Open Stack

On 19/06/13 06:01, Adrian Otto wrote:
> Angus,
>
> I'm glad you are asking good questions. I have additional input for you to consider below.
>
> On Jun 18, 2013, at 6:09 PM, Angus Salkeld <asalkeld at redhat.com> wrote:
>
>> On 18/06/13 23:32 +0000, Adrian Otto wrote:
>>> Yes. I think having a POST method in the API makes perfect sense. Assuming we reach agreement on that, the next question that comes up is:
>>
>> Err, I am not convinced.
>>
>> Before that, I think it's worth highlighting the different proposals/requirements here:
>>
>> 1) heat needs a way to setup an autoscaling group
>> 2) autoscaling needs a way of telling heat to scale up/down
>> 3) People might want to integrate other orchestration engines with Heat
>>
>> So one by one:
>> 1) autoscaling has a post api that the Heat autoscaling group resource
>>    posts to, Heat will provide a webhook in the event of a scaling action.
>> 2) when autoscaling determines that there should be a scaling action
>>    it calls the webhook. The reason I suggest a webhook is in-instance
>>    applications might want to scale their applications (like
>>    openshift-gears) - so don't assume a heat endpoint.
>>    Also we can have this as an action not a resource-create.
>>    PUT /$tenant/stacks/$stack/resources/<autoscale-group>
>>    (with an action of scale-up/down)
>>
>> I think we should discourage (make it impossible) for users from modifing
>> stacks outside of stack-update.
>>
>> To me one of the most powerful and apealing things of Heat is the
>> ability to reproducibly re-create a stack from a template. This
>> new public API is going to make this difficult.
>
> Adding an API only makes it difficult if you decide to bypass templates and use the API. You can still be disciplined and keep your templates updated to achieve the reproducible goal. Yes, and API of this sort is a sharp instrument, but it can be useful if applied properly.

A giant -2 to that. We should be designing technology in such a way as 
to guide users into using it in ways that meet their goals, not painting 
a target on their foot, handing them a shotgun and telling them to go 
forth and "be disciplined". In my considered opinion this is one of the 
core responsibilities we, as engineers, have to society.
>
> The ability to quickly and easily add or modify a resource in a stack/assembly is very useful, particularly in cases where an orchestration was partially successful, but gets stuck. Imagine if it's something that takes 4 hours to conduct from beginning to end, and you got 3.8 hours into it when something jammed. It would be *really* handy to be able to un-jam it without starting over from scratch. You'd still need to fix your template if you wanted to tweak to persist for the next time you use the template.

That use case is a genuine problem, and Clint has already suggested 
(and I support) allowing retries to help mitigate it. There may be other 
things we can do too.

I'm not sure if it's possible to write an orchestration system that is 
robust in the face of users changing stuff in the middle of a 
deployment, but I am pretty sure I am not smart enough to write it and I 
am completely sure that Heat is a million miles from ever being that system.
>
> It's also useful for situations like rolling updates, for when a complete redeployment is not practical, affordable, or desirable. Think of this as the point of intersection between a configuration management system and the orchestration system. Yes, you can accomplish this with stack-update, but that could be rather awkward or impractical depending on the size of the stack/assembly. Imagine something with 2000 nodes in it… perhaps a large scale triple-o use case. Sometimes groupings are sufficient, and sometimes they are not.

I don't understand this. Stack updates do not do a complete 
redeployment. Either you want to change something on all 2000 nodes, in 
which case doing it manually doesn't buy you anything, or you don't, in 
which case the number of unchanged nodes is irrelevant.
>
>> Also creating a resource is not a trivial issue, the user would have
>> create the resources in the correct order (with correct inter-resource
>> references etc..) mostly throwing away the point of orchestation in
>> the first place. If you are doing this you may as well talk directly
>> to nova/cinder/networking directly.
>
> Using the low level API's would bypass the workflow features of Heat, which we plan to contribute to considerably. We don't want to do that. We want to leverage those workflow features. There is value in pulling strings through Heat, even if all you are doing is adding a node. There is value to having an expression of the complete deployment, and all of the related resources that's consistent with what's happened with the lower level APIs (such as nova/cinder/networking). That value does not diminish when you have a need to surgically alter a stack.

Stack update can already surgically alter a stack. The difference 
between stack update and this proposal is that stack updates can be 
orchestrated in a robust manner, and this proposal pushes all of the 
orchestration work onto the user (who has the added handicap of not 
being able to see what else is going on).

cheers,
Zane.
>
>>> How to do you modify resources that have been created with a POST?
>>>
>>> You mention HTTP PUT as an answer to that. Unfortunately PUT is only really useful for doing a full resource replacement, not just tweaking something that's already there. For that, you really want HTTP PATCH (http://tools.ietf.org/html/rfc5789). You can make this really elegant for JSON with JSON Patch (http://tools.ietf.org/html/draft-ietf-appsawg-json-patch-10).
>>>
>>> We should note that offering API methods to adjust a stack (aka: assembly) means that there will be a divergence between what's described in the original template, and the actual running state of the stack/assembly created by the template, well beyond the results of an autoscale policy. In fact, it would be possible to build a stack/assembly with no template at all, if the right API methods are present. There are good use cases for this, particularly for higher level compatibility layers where it would be awkward to generate permutations of templates to immediately feed into an API, rather than just use an API method for adjusting the stack/assembly in place. it would be much more elegant, for example, to implement a CAMP implementation on top of Heat if Heat had a REST API for creating and managing individual resources within a stack/assembly. This same argument applies to integrating any other orchestration or configuration management system with Heat.
>>
>> So this is "3)":
>> As I was alluding to above, if you are integrating at this level what
>> value is Heat providing to you? If you are just using it for resource
>> create this is a thin layer over the python clients. I'd suggest just
>> using the resources as python plugins. I think exposing this would
>> bring more harm than good.
>
> The value is in the expression of the stack/assembly for the purposes of system management, task-flow/workflow, application lifecycle management, and (future) the ability to manage a multi-cloud deployment with a single point of control.
>
> Yes, sharp instruments can be dangerous. They can also be very handy when you need one.
>
> Thanks,
>
> Adrian