Open Stack

To me u just made state consistency be a lock by another name.

A lock protects a region of code from being mutually accessed, u can do the same with state consistency. In the end though u need some reliable transactional consistent storage (a database, zookeeper, other) to store that state (which itself is using locks). So to me they are similar, state consistency is nice in that it becomes extremely obvious what the states and transitions are (and includes other very nice benefits, like having a log of what states occurred). The question to me becomes what happens to that state consistency when its running in a distributed system, which all of openstack is running in. At that point u need a way to ensure multiple servers (going through various states) are not manipulating the same resources at the same time (delete volume from cinder, while attaching it in nova). Those 2 separate services do not likely share the same state transitions (and will likely not as they become tightly coupled at that point). So then u need some type of coordination system to ensure the ordering of these 2 resource actions is done in a consistent manner. To me this starts to involve zookeeper (or something similar) since its in the end just distributed coordination (which is what these systems were designed for).

My 2 cents.

-Josh

From: Georgy Okrokvertskhov <gokrokvertskhov at mirantis.com<mailto:gokrokvertskhov at mirantis.com>>
Reply-To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Date: Wednesday, October 30, 2013 11:04 AM
To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Subject: Re: [openstack-dev] [Heat] Locking and ZooKeeper - a space oddysey

Hi Clint,

I think you rose a point here. We implemented distributed engine in Murano without locking mechanism by keeping state consistent on each step. We extracted this engine from Murano and plan to put it as a part of Mistral project for task management and execution. Working Mistral implementation will appear during IceHouse development. We are working closely with taskflow team, so I think you can expect to have distributed task execution support in taskflow library natively or through Mistral.

I am not against ZooKeeper but I think that for OpenStack service it is better to use oslo library shared with other projects instead of adding some custom locking mechanism for one project.

Thanks
Georgy


On Wed, Oct 30, 2013 at 10:42 AM, Clint Byrum <clint at fewbar.com<mailto:clint at fewbar.com>> 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?

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--
Georgy Okrokvertskhov
Technical Program Manager,
Cloud and Infrastructure Services,
Mirantis
http://www.mirantis.com<http://www.mirantis.com/>
Tel. +1 650 963 9828
Mob. +1 650 996 3284
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