Comment 3 for bug 1488986

So, it's pretty hard to explain in one small comment how the model is behaving, but please consider that we have 'sort of' two-phase commits when booting an instance.

When a request comes in, you're right, the instances are elected iteratively by decrementing the resource usage of the elected node in HostState.consume_from_instance(). That means that when you're asking for 10 instances of the same type, the corresponding HostState(s) will be decremented before the next filters call which should provide a good way for ensuring consistency. That's only when the 10 instances are elected that the scheduler gives the answer back to the *conductor* which calls the respective compute managers (ie. your step #3 is incorrect since Juno).

Now, that HostState model is something kept in-memory and only refreshed when a new request comes in. That means that if you have two schedulers running separately (or when you have 2 concurrent requests coming in), then yes you could have race conditions.

That's not really a problem in general, because if your could is enough sized, it will go to the compute manager which will use a context manager called "instance_claim()" for ensuring that its *OWN* internal representation is correct (and that method is thread-safe in the context). If the scheduler decision was incorrect, then it raises an error which is catched by the compute manager which calls again the conductor to ask for a reschedule (by excluding the wrong host).

So, see, when we have races, we have retries (that's the 2PC I mentioned). That's not perfect, in particular when the cloud is a bit full, and that's why we're working towards resolving that thru multiple possibilities :

https://review.openstack.org/#/c/192260/7/doc/source/scheduler_evolution.rst,cm

To be honest, I don't see clear actionable items in your bug request. I'd rather propose you to join the scheduler meetings happening every Mondays at 1400 UTC if you wish to help us and contribute.