Autoscaling Group Rolling Update Hooks

Hooks are included in Kilo, but don’t address a helpful use for hooks. Currently, there are pre-create and pre-update hooks, but this would add a special hook to OS::Heat::AutoScalingGroup to set a hook before each batch in a rolling update.

Problem description

Working with TripleO it’s often desirable to pause to check that an update is doing what you want (which is why hooks exist at all), and the “pause_time” provided by the rolling_updates policy can be used for a similar purpose.

The problem is that you may not be able to sufficiently test the result of a rolling update batch within the pause_time set, but you don’t have a way to signal to heat to add more time. Or the pause_time may be excessively long, making the update time too slow.

Being able to set a breakpoint between batches is a much better solution, so the operator can take an arbitrarily long (up to stack timeout) or short time to confirm the upgrade went as planned.

Proposed change

To make debugging and verifying rolling updates easier, I propose adding a ‘batch_hook’ parameter to rolling_updates like below.:

my_asg:
  type: "OS::Heat::AutoScalingGroup"
  properties:
    desired_capacity: 4
    ...
    rolling_updates:
      batch_hook: true
      min_in_service: 1
      ...

The batch_hook option and pause_time will be mutually exclusive, since it doesn’t make much sense to have both a set pause time and hooks between batches.

This will be confined to AutoScalingGroup, won’t break any existing templates, and won’t affect other grouped resources.

Alternatives

1. The name “batch_hook” seems descriptive enough for me, but another option for the parameter name would be “pre_batch_hook” to denote that the hook is set before each batch (not after).

2. Another alternative would be to add a hook type that would be set by in the environment, not in the rolling_update policy. I think localizing this to the AutoScalingGroup scaling policy is a better choice and use stack updates to toggle the hooks for each group.

Implementation

Assignee(s)

Primary assignee:

sb~p (Ryan Brown)

Milestones

liberty-1

Work Items

Dependencies

None