[openstack-dev] [Heat] Continue discussing multi-region orchestration

Zane Bitter zbitter at redhat.com
Tue Nov 19 21:38:23 UTC 2013

On 19/11/13 19:03, Clint Byrum wrote:
> Excerpts from Zane Bitter's message of 2013-11-15 12:41:53 -0800:
>> Good news, everyone! I have created the missing whiteboard diagram that
>> we all needed at the design summit:
>> https://wiki.openstack.org/wiki/Heat/Blueprints/Multi_Region_Support_for_Heat/The_Missing_Diagram
>> I've documented 5 possibilities. (1) is the current implementation,
>> which we agree we want to get away from. I strongly favour (2) for the
>> reasons listed. I don't think (3) has many friends. (4) seems to be
>> popular despite the obvious availability problem and doubts that it is
>> even feasible. Finally, I can save us all some time by simply stating
>> that I will -2 on sight any attempt to implement (5).
>> When we're discussing this, please mention explicitly the number of the
>> model you are talking about at any given time.
>> If you have a suggestion for a different model, make your own diagram!
>> jk, you can sketch it or something for me and I'll see if I can add it.
> Thanks for putting this together Zane. I just now got around to looking
> closely.
> Option 2 is good. I'd love for option 1 to be made automatic by making
> the client smarter, but parsing templates in the client will require
> some deep thought before we decide it is a good idea.
> I'd like to consider a 2a, which just has the same Heat engines the user
> is talking to being used to do the orchestration in whatever region
> they are in. I think that is actually the intention of the diagram,
> but it looks like there is a "special" one that talks to the engines
> that actually do the work.

Yes, I think you're saying the same thing as the diagram was intended to 
convey. (Each orange box is meant to be a Heat engine.)

So the user talks to the Heat endpoint in a region of their choice 
(doesn't matter which, they're all the same). When a OS::Heat::Stack 
resource has Region and/or Endpoint specified, it will use 
python-heatclient to connect to the appropriate engine for that nested 

> 2 may morph into 3 actually, if users don't like the nested stack
> requirement for 2, we can do the work to basically make the engine create
> a nested stack per region. So that makes 2 a stronger choice for first
> implementation.

Yeah, if you run your own standalone Heat engine, you've effectively 
created 3 with no code changes from 2 afaict. I wouldn't recommend that 
operators deploy this way though.

> 4 has an unstated pro, which is that attack surface is reduced. This
> makes more sense when you consider the TripleO case where you may want
> the undercloud (hardware cloud) to orchestrate things existing in the
> overcloud (vm cloud) but you don't want the overcloud administrators to
> be able to control your entire stack.
> Given CAP theorem, option 5, the global orchestrator, would be doable
> with not much change as long as partition tolerance were the bit we gave

Well, in the real world partitions _do_ happen, so the choice is to give 
up consistency, availability or both.

> up. We would just have to have a cross-region RPC bus and database. Of
> course, since regions are most likely to be partitioned, that is not
> really a good choice. Trading partition tolerance for consistency lands
> us in the complexity black hole. Trading out availability makes it no
> better than option 4.

Yep, exactly.


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