[openstack-dev] [Heat] Multi region support for Heat

Bartosz Górski bartosz.gorski at ntti3.com
Mon Jul 29 15:16:09 UTC 2013

On 07/29/2013 09:51 AM, Zane Bitter wrote:
> On 29/07/13 02:04, Angus Salkeld wrote:
>> On 26/07/13 09:43 -0700, Clint Byrum wrote:
>>> Excerpts from Zane Bitter's message of 2013-07-26 06:37:09 -0700:
>>>> On 25/07/13 19:07, Bartosz Górski wrote:
>>>> > We want to start from something simple. At the beginning we are
>>>> assuming
>>>> > no dependencies between resources from different region. Our 
>>>> first use
>>>> > case (the one on the wikipage) uses this assumptions. So this is
>>>> why it
>>>> > can be easily split on two separate single region templates.
>>>> >
>>>> > Our goal is to support dependencies between resources from different
>>>> > regions. Our second use case (I will add it with more details to the
>>>> > wikipage soon) is similar to deploying two instances (app server 
>>>> + db
>>>> > server) wordpress in two different regions (app server in the first
>>>> > region and db server in the second). Regions will be connected to 
>>>> each
>>>> > other via VPN connection . In this case configuration of app server
>>>> > depends on db server. We need to know IP address of created DB
>>>> server to
>>>> > properly configure App server. It forces us to wait with creating 
>>>> app
>>>> > server until db server will be created.
>>>> That's still a fairly simple case that could be handled by a pair of
>>>> OS::Heat::Stack resources (one provides a DBServerIP output it is 
>>>> passed
>>>> as a parameter to the other region using {'Fn::GetAtt':
>>>> ['FirstRegionStack', 'Outputs.DBServerIP']}. But it's possible to
>>>> imagine circumstances where that approach is at least suboptimal (e.g.
>>>> when creating the actual DB server is comparatively quick, but we have
>>>> to wait for the entire template, which might be slow).
>> How about we add an actual heat resource?
>> So you could aggregate stacks.
>> We kinda have one with "OS::Heat::Stack", but it doesn't use
> (aside: this doesn't actually exist yet, we only have 
> AWS::CloudFormation::Stack at present.)
>> python-heatclient. We could solve this by adding an "endpoint"
>>   property to the "OS::Heat::Stack" resource. Then if it is not
>> local then it uses python-heatclient to create the nested stack
>> remotely.
> Yes, that's what I was trying (and failing) to suggest.
+1. I think it is really good idea and I still like Steve's context 
concept. We should use both of them.
So it should be possible to create context as a resource and for each 
nested stack specify context.
>> Just a thought.
>> -Angus
>>> If you break that stack up into two stacks, db and "other slow stuff"
>>> then you can get the Output of the db stack earlier, so that is a
>>> solvable problem.
> +1
>>>> > More complicated use case with load balancers and more regions are
>>>> also
>>>> > in ours minds.
>>>> Good to know, thanks. I'll look forward to reading more about it on 
>>>> the
>>>> wiki.
>>>> What I'd like to avoid is a situation where anything _appears_ to be
>>>> possible (Nova server and Cinder volume in different regions? Sure!
>>>> Connect 'em together? Sure!), and the user only finds out later 
>>>> that it
>>>> doesn't work. It would be much better to structure the templates in 
>>>> such
>>>> a way that only things that are legitimate are expressible. That's not
>>>> an achievable goal, but IMO we want to be much closer to the latter 
>>>> than
>>>> the former.
>>> These are all predictable limitations and can be handled at the parsing
>>> level.  You will know as soon as you have template + params whether or
>>> not that cinder volume in region A can be attached to the nova server
>>> in region B.
> That's true, but IMO it's even better if it's obvious at the time you 
> are writing the template. e.g. if (as is currently the case) there is 
> no mechanism within a template to select a region for each resource, 
> then it's obvious you have to write separate templates for each region 
> (and combine them somehow).
I agree with you. It will be more obvious from the beginning what is 
possible and what is not.
>>> I'm still convinced that none of this matters if you rely on a single
>>> Heat
>>> in one of the regions. The whole point of multi region is to eliminate
>>> a SPOF.
> So the idea here would be that you spin up a master template in one 
> region, and this would contain OS::Heat::Stack resources that use 
> python-heatclient to connect to Heat APIs in other regions to spin up 
> the constituent stacks in each region. If a region goes down, even if 
> it is the one with your master template, that's no problem because you 
> can still interact with the constituent stacks directly in whatever 
> region(s) you _can_ reach.
> So it solves the non-obviousness problem and the 
> single-point-of-failure problem in one fell swoop. The question for me 
> is whether there are legitimate use cases that this would shut out.
I want to be sure that we are on the same page. By master template you 
mean a template that has only nested stacks as resources? Or also other 
types of resources (like single server) which will be created in the 
region where heat engine is located to which the master template was 
sent? I think it would be great if master template contains only of 
nested stacks as resources. For each nested stack a context is specified 
(tenant/project, endpoint/region, etc.) and heat uses python-client to 
create all of them (even for those from the same region where heat 
engine is located). In this situation it will be possible to try delete 
created multi region stack using different heat engine (located in 
different region).
> cheers,
> Zane.
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