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We have a need for a distributed lock in the VMware driver, which I
suspect isn't unique. Specifically it is possible for a VMware datastore
to be accessed via multiple nova nodes if it is shared between
clusters[1]. Unfortunately the vSphere API doesn't provide us with the
primitives to implement robust locking using the storage layer itself,
so we're looking elsewhere.

The closest we seem to have in Nova currently are service groups, which
currently have 3 implementations: DB, Zookeeper and Memcached. The
service group api currently provides simple membership, but for locking
we'd be looking for something more.

I think the api we'd be looking for would be something along the lines of:

Foo.lock(name, fence_info)
Foo.unlock(name)

Bar.fence(fence_info)

Note that fencing would be required in this case. We believe we can
fence by terminating the other Nova's vSphere session, but other options
might include killing a Nova process, or STONITH. These would be
implemented as fencing drivers.

Although I haven't worked through the detail, I believe lock and unlock
would be implementable in all 3 of the current service group drivers.
Fencing would be implemented separately.

My questions:

* Does this already exist, or does anybody have patches pending to do
something like this?
* Are there other users for this?
* Would service groups be an appropriate place, or a new distributed
locking class?
* How about if we just used zookeeper directly in the driver?

Matt

[1] Cluster ~= hypervisor
-- 
Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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