[openstack-dev] Moving task flow to conductor - concern about scale

Dan Smith dms at danplanet.com
Fri Jul 19 14:15:22 UTC 2013

> There's nothing I've seen so far that causes me alarm,  but then
> again we're in the very early stages and haven't moved anything
> really complex.

The migrations (live, cold, and resize) are moving there now. These are
some of the more complex stateful operations I would expect conductor
to manage in the near term, and maybe ever.

> I just don't buy into this line of thinking - I need more than one
> API node for HA as well - but that doesn't mean that therefore I want
> to put anything else that needs more than one node in there.
> I don't even think these do scale-with-compute in the same way;  DB
> proxy scales with the number of compute hosts because each new host
> introduces an amount of DB load though its periodic tasks.    Task

> to create / modify servers - and that's not directly related to the
> number of hosts.     

Unlike API, the only incoming requests that generate load for the
conductor are things like migrations, which also generate database

> So rather than asking "what doesn't work / might not work in the
> future" I think the question should be "aside from them both being
> things that could be described as a conductor - what's the
> architectural reason for wanting to have these two separate groups of
> functionality in the same service ?"

IMHO, the architectural reason is "lack of proliferation of services and
the added complexity that comes with it." If one expects the
proxy workload to always overshadow the task workload, then making
these two things a single service makes things a lot simpler.

> If they were separate services and it turns out that I can/want/need
> to run the same number of both then I can pretty easily do that  -
> but the current approach is removing what to be seems a very
> important degree of freedom around deployment on a large scale system.

I guess the question, then, is whether other folks agree that the
scaling-separately problem is concerning enough to justify at least an
RPC topic split now which would enable the services to be separated
later if need be.

I would like to point out, however, that the functions are being split
into different interfaces currently. While that doesn't reach low
enough on the stack to allow hosting them in two different places, it
does provide organization such that if we later needed to split them, it
would be a relatively simple (hah) matter of coordinating an RPC
upgrade like anything else.


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