[openstack-dev] [heat] autoscaling and load balancers

Miguel Grinberg miguel.s.grinberg at gmail.com
Wed Apr 8 22:03:12 UTC 2015

Zane, replies inline.

On Wed, Apr 8, 2015 at 3:46 PM, Zane Bitter <zbitter at redhat.com> wrote:

> On 07/04/15 22:02, Miguel Grinberg wrote:
>> Hi,
>> The OS::Heat::AutoScalingGroup resource is somewhat limited at this
>> time, because when a scaling even occurs it does not notify dependent
>> resources, such as a load balancer, that the pool of instances has
>> changed.
> As Thomas mentioned, the 'approved' way to solve this is to make your
> scaled unit a stack, and include a Neutron PoolMember resource in it.

LBAAS is an optional, now even external component, not part of the Neutron
API. Many installations don't have it. Allowing the use of custom load
balancers is a desirable option, in my opinion, more so while LBAAS is not
core neutron functionality.

>  The AWS::AutoScaling::AutoScalingGroup resource, on the other side, has
>> a LoadBalancerNames property that takes a list of
>> AWS::ElasticLoadBalancing::LoadBalancer resources that get updated
>> anytime the size of the ASG changes.
> Which is an appalling hack.
> Yes. This is hacky, but it seems it models the AWS load balancing APIs, so
there isn't much that can be done here, right?

> (If it called the Neutron LBaaS API, like the equivalent in CloudFormation
> does with ELB, it would be OK. But in reality, as you know, it's a hack
> that makes calls directly to another resource plugin within Heat.)
>  I'm trying to implement this notification mechanism for HOT templates,
>> but there are a few aspects that I hope to do better.
>> 1. A HOT template can have get_attr function calls that invoke
>> attributes of the ASG. None of these update when the ASG resizes at this
>> time, a scaling even does a partial update that only affects the ASG
>> resource. I would like to address this.
> In the medium-term I think this is something that I believe Convergence
> will be able to solve for us. I'm not sure that it's worth putting in a
> short-term work-around for.

Here is where we disagree. In my opinion this is broken functionality.
After a scaling event there are resources that go stale because they are
never told that the ASG resized. This to me is clearly a bug that deserves
fixing, even if in the future a better/nicer fix can be crafted.

>  2. The AWS solution relies on the well known LoadBalancer resource, but
>> often load balancers are just regular instances that get loaded with a
>> load balancer such as haproxy in a custom way. I'd like custom load
>> balancers to also update when the ASG resizes.
> TBH the correct answer for load balancers specifically is use the Neutron
> LBaaS API, end of story.

This does not help me, as I don't have LBAAS. But as a said above, even if
I had it, I may want to use my own load balancer, why not let me use my own
if that is what I need for my project? Or what if I had another resource
type that is not a load balancer, maybe a custom resource from a plugin
that wants to be notified when the ASG resizes? If this can be done for
regular stack updates, my opinion is that it should also work for these
special signal-triggered updates to the ASG.

> But you're right that there are many uses for a more generic notification
> mechanism. (For example, in OpenShift we need to notify the controller when
> we add or remove nodes.) The design goal for ASG was always that we would
> have an arbitrary scaled unit (defined by a template) and an arbitrary
> non-scaled unit that could receive notifications about changes to the
> scaling group. So far we have delivered on only the first part of that
> promise.
> My vision for the second part has always been that we'd use hooks, the
> initial implementation of which Tomas has landed in Kilo. We'll need to
> implement some more hook types to do it - post-create, post-update and
> pre-delete at a minimum. We also need some way of notifying the user
> asynchronously about when the hooks are triggered, so that they can take
> whatever action (e.g. add to load balancer) before calling the API to clear
> the hook. (At the moment the only way to find out when your hook should run
> is by polling the Heat API.)

I'm not really sure I understand how this would work. If I have a resource
that sets one of its properties to { get_attr: [my_asg, size] }, then on a
stack-update I don't need a hook to update my resource, it automatically
updates. On an alarm triggered resize it will not, only because the update
is partial in that case. If I add a post-update hook to that, then I may be
able to get the resource to update on a resize event, but on a regular
stack-update now the update will happen twice, once due to the normal
update process, then again with the hook.

To make this work I would have to not use get_attr, and somehow get this
resource to obtain whatever attribute it needs from the ASG using some
other way, like maybe the Heat API. Which is all fine, but get_attr is a
valid option I have as a stack developer, and it is currently broken.

I know you disagree with my view, but in my opinion the problem, as I
mentioned before, is that the resize event of the ASG does a partial
update, which leaves the stack in an inconsistent state.

> In my ideal world, the notification mechanism (or at least one of them) is
> a message to a Zaqar queue/topic (whatever you want to call it) specified
> by the user. So someone e.g. running their own HAProxy (don't do this ;)
> could put a little micro-daemon on the same box that listened to Zaqar for
> notifications and update the HAProxy config appropriately.
> Also in my ideal world, a Mistral workflow could be triggered (and seeded
> with parameter values) by the exact same message queue, so that the user
> can run any action that Mistral can support without having to have a server
> around to run it. And we'd use the same system for e.g. Ceilometer alarms
> talking to scaling policies, so that one could also insert a Mistral
> workflow into the process. Things are actually pretty awesome in my ideal
> world.

I really have no objection to this, sounds pretty good and I would likely
use it when it is available. But this is future looking, and I'm trying to
address a very specific problem in current releases.

>  The ResourceGroup is an interesting resource. It is much simpler than
>> the ASG. In particular, the only way to scale the ResourceGroup is by
>> issuing a stack-update with a new size. This indirectly solves #1 and #2
>> above, because when a full update is issued any references to the
>> ResourceGroup get updated as well.
> It doesn't really solve the problem, because you could still manually
> update the nested stack that the ResourceGroup manages. It just entirely
> lacks the feature that makes it easy to run in to the problem. And not in a
> good way.
Not sure I understand this. You have a list of nested stacks, as many as
the size property of the resource group dictates. You can update them and
that's fine. I guess you can delete one and that is probably not fine, in
the same way you can delete instances from the ASG pool without the ASG
resource knowing, or actually modify or delete any native entities without
the heat resource that owns it knowing. That still does not cancel the fact
that if you play by the rules, the ResourceGroup is much more reliable than
the ASG because it can only be updated in a stack-update operation.

>  In my opinion, the best way to address #1 and #2 above so that they work
>> for the ASG as they work for the RG, is to change what happens when
>> there is a scaling event. When the ScalingPolicy resource gets a signal,
>> it reaches directly to the ASG by calling asg.adjust() (or in the near
>> future by sending a signal to it, when a currently proposed patch
>> merges) with the new size. This bypasses the update mechanism, so only a
>> partial update occurs, just the ASG resource itself is updated. I would
>> like this to be a full stack update, so that all references get updated
>> with the new ASG size. This will address #1 and #2.
> -1

This I disagree with. The partial update leaves the stack in an
inconsistent state. It's a bug that should be straightforward to fix,
without altering any plans for the future that can make the use of load
balancers more friendly to users.

> The way to think about autoscaling is as a separate service that delegates
> the creation and deletion of its members to and maintains its state in a
> Heat stack. It *isn't* of course, but nor will it ever be if people
> continue to think about it as a resource plugin that is free to reach in to
> its parent stack and start messing with other things.
> Apart from being a layering violation, anything that relies on updating
> the parent stack *after* a scaling operation is complete simply doesn't
> work. When scaling down, you want the changes to be made *before* updating
> the scaling group. In the general case - a batched rolling update - there
> are multiple changes that need to be made mostly *during* the scaling group
> update.
>  But there is an alternative to this. I guess we could copy the update
>> mechanism used on the AWS side, which is also partial, but at least
> -2! This is what we most wanted to avoid in the native resources.

I'm fine with this, I don't really like the solution myself that much.

>  covers the load balancers, given in the LoadBalancerNames property. We
>> can have a "load_balancer_names" equivalent property for the
>> OS::Heat::ASG resource, and we can then trigger the updates of the load
>> balancer(s) exactly like the AWS side does it. For this option, I would
>> like to extend the load balancer update mechanism to work on custom load
>> balancers, as it currently works with the well known load balancer
>> resources. I have implemented this approach and is currently up for
>> review: https://review.openstack.org/#/c/170634/. I honestly prefer the
>> full update, seems cleaner to me.
>> Anyway, sorry for the long email. If you can provide guidance on which
>> of the approaches are preferred, or if you have other ideas, I would
>> appreciate it.
> Long emails are good, thanks for writing this up :)
> cheers,
> Zane.
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