acquire the
> lock for in-progress resources that were handled by the engine which
> died. They will then run the tasks to completion.
> 
> Another option is to use a coordination library like the community owned
> tooz (http://docs.openstack.org/developer/tooz/) which supports
> distributed locking and leader election. We use it to elect a leader
> among heat engines and that will be responsible for running periodic
> tasks for checking state of each engine and distributing the tasks to
> other engines when one fails. The advantage, IMHO, will be simplified
> heat code. Also, we can move the timeout task to the leader which will
> run time out for all the stacks and sends signal for aborting operation
> when timeout happens. The downside: an external resource like
> Zookeper/memcached etc are needed for leader election.
> 

It's becoming increasingly clear that OpenStack services in general need
to look at distributed locking primitives. There's a whole spec for that
right now:

https://review.openstack.org/#/c/209661/

I suggest joining that conversation, and embracing a DLM as the way to
do this.

Also, the leader election should be per-stack, and the leader selection
should be heavily weighted based on a consistent hash algorithm so that
you get even distribution of stacks to workers. You can look at how
Ironic breaks up all of the nodes that way. They're using a similar lock
to the one Heat uses now, so the two projects can collaborate nicely on
a real solution.