[openstack-dev] [nova][mistral] Automatic evacuation as a long running task

Matthew Booth mbooth at redhat.com
Tue Oct 6 15:34:26 UTC 2015

Hi, Roman,

Evacuated has been on my radar for a while and this post has prodded me to
take a look at the code. I think it's worth starting by explaining the
problems in the current solution. Nova client is currently responsible for
doing this evacuate. It does:

1. List all instances on the source host
2. Initiate evacuate for each instance

Evacuating a single instance does:

1. Set instance task state to rebuilding
2. Create a migration record with source and dest if specified

3. Call the scheduler to get a destination host if not specified
4. Get the migration object from the db

5. Rebuild the instance on dest
6. Update instance.host to dest

Examining single instance evacuation, the first obvious thing to look at is
what if 2 happen simultaneously. Because step 1 is atomic, it should not be
possible to initiate 2 evacuations simultaneously of a single instance.
However, note that this atomic action hasn't updated the instance host,
meaning the source host remains the owner of this instance. If the
evacuation process fails to complete, the source host will automatically
delete it if it comes back up because it will find a migration record, but
it will not be rebuilt anywhere else. Evacuating it again will fail,
because its task state is already rebuilding.

Also, let's imagine that the conductor crashes. There is not enough state
for any tool, whether internal or external, to be able to know if the
rebuild is ongoing somewhere or not, and therefore whether it is safe to
retry even if that retry would succeed, which it wouldn't.

Which is to say that we can't currently robustly evacuate one instance!

Looking at the nova client side, there is an obvious race there: there is
no guarantee in step 2 that instances returned in step one have not already
been evacuated by another process. We're protected here, though because
evacuating a single instance twice will fail the second time. Note that the
process isn't idempotent, though, because an evacuation which falls into a
hole will never be retried.

Moving on to what evacuated does. Evacuated uses rabbit to distribute jobs
reliably. There are 2 jobs in evacuated:

1. Evacuate host:
  1.1 Get list of all instances on the source host from Nova
  1.2 Send an evacuate vm job for each instance
2. Evacuate vm:
  2.1 Tell Nova to start evacuating an instance

Because we're using rabbit as a reliable message bus, the initiator of one
of the tasks knows that it will eventually run to completion at least once.
Note that there's nothing to prevent the task being executed more than once
per call, though. A task may crash before sending an ack, or may just be
really slow. However, in both cases, for exactly the same reasons as for
the implementation in nova client, running more than once should not race.
It is still not idempotent, though, again for exactly the same reasons as
nova client.

Also notice that, exactly as in the nova client implementation, we are not
asserting that an instance has been evacuated. We are only asserting that
we called nova.evacuate, which is to say that we got as far as step 2 in
the evacuation sequence above.

In other words, in terms of robustness, calling evacuated's evacuate host
is identical to asserting that nova client's evacuate host ran to
completion at least once, which is quite a lot simpler to do. That's still
not very robust, though: we don't recover from failures, and we don't
ensure that an instance is evacuated, only that we started an attempt to
evacuate at least once. I'm obviously not satisfied with nova client,
however as the implementation is simpler I would favour it over evacuated.

I believe we can solve this problem, but I think that without fixing
single-instance evacuate we're just pushing the problem around (or creating
new places for it to live). I would base the robustness of my
implementation on a single principal:

  An instance has a single owner, which is exclusively responsible for
rebuilding it.

In outline, I would redefine the evacuate process to do:

1. Call the scheduler to get a destination for the evacuate if none was
2. Atomically update instance.host to this destination, and task state to

3. Rebuild the instance.

This would be supported by a periodic task on the compute host which looks
for rebuilding instances assigned to this host which aren't currently
rebuilding, and kicks off a rebuild for them. This would cover the compute
going down during a rebuild, or the api going down before messaging the

Implementing this gives us several things:

1. The list instances, evacuate all instances process becomes idempotent,
because as soon as the evacuate is initiated, the instance is removed from
the source host.
2. We get automatic recovery of failure of the target compute. Because we
atomically moved the instance to the target compute immediately, if the
target compute also has to be evacuated, our instance won't fall through
the gap.
3. We don't need an additional place for the code to run, because it will
run on the compute. All the work has to be done by the compute anyway. By
farming the evacuates out directly and immediately to the target compute we
reduce both overhead and complexity.

The coordination becomes very simple. If we've run the nova client
evacuation anywhere at least once, the actual evacuations are now Sombody
Else's Problem (to quote h2g2), and will complete eventually. As evacuation
in any case involves a forced change of owner it requires fencing of the
source and implies an external agent such as pacemaker. The nova client
evacuation can run in pacemaker.


On Fri, Oct 2, 2015 at 2:05 PM, Roman Dobosz <roman.dobosz at intel.com> wrote:

> Hi all,
> The case of automatic evacuation (or resurrection currently), is a topic
> which surfaces once in a while, but it isn't yet fully supported by
> OpenStack and/or by the cluster services. There was some attempts to
> bring the feature into OpenStack, however it turns out it cannot be
> easily integrated with. On the other hand evacuation may be executed
> from the outside using Nova client or Nova API calls for evacuation
> initiation.
> I did some research regarding the ways how it could be designed, based
> on Russel Bryant blog post[1] as a starting point. Apart from it, I've
> also taken high availability and reliability into consideration when
> designing the solution.
> Together with coworker, we did first PoC[2] to enable cluster to be able
> to perform evacuation. The idea behind that PoC was simple - providing
> additional, small service which would trigger and supervise the
> evacuation process, which would be triggered from the outside (in this
> example we were using Pacemaker fencing facility, but it might be
> anything) using RabbitMQ directly. Those services are running on the
> control plane in AA fashion.
> That work well for us. So we started exploring other possibilities like
> oslo.messaging just to use it in the same manner as we did in the poc.
> It turns out that the implementation will not be as easy, because there
> is no facility in the oslo.messaging for letting sending an ACK from the
> client after the job is done (not as soon as it gets the message). We
> also looked at the existing OpenStack projects for a candidate which
> provide service for managing long running tasks.
> There is the Mistral project, which gives us almost all the features we
> need. The one missing feature is the HA of the Mistral tasks execution.
> The question is, how such problem (long running tasks) could be resolved
> in OpenStack?
> [1]
> http://blog.russellbryant.net/2014/10/15/openstack-instance-ha-proposal/
> [2] https://github.com/dawiddeja/evacuationd
> --
> Cheers,
> Roman Dobosz
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