Restrict Stack Update Scope

https://blueprints.launchpad.net/heat/+spec/stack-update-restrict

When updating a stack, there is currently no way to stop an update from destroying a given resource.

Problem description

Users can (and do) worry about stack update doing wonky things. The update-preview endpoint addresses this partially by showing what will probably happen. The limitation of the preview function is that resources can raise UpdateReplace exceptions at any time, making it impossible to be certain of the results of an update until it is performed.

Proposed change

Use the existing ‘update_policy’ resource attribute to let users protect certain resources from being replaced during updates.

If the update_policy can’t be satisfied, heat will move the stack to ‘UPDATE_FAILED’ and halt. If at all possible, constraints should be validated before applying the update, thus moving the stack straight to ‘UPDATE_FAILED’ when the update_policy is incorrect. After the update fails, the user can adjust the restrictions and try again.

The update_policy attribute is already used for CloudFormation autoscaling preferences, which are nested into the keys “AutoScalingScheduledAction” and “AutoScalingRollingUpdate”. CFN preferences would be unaffected by the HOT version of update policies.

A user would specify per-resource how aggressive an update can be with a resource. The restrictions could be on updating the resource at all, or just on destroying the resource (including UpdateReplace).

The base cases here are:

  • Restrict destroy/replace
  • Restrict nondestructive updates
  • Restrict both
  • Restrict nothing
  • Omit the update_policy entirely

The keys for these restrictions would be nested into an ‘actions’ key as below.

resources:
  myresource:
    type: Foo::Bar::Baz
    update_policy:
      allow:
        update: <bool>
        replace: <bool>

The reason for nesting the allowed actions is to avoid adding top level keys if there are more actions that users want to restrict in the future.

A user would be able to add or remove restrictions by updating the resource template. The new restrictions would be effective for the current update. For example, a resource that would otherwise be replaced would be protected if it had an update policy added in the current update.

Conflicting directives are possible, for example in nested stacks. If an inner resource has “replace: true” but the outer scope has “replace: false” then heat will transfer the stack to UPDATE_FAILED to surface the problem to the user.

Alternatives

An alternatives way to handle conflicting directives may be to honor the most conservative applicable policy. This method would be much more confusing for users, so failing the update would be preferable.

Pitfalls

Implementation

Assignee(s)

Milestones

Targeted for Kilo

Work Items

  • Add an actions key to update_policy

Dependencies

update-dry-run

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