SSA dryrun detects changes of other controllers

[sbueringer]

I played a bit around with SSA and took a closer look at dryrun.

It looks like SSA dryrun currently detects changes made by other controllers and thus triggers SSA even when it's not necessary.

Example:

• Topology Controller: Reconcile: getCurrentState (original)
• KCP controller: takes ownership of additional conditions (managed fields of KCP change)
• Topology Controller: Reconcile: reconcileState (original=>modified)
  • dryrun SSA with modified (picks up changes made by other controllers (i.e. managed field change)
  • diff between original and modified => contains diff made by other controllers (i.e. managedField changes for condition ownership)

I think we should reduce the cases where this can happen to avoid running additional SSA's and potentially even template rotations (although I'm not aware of any infra / bootstrap provider controller which reconciles templates).

I would propose optimizing this by dropping managed fields of other controllers before we diff.

Of course this doesn't catch cases where other controllers change metadata or spec and not only the managed fields.

/kind bug
/area topology
[One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels]

[sbueringer]

/assign