Open Stack

On 09/30/2015 03:10 AM, Anant Patil wrote:
> 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.

Implementing our own locking system, even a "simple" one, sounds like a 
recipe for major bugs to me. I agree with your assessment that tooz is a 
better long-run decision.

> 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.

That's not necessarily true. For single-node installations (devstack, 
TripleO underclouds, etc) tooz offers file and IPC backends that don't 
need an extra service. Tooz's MySQL/PostgreSQL backends only provide 
distributed locking functionality, so we may need to depend on the 
memcached/redis/zookeeper backends for multi-node installs.

Even if tooz doesn't provide everything we need, I'm sure patches would 
be welcome.

> In the long run, IMO, using a library like tooz will be useful for heat.
> A lot of boiler plate needed for locking and running centralized tasks
> (such as timeout) will not be needed in heat. Given that we are moving
> towards distribution of tasks and horizontal scaling is preferred, it
> will be advantageous to use them.
>
> Please share your thoughts.
>
> - Anant
-- 
Ryan Brown / Senior Software Engineer, Openstack / Red Hat, Inc.