Reorg AutoScalingGroup Implementation

Include the URL of your launchpad blueprint:

This specs is about reorganize AutoScalingGroup implementation which includes the following resource types:

  • AWS::AutoScaling::LaunchConfiguration
  • AWS::AutoScaling::AutoScalingGroup
  • AWS::AutoScaling::ScalingPolicy
  • OS::Heat::InstanceGroup
  • OS::Heat::AutoScalingGroup
  • OS::Heat::ScalingPolicy
  • OS::Heat::ResourceGroup

The goal is to 1) reorganize the class hierarchy; 2) split and relocate sources into subdirectories to better reflect resources’ name space; 3) make it easier for future enhancements to each resource types.

Problem description

The current class hierarchy of resource groups and scaling groups is something like the diagram shown below:

 | StackResource
 |   |
 |   +--> ResourceGroup [OS::Heat::ResourceGroup]
 |   |
 |   +--> InstanceGroup [OS::Heat::InstanceGroup]
 |         |
 +---------+--> AutoScalingGroup [AWS::AutoScaling::AutoScalingGroup]
 |                |
 |                +--> AutoScalingResourceGroup [OS::Heat:AutoScalingGroup]
 | SignalResponder
 |   |
 +---+--> ScalingPolicy [AWS::AutoScaling::ScalingPolicy]
            +--> AutoScalingPolicy [OS::Heat::ScalingPolicy]

Besides this hierarchy, there are utility functions located in the modules like heat.scaling.template.

One of the problems of this design is related to namespace as pointed out by an existing blueprint:

Another problem is that having all classes implemented in almost one file is making the implementation difficult to digest or improve. For example, it may make a better sense to have InstanceGroup a subclass of ResourceGroup. For another example, it doesn’t make much sense to have AutoScalingResourceGroupa subclass of InstanceGroup because the subclass is more open to other resource types as its members.

Proposed change

  1. Reorganize Class Hierarchy

The proposed change is to reorganize the class hierarchy to be something like shown in the diagram below:

 |                           StackResource
 |                                  |
 |                            ResourceGroup
 |                       [OS::Heat::ResourceGroup]
 |                                  |
 |              +-------------------+---------------+
 |              |                                   |
 +-->    AutoScalingGroup                      InstanceGroup
 | [OS::Heat::AutoScalingGroup]          [OS::Heat::InstanceGroup]
 |                                                  |
 |                                                  |
 +---------------------------------------> AWSAutoScalingGroup
 |                                 [AWS::AutoScaling:AutoScalingGroup]
 |                 SignalResponder
 |                        |
              |                               |
      AWSAutoScalingPolicy             AutoScalingPolicy
 [AWS::AutoScaling::ScalingPolicy]  [OS::Heat::ScalingPolicy]

This change will break the subclass relationships between OpenStack and AWS implementation.

As for utility/helper classes, e.g. CooldownMixin, the first step is to separate them into independent classes, followed by further refactoring them into utility functions when appropriate.

  1. Relocate Source Files

The AWS version will be relocated into heat/engine/resources/aws subdirectory, including the LaunchConfiguration implementation. The OpenStack version will be relocated into heat/engine/resources/openstack subdirectory.

The shared parent class ResourceGroup will remain in heat/engine/resources, while the CooldownMixin class will be relocated into heat/scaling subdirectory. The eventual layout of the modules and classes would look like this:

  +--  [ResourceGroup]
  +-- [InstanceGroup]
  +-- aws/
  |    |
  |    +--- [AWSAutoScalingGroup]
  |    +--- [AWSAutoScalingPolicy]
  |    +--- [LaunchConfiguration]
  +-- openstack/
       +-- [AutoScalingGroup]
       +-- [AutoScalingPolicy]

  +-- [CooldownMixin]
  +-- (possibily other shared utility classes)

This reshuffling is optional. We will determine whether reshuffling is necessary indeed after the cleanup work is done.


Since this is a pure implementation level change, one rule of thumb is that “we don’t break userland”.

We can have AWS AutoScalingPolicy extend Heat AutoScalingPolicy. However that may mean that any future changes to Heat implementation must be very careful, in case those changes may break the conformance of the AWS version to its Amazon specification.

The same applies to the two versions of AutoScalingGroup. Hopefully, we may extract common code into ResourceGroup level to minimize code duplication. However, having a subclass relationship between these two classes is not a good design in the long term. The goal of the AWS version is to closely follow the Amazon development while the goal of the Heat version is more about user needs in the context of OpenStack. So the current thought is to split the implmentation although it may imply some code duplication.



Primary assignee:

There could be other contributors interested in helping out as well.


Target Milestone for completion:

Work Items

  • Extract common code to parent classes
  • Split AWS version and OS version of resources
  • Modify test cases


No new dependencies to other libraries will be introduced.

This work may disturb several on-going work related to AutoScalingGroups. The following work will have to be rebased on this change.

  1. Scaling group scale-down plugpoint
  2. LaunchConfiguration bdm
  3. Balancing ScalingGroup across AZs

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