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What you describe is a spike. It's a grand plan, and you don't need
anyone's permission, so huzzah for the spike!

As far as what should be improved, I hear a lot that having multiple
schedulers does not scale well, so I'd suggest that as a primary target
(maybe measure the _current_ problem, and then set the target as a 10x
improvement over what we have now).

Things to consider while pushing on that goal:

* Do not backslide the resilience in the system. The code is just now
starting to be fault tolerant when talking to RabbitMQ, so make sure
to also consider how tolerant of failures this will be. Cassandra is
typically chosen for its resilience and performance, but Cassandra does
a neat trick in that clients can switch its CAP theorem profile from
Consistent and Available (but slow) to Available and Performant when
reading things. That might be useful in the context of trying to push
the performance _UP_ for schedulers, while not breaking anything else.

* Consider the cost of introducing a brand new technology into the
deployer space. If there _is_ a way to get the desired improvement with,
say, just MySQL and some clever sharding, then that might be a smaller
pill to swallow for deployers.

Anyway, I wish you well on this endeavor and hope to see your results
soon!

Excerpts from Ed Leafe's message of 2015-07-15 07:18:42 -0700:
> Hash: SHA512
> 
> Changing the architecture of a complex system such as Nova is never
> easy, even when we know that the design isn't working as well as we
> need it to. And it's even more frustrating because when the change is
> complete, it's hard to know if the improvement, if any, was worth it.
> 
> So I had an idea: what if we ran a test of that architecture change
> out-of-tree? In other words, create a separate deployment, and rip out
> the parts that don't work well, replacing them with an alternative
> design. There would be no Gerrit reviews or anything that would slow
> down the work or add load to the already overloaded reviewers. Then we
> could see if this modified system is a significant-enough improvement
> to justify investing the time in implementing it in-tree. And, of
> course, if the test doesn't show what was hoped for, it is scrapped
> and we start thinking anew.
> 
> The important part in this process is defining up front what level of
> improvement would be needed to make considering actually making such a
> change worthwhile, and what sort of tests would demonstrate whether or
> not whether this level was met. I'd like to discuss such an experiment
> next week at the Nova mid-cycle.
> 
> What I'd like to investigate is replacing the current design of having
> the compute nodes communicating with the scheduler via message queues.
> This design is overly complex and has several known scalability
> issues. My thought is to replace this with a Cassandra [1] backend.
> Compute nodes would update their state to Cassandra whenever they
> change, and that data would be read by the scheduler to make its host
> selection. When the scheduler chooses a host, it would post the claim
> to Cassandra wrapped in a lightweight transaction, which would ensure
> that no other scheduler has tried to claim those resources. When the
> host has built the requested VM, it will delete the claim and update
> Cassandra with its current state.
> 
> One main motivation for using Cassandra over the current design is
> that it will enable us to run multiple schedulers without increasing
> the raciness of the system. Another is that it will greatly simplify a
> lot of the internal plumbing we've set up to implement in Nova what we
> would get out of the box with Cassandra. A third is that if this
> proves to be a success, it would also be able to be used further down
> the road to simplify inter-cell communication (but this is getting
> ahead of ourselves...). I've worked with Cassandra before and it has
> been rock-solid to run and simple to set up. I've also had preliminary
> technical reviews with the engineers at DataStax [2], the company
> behind Cassandra, and they agreed that this was a good fit.
> 
> At this point I'm sure that most of you are filled with thoughts on
> how this won't work, or how much trouble it will be to switch, or how
> much more of a pain it will be, or how you hate non-relational DBs, or
> any of a zillion other negative thoughts. FWIW, I have them too. But
> instead of ranting, I would ask that we acknowledge for now that:
> 
> a) it will be disruptive and painful to switch something like this at
> this point in Nova's development
> b) it would have to provide *significant* improvement to make such a
> change worthwhile
> 
> So what I'm asking from all of you is to help define the second part:
> what we would want improved, and how to measure those benefits. In
> other words, what results would you have to see in order to make you
> reconsider your initial "nah, this'll never work" reaction, and start
> to think that this is will be a worthwhile change to make to Nova.
> 
> I'm also asking that you refrain from talking about why this can't
> work for now. I know it'll be difficult to do that, since nobody likes
> ranting about stuff more than I do, but right now it won't be helpful.
> There will be plenty of time for that later, assuming that this
> experiment yields anything worthwhile. Instead, think of the current
> pain points in the scheduler design, and what sort of improvement you
> would have to see in order to seriously consider undertaking this
> change to Nova.
> 
> I've gotten the OK from my management to pursue this, and several
> people in the community have expressed support for both the approach
> and the experiment, even though most don't have spare cycles to
> contribute. I'd love to have anyone who is interested become involved.
> 
> I hope that this will be a positive discussion at the Nova mid-cycle
> next week. I know it will be a lively one. :)
> 
> [1] http://cassandra.apache.org/
> [2] http://www.datastax.com/
>