Open Stack

I have a different point of view. First I will offer some assertions:

A-1) We need to keep it simple.
	A-1.1) Systems that are hard to comprehend are hard to debug, and that's bad.
	A-1.2) Complex systems tend to be much more brittle than simple ones.

A-2) Scale-up operations need to be as-fast-as-possible. 
	A-2.1) Auto-Scaling only works right if your new capacity is added quickly when your controller detects that you need more. If you spend a bunch of time goofing around before actually adding a new resource to a pool when its under staring. 
	A-2.2) The fewer network round trips between "add-more-resources-now" and "resources-added" the better. Fewer = less brittle.

A-3) The control logic for scaling different applications vary. 
	A-3.1) What metrics are watched may differ between various use cases. 
	A-3.2) The data types that represent sensor data may vary.
	A-3.3) The policy that's applied to the metrics (such as max, min, and cooldown period) vary between applications. Not only the values vary, but the logic itself.
	A-3.4) A scaling policy may not just be a handful of simple parameters. Ideally it allows configurable logic that the end-user can control to some extent.

A-4) Auto-scale operations are usually not orchestrations. They are usually simple linear workflows.
	A-4.1) The Taskflow project[1] offers a simple way to do workflows and stable state management that can be integrated directly into Autoscale.
	A-4.2) A task flow (workflow) can trigger a Heat orchestration if needed.

Now a mental tool to think about control policies:

Auto-scaling is like steering a car. The control policy says that you want to drive equally between the two lane lines, and that if you drift off center, you gradually correct back toward center again. If the road bends, you try to remain in your lane as the lane lines curve. You try not to weave around in your lane, and you try not to drift out of the lane.

If your controller notices that you are about to drift out of your lane because the road is starting to bend, and you are distracted, or your hands slip off the wheel, you might drift out of your lane into nearby traffic. That's why you don't want a Rube Goldberg Machine[2] between you and the steering wheel. See assertions A-1 and A-2.

If you are driving an 18-wheel tractor/trailer truck, steering is different than if you are driving a Fiat. You need to wait longer and steer toward the outside of curves so your trailer does not lag behind on the inside of the curve behind you as you correct for a bend in the road. When you are driving the Fiat, you may want to aim for the middle of the lane at all times, possibly even apexing bends to reduce your driving distance, which is actually the opposite of what truck drivers need to do. Control policies apply to other parts of driving too. I want a different policy for braking than I use for steering. On some vehicles I go through a gear shifting workflow, and on others I don't. See assertion A-3.

So, I don't intend to argue the technical minutia of each design point, but I challenge you to make sure that we (1) arrive at a simple system that any OpenStack user can comprehend, (2) responds quickly to alarm stimulus, (3) is unlikely to fail, (4) can be easily customized with user-supplied logic that controls how the scaling happens, and under what conditions.

It would be better if we could explain Autoscale like this:

Heat -> Autoscale -> Nova, etc.
-or-
User -> Autoscale -> Nova, etc.

This approach allows use cases where (for whatever reason) the end user does not want to use Heat at all, but still wants something simple to be auto-scaled for them. Nobody would be scratching their heads wondering why things are going in circles.

>From an implementation perspective, that means the auto-scale service needs at least a simple linear workflow capability in it that may trigger a Heat orchestration if there is a good reason for it. This way, the typical use cases don't have anything resembling circular dependencies. The source of truth for how many members are currently in an Autoscaling group should be the Autoscale service, not in the Heat database. If you want to expose that in list-stack-resources output, then cause Heat to call out to the Autoscale service to fetch that figure as needed. It is irrelevant to orchestration. Code does not need to be duplicated. Both Autoscale and Heat can use the same exact source code files for the code that launches/terminates instances of resources.

References:
[1] https://wiki.openstack.org/wiki/TaskFlow
[2] http://en.wikipedia.org/wiki/Rube_Goldberg_machine

Thanks,

Adrian


On Aug 16, 2013, at 11:36 AM, Zane Bitter <zbitter at redhat.com> wrote:

> On 16/08/13 00:50, Christopher Armstrong wrote:
>> *Introduction and Requirements*
>> 
>> So there's kind of a perfect storm happening around autoscaling in Heat
>> right now. It's making it really hard to figure out how I should compose
>> this email. There are a lot of different requirements, a lot of
>> different cool ideas, and a lot of projects that want to take advantage
>> of autoscaling in one way or another: Trove, OpenShift, TripleO, just to
>> name a few...
>> 
>> I'll try to list the requirements from various people/projects that may
>> be relevant to autoscaling or scaling in general.
>> 
>> 1. Some users want a service like Amazon's Auto Scaling or Rackspace's
>> Otter -- a simple API that doesn't really involve orchestration.
>> 2. If such a API exists, it makes sense for Heat to take advantage of
>> its functionality instead of reimplementing it.
> 
> +1, obviously. But the other half of the story is that the API is likely be implemented using Heat on the back end, amongst other reasons because that implementation already exists. (As you know, since you wrote it ;)
> 
> So, just as we will have an RDS resource in Heat that calls Trove, and Trove will use Heat for orchestration:
> 
>  user => [Heat =>] Trove => Heat => Nova
> 
> there will be a similar workflow for Autoscaling:
> 
>  user => [Heat =>] Autoscaling -> Heat => Nova
> 
> where the first, optional, Heat stack contains the RDS/Autoscaling resource and the backend Heat stack contains the actual Nova instance(s).
> 
> One difference might be that the Autoscaling -> Heat step need not happen via the public ReST API. Since both are part of the Heat project, I think it would also be OK to do this over RPC only.
> 
>> 3. If Heat integrates with that separate API, however, that API will
>> need two ways to do its work:
> 
> Wut?
> 
>>    1. native instance-launching functionality, for the "simple" use
> 
> This is just the simplest possible case of 3.2. Why would we maintain a completely different implementation?
> 
>>    2. a way to talk back to Heat to perform orchestration-aware scaling
>> operations.
> 
> [IRC discussions clarified this to mean scaling arbitrary resource types, rather than just Nova servers.]
> 
>> 4. There may be things that are different than AWS::EC2::Instance that
>> we would want to scale (I have personally been playing around with the
>> concept of a ResourceGroup, which would maintain a nested stack of
>> resources based on an arbitrary template snippet).
>> 5. Some people would like to be able to perform manual operations on an
>> instance group -- such as Clint Byrum's recent example of "remove
>> instance 4 from resource group A".
>> 
>> Please chime in with your additional requirements if you have any! Trove
>> and TripleO people, I'm looking at you :-)
>> 
>> 
>> *TL;DR*
>> 
>> Point 3.2. above is the main point of this email: exactly how should the
>> autoscaling API talk back to Heat to tell it to add more instances? I
>> included the other points so that we keep them in mind while considering
>> a solution.
>> 
>> *Possible Solutions*
>> 
>> I have heard at least three possibilities so far:
>> 
>> 1. the autoscaling API should maintain a full template of all the nodes
>> in the autoscaled nested stack, manipulate it locally when it wants to
>> add or remove instances, and post an update-stack to the nested-stack
>> associated with the InstanceGroup.
> 
> This is what I had been thinking.
> 
>> Pros: It doesn't require any changes to Heat.
>> 
>> Cons: It puts a lot of burden of state management on the autoscale API,
> 
> All other APIs need to manage state too, I don't really have a problem with that. It already has to handle e.g. the cooldown state; your scaling strategy (uh, for the service) will be determined by that.
> 
>> and it arguably spreads out the responsibility of "orchestration" to the
>> autoscale API.
> 
> Another line of argument would be that this is not true by definition ;)
> 
>> Also arguable is that automated agents outside of Heat
>> shouldn't be managing an "internal" template, which are typically
>> developed by devops people and kept in version control.
>> 
>> 2. There should be a new custom-built API for doing exactly what the
>> autoscaling service needs on an InstanceGroup, named something
>> unashamedly specific -- like "instance-group-adjust".
> 
> +1 to having a custom (RPC-only) API if it means forcing some state out of the autoscaling service.
> 
> -1 for it talking to an InstanceGroup - that just brings back all our old problems about having "resources" that don't have their own separate state and APIs, but just exist inside of Heat plugins. Those are the cause of all of the biggest design problems in Heat. They're the thing I want the Autoscaling API to get rid of. (Also, see below.)
> 
>> Pros: It'll do exactly what it needs to do for this use case; very
>> little state management in autoscale API; it lets Heat do all the
>> orchestration and only give very specific delegation to the external
>> autoscale API.
>> 
>> Cons: The API grows an additional method for a specific use case.
>> 
>> 3. the autoscaling API should update the "Size" Property of the
>> InstanceGroup resource in the stack that it is placed in. This would
>> require the ability to PATCH a specific piece of a template (an
>> operation isomorphic to update-stack).
> 
> -1
> 
> Half the point of a separate autoscaling API is that an autoscaling group shouldn't _need_ to exist only in the context of a template.
> 
> So, to get around this I think you're proposing something where the two ways of interacting with Autoscaling are (with and without Heat):
> 
>  user => Heat <=> Autoscaling
>            \
>             ------------------> Heat => Nova
> 
>  user => Autoscaling => Heat -> Heat => Nova
> 
> Instead of:
> 
>  user => Heat => Autoscaling => Heat => Nova
> 
>  user =========> Autoscaling => Heat => Nova
> 
> 
> This is two lots of code to write & test and, from the user's point of view, a bizarre and unprecedented conceptual inversion of where Orchestration belongs in the system. (Note that the backend Heat is an implementation detail hidden from the user in either case.)
> 
> cheers,
> Zane.
> 
>> Pros: The API modification is generic, simply a more optimized version
>> of update-stack; very little state management required in autoscale API.
>> 
>> Cons: This would essentially require manipulating the user-provided
>> template.  (unless we have a concept of "private properties", which
>> perhaps wouldn't appear in the template as provided by the user, but can
>> be manipulated with such an update stack operation?)
>> 
>> 
>> *Addenda*
>> 
>> Keep in mind that there are use cases which require other types of
>> manipulation of the InstanceGroup -- not just the autoscaling API. For
>> example, see Clint's #5 above.
>> 
>> 
>> Also, about implementation: Andrew Plunk and I have begun work on Heat
>> resources for Rackspace's Otter, which I think will be a really good
>> proof of concept for how this stuff should work in the Heat-native
>> autoscale API. I am trying to gradually work the design into the native
>> Heat autoscaling design, and we will need to solve the
>> autoscale-controlling-InstanceGroup issue soon.
>> 
>> --
>> IRC: radix
>> Christopher Armstrong
>> Rackspace
>> 
>> 
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> 
> 
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