Open Stack

On 10/30/2013 10:42 AM, Clint Byrum wrote:
> So, recently we've had quite a long thread in gerrit regarding locking
> in Heat:
>
> https://review.openstack.org/#/c/49440/
>
> In the patch, there are two distributed lock drivers. One uses SQL,
> and suffers from all the problems you might imagine a SQL based locking
> system would. It is extremely hard to detect dead lock holders, so we
> end up with really long timeouts. The other is ZooKeeper.
>
> I'm on record as saying we're not using ZooKeeper. It is a little
> embarrassing to have taken such a position without really thinking things
> through. The main reason I feel this way though, is not because ZooKeeper
> wouldn't work for locking, but because I think locking is a mistake.
>
> The current multi-engine paradigm has a race condition. If you have a
> stack action going on, the state is held in the engine itself, and not
> in the database, so if another engine starts working on another action,
> they will conflict.
>
> The locking paradigm is meant to prevent this. But I think this is a
> huge mistake.
>
> The engine should store _all_ of its state in a distributed data store
> of some kind. Any engine should be aware of what is already happening
> with the stack from this state and act accordingly. That includes the
> engine currently working on actions. When viewed through this lense,
> to me, locking is a poor excuse for serializing the state of the engine
> scheduler.
>
> It feels like TaskFlow is the answer, with an eye for making sure
> TaskFlow can be made to work with distributed state. I am not well
> versed on TaskFlow's details though, so I may be wrong. It worries me
> that TaskFlow has existed a while and doesn't seem to be solving real
> problems, but maybe I'm wrong and it is actually in use already.
>
> Anyway, as a band-aid, we may _have_ to do locking. For that, ZooKeeper
> has some real advantages over using the database. But there is hesitance
> because it is not widely supported in OpenStack. What say you, OpenStack
> community? Should we keep ZooKeeper out of our.. zoo?

I will -2 any patch that adds zookeeper as a dependency to Heat.

The rest of the idea sounds good though.  I spoke with Joshua about 
TaskFlow Friday as a possibility for solving this problem, but TaskFlow 
presently does not implement a distributed task flow. Joshua indicated 
there was a celerity review at https://review.openstack.org/#/c/47609/, 
but this would introduce a different server dependency which suffers 
from the same issues as Zookeeper, not to mention incomplete AMQP server 
support for various AMQP implementations.  Joshua indicated using a pure 
AMQP implementation would be possible for this job but is not implemented.

I did get into a discussion with him about the subject of breaking the 
tasks in the flow into "jobs", which led to the suggestion that the 
parser should be part of the API server process (then the engine could 
be responsible for handling the various jobs Heat needs). Sounds like 
poor abstraction, not to mention serious rework required.

My take from our IRC discussion was that TaskFlow is not a job 
distribution system (like Gearman) but an in-process workflow manager.  
These two things are different.  I was unclear if Taskflow could be made 
to do both, while also operating under already supported AMQP server 
infrastructure that all of OpenStack relies on currently.  If it could, 
that would be fantastic, as we would only have to introduce a library 
dependency vs a full on server dependency with documentation, HA and 
scalability concerns.

Regards
-steve

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