interface: drift and configuration difference related status API changes

[michaelawyu]

Description of your changes

This PR includes drafted changes to the status API regarding drift and configuration difference reporting.

I have:

• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

N/A

Special notes for your reviewer

Please see the design doc for more information regarding this PR.

[michaelawyu]

A few notes on this PR:

This PR will report resources that have drifts and those that have configuration differences in separate status fields; it will also report the drift/diff details in the structured form of JSON patches to allow better readability.

There has been discussions regarding:

a) report diffs as a CRP condition, with the diff details being explained as a condition message; i.e., drifts will have per-resource details, diffs will only have a cluster-scoped summary.

This PR:

status:
  placementStatuses:
  - clusterName: bravelion
    driftedPlacements:
    - resourceIdentifier:
        kind: Deployment
        name: app
      observedDrifts: ...
    diffedPlacements:
    - resourceIdentifier:
        kind: ConfigMap
        name: app
      observedDiffs: ...
    ...
  ...

The condition form:

status:
  placementStatuses:
  - clusterName: bravelion
    driftedPlacements:
    - resourceIdentifier:
        kind: Deployment
        name: app
    conditions:
    - type: ResourceDiffed
      message: ConfigMap resources `work/app` and 3 more are observed to have configuration differences
    ...
  ...

b) instead of using structured drift/diff details in the JSON patch format, we produces a summary string of JSON patches as the drift diff details

Structured form (this PR):

observedDrifts:
  - op: replace
     field: /spec/replicas
     oldValue: 1
     value: 2
  - op: add
     field: /spec/revisionHistoryLimit
     value: 5
  ...

Summary form:

drifts have been found in the resource: if applied, in the live state "/spec/replicas" (value: "2", oldValue: "1") and 4 more path(s) will be replaced; "/spec/revisionHistoryLImit" (value: "5") and 1 more path(s) will be added.

[michaelawyu]

The concern is mostly that we would like to strike a balance between simplicity (CRP status is already an overburdened struct) and usefulness/being informative (users can find out exactly where goes wrong).

It is true that K8s objects are not the best place to put drift/diff details; but we do not have an CLI/UI available yet.

Of course none of these are function blockers.

[ryanzhang-oss]

does this object exist if the apply type is not "reportDiff"

[michaelawyu]

Hi Ryan! If the applyStrategy is not reportDiff, and there are no resources pending takeover actions, this field will be omitted.

[circy9]

I am wondering whether you still need the "op" if you show both values. BTW, CurrentValue vs Value is confusing. Can we use hub value and member value?

Path: "/name" ValueInHub: "foo" ValueInMember: "bar"
Path: "/age" ValueInHub: "10" ValueInMember: nil
Path: "/occupation" ValueInHub: nil ValueInMember: "engineer"

[michaelawyu]

Thanks Liqian! I have made the requested changes. 🙏

[michaelawyu]

Due to timeline complications, we will merge this PR first to unblock progress. Will open future PR to address any further concerns; many apologies for the inconvenience.