add KEP 238, to add controller revision

keps/238-controller-revision/README.md

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+# KEP-238: Controller-Revision
+
+<!--
+This is the title of your KEP. Keep it short, simple, and descriptive. A good
+title can help communicate what the KEP is and should be considered as part of
+any review.
+-->
+
+<!--
+A table of contents is helpful for quickly jumping to sections of a KEP and for
+highlighting any additional information provided beyond the standard KEP
+template.
+
+Ensure the TOC is wrapped with
+  <code>&lt;!-- toc --&rt;&lt;!-- /toc --&rt;</code>
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+
+<!-- toc -->
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [User Stories (Optional)](#user-stories-optional)
+    - [Story 1](#story-1)
+    - [Story 2](#story-2)
+  - [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
+  - [Risks and Mitigations](#risks-and-mitigations)
+- [Design Details](#design-details)
+  - [LWS controller](#lws-controller)
+    - [Case 1: No exsting controller revisions in history](#case-1-no-exsting-controller-revisions-in-history)
+    - [Case 2 &amp; 3: Regular create and update](#case-2--3-regular-create-and-update)
+  - [Pod Controller](#pod-controller)
+  - [Controller Revision Implementation](#controller-revision-implementation)
+  - [Test Plan](#test-plan)
+      - [Prerequisite testing updates](#prerequisite-testing-updates)
+      - [Unit tests](#unit-tests)
+      - [Integration tests](#integration-tests)
+      - [e2e tests](#e2e-tests)
+  - [Graduation Criteria](#graduation-criteria)
+- [Implementation History](#implementation-history)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+<!-- /toc -->
+
+## Summary
+
+<!--
+This section is incredibly important for producing high-quality, user-focused
+documentation such as release notes or a development roadmap. It should be
+possible to collect this information before implementation begins, in order to
+avoid requiring implementors to split their attention between writing release
+notes and implementing the feature itself. KEP editors and SIG Docs
+should help to ensure that the tone and content of the `Summary` section is
+useful for a wide audience.
+
+A good summary is probably at least a paragraph in length.
+
+Both in this section and below, follow the guidelines of the [documentation
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+updates.
+
+[documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md
+-->
+
+This KEP aims to add controller revision to store previous states of the LWS object.
+It fixes bug #238, and opens the road for future rollback support.
+
+## Motivation
+
+<!--
+This section is for explicitly listing the motivation, goals, and non-goals of
+this KEP.  Describe why the change is important and the benefits to users. The
+motivation section can optionally provide links to [experience reports] to
+demonstrate the interest in a KEP within the wider Kubernetes community.
+
+[experience reports]: https://github.com/golang/go/wiki/ExperienceReports
+-->
+If a replica is restarted during rolling update, and the replica hasn't been updated
+yet, the worker pod spec that is used is the updated one, while the leader pod spec 
+is the original one. We can fix this by storing the worker pod spec using controller revision.
+
+Another issue is that when upgrading between different LWS controller versions, a rolling update is triggered, even when the deployed LWS object hasn't changed. This can be fixed by storing the
+LWS spec in the revision, then applying the patch and making a semantic comparison, instead of a 
+string one like it is done currently when computing the template hash.
+
+### Goals
+
+<!--
+List the specific goals of the KEP. What is it trying to achieve? How will we
+know that this has succeeded?
+-->
+
+ - Stores the state of the LWS object in order to use the correct version when recreating a replica.
+ - Change the mechanism with which we determine whether an LWS object has changed.
+
+
+### Non-Goals
+
+<!--
+What is out of scope for this KEP? Listing non-goals helps to focus discussion
+and make progress.
+-->
+
+This KEP does not add rollback support, though the design takes into account that it will be 
+added in the future.
+
+## Proposal
+We propose adding controller revision to the LWS controller. This allows us to store previous
+iterations of the LWS object, in order to make it possible to recreate pods with the right pod 
+spec when restarted during rolling update, and be able to compare different LWS objects to determine if the spec has changed. 
+
+<!--
+This is where we get down to the specifics of what the proposal actually is.
+This should have enough detail that reviewers can understand exactly what
+you're proposing, but should not include things like API designs or
+implementation. What is the desired outcome and how do we measure success?.
+The "Design Details" section below is for the real
+nitty-gritty.
+-->
+
+### User Stories (Optional)
+
+<!--
+Detail the things that people will be able to do if this KEP is implemented.
+Include as much detail as possible so that people can understand the "how" of
+the system. The goal here is to make this feel real for users without getting
+bogged down.
+-->
+
+#### Story 1
+
+#### Story 2
+
+### Notes/Constraints/Caveats (Optional)
+
+<!--
+What are the caveats to the proposal?
+What are some important details that didn't come across above?
+Go in to as much detail as necessary here.
+This might be a good place to talk about core concepts and how they relate.
+-->
+
+### Risks and Mitigations
+
+<!--
+What are the risks of this proposal, and how do we mitigate? Think broadly.
+For example, consider both security and how this will impact the larger
+Kubernetes ecosystem.
+
+How will security be reviewed, and by whom?
+
+How will UX be reviewed, and by whom?
+
+Consider including folks who also work outside the SIG or subproject.
+-->
+
+## Design Details
+There are a few constraints that need to be taken into account in the design:
+- The label `templateHash` is not a reliable way to determine whether or not an LWS object has been updated, as seen in #281
+- The same LWS object can generate two different controllerRevision names
+	- For instance, everytime the LWS is queried, it will have different `lws.ResourceVersion`, causing two revisions to be named differently even if they have the same LWS object
+- Adding a new default pod label will trigger rolling update when updating from a different LWS controller version, so no new pod labels can be added.
+
+Taking those into account, we'll combine controller revisions and template hashes. Each controller revision will have a templateHash as a label, essentially creating a map of revisions with template hashes as keys. 
+
+
+### LWS controller
+In order to fix #281, we will use the hash key in the leaderSts to make a controller revision lookup, and get the LWS object that was used to create it. To determine if an update has happened, we'll compare the fields that are used to generate the template hash.
+
+
+```golang
+func templateUpdated(sts, lws) bool {
+	controllerRevision := GetLeaderWorkerSetRevisionFromTemplateHash(sts.Labels[templateHash])
+	baselineLws:= controllerutils.ApplyRevision(lws, controllerRevision)
+	return !utils.EqualLeaderWorkerTemplates(baselineLws, lws)
+}
+```
+
+There are three cases where we need to create a new controllerRevision: 
+- There are no existing controller revisions in the history
+- A new LWS object is created
+- An update has been made to the LWS object
+
+#### Case 1: No exsting controller revisions in history
+This is the case when upgrading from a version that did not have controller revisions. Since a controller revision is needed to determine whether or not an update has happened, it will be created in `rollingUpdateParameters`, so that way an update isn't triggered until a controller revision has been created.
+
+```golang
+func rollingUpdateParameters() {
+	if !existingControllerRevisions {
+		createLeaderWorkerSetRevision(lws, leaderSts.labels[templateHash])
+		return
+	}
+
+	if templateUpdated(leaderSts, lws) {
+		// triggers the rolling update
+	}
+}
+```
+
+#### Case 2 & 3: Regular create and update
+Because an update can be triggered by changing the value of a label, what templateHash value is used when building the leader sts also has to be safeguarded.
+
+```golang
+func SSAwithStatefulSet(lws, partition, lws) {
+	templateHash := utils.LeaderWorkerTemplateHash(lws)
+	if leaderStsExists && !templateUpdated(leaderSts, lws){
+		templatehash := leaderSts.Labels[templateHash]
+	}
+	createLeaderWorkerSetRevision(lws, templateHash)
+	constructLeaderStatefulSetApplyConfiguration(lws, partition, replicas, templateHash)
+}
+```
+
+Once the update has been determined to be done, we'll reset the history to only have the revision of the current LWS object  in the history.
+
+```golang
+func updateConditions() {
+	if updatedAndReadyCount == int(*lws.Spec.Replicas) {
+		conditions = append(conditions, makeCondition(leaderworkerset.LeaderWorkerSetAvailable))
+		truncateHistory()
+	}
+}
+```
+
+### Pod Controller
+Now that there is a map between the template hash of the leader and controller revision, a lookup can be done to select the worker pod spec that will be used.
+
+
+```golang
+func Reconcile() {
+	controllerRevision := GetLeaderWorkerSetRevisionFromTemplateHash(pod.Labels[templateHash])
+	constructWorkerStatefulSetApplyConfiguration(controllerRevision)
+}
+
+func constructWorkerStatefulSetApplyConfiguration(currentRevision) {
+	currentLws := controllerutils.ApplyRevision(lws, controllerRevision)
+	podTemplateSpec := **currentLws.WorkerTemplate.DeepCopy()
+}
+```
+
+### Controller Revision Implementation
+A new package will be created for the functions that interact with controllerRevision, similar to how it is done in [upstream](https://github.com/kubernetes/kubernetes/blob/cb31e42b85d0a1e2be6639ecd4f7b9414887552a/pkg/controller/history/controller_history.go).
+
+Functions for creating patches, applying revisions, and truncating history will be added to `controller_utils` since both the pod and lws controllers need to access these functions.
+
+We'll store the whole LWS spec to make it easier to compare LWS objects when determining if it has been updated.
+
+In order to ensure that there only exists one revision per templateHash, two controller revisions will be determined to be equal if they have the same template hash label.
+
+
+
+<!--
+This section should contain enough information that the specifics of your
+change are understandable. This may include API specs (though not always
+required) or even code snippets. If there's any ambiguity about HOW your
+proposal will be implemented, this is the place to discuss them.
+-->
+
+### Test Plan
+
+<!--
+**Note:** *Not required until targeted at a release.*
+The goal is to ensure that we don't accept enhancements with inadequate testing.
+
+All code is expected to have adequate tests (eventually with coverage
+expectations). Please adhere to the [Kubernetes testing guidelines][testing-guidelines]
+when drafting this test plan.
+
+[testing-guidelines]: https://git.k8s.io/community/contributors/devel/sig-testing/testing.md
+-->
+
+[X] I/we understand the owners of the involved components may require updates to
+existing tests to make this code solid enough prior to committing the changes necessary
+to implement this enhancement.
+
+##### Prerequisite testing updates
+
+<!--
+Based on reviewers feedback describe what additional tests need to be added prior
+implementing this enhancement to ensure the enhancements have also solid foundations.
+-->
+
+##### Unit tests
+- Test controller revision implemenation functions
+<!--
+In principle every added code should have complete unit test coverage, so providing
+the exact set of tests will not bring additional value.
+However, if complete unit test coverage is not possible, explain the reason of it
+together with explanation why this is acceptable.
+-->
+
+<!--
+Additionally, for Alpha try to enumerate the core package you will be touching
+to implement this enhancement and provide the current unit coverage for those
+in the form of:
+- <package>: <date> - <current test coverage>
+The data can be easily read from:
+https://testgrid.k8s.io/sig-testing-canaries#ci-kubernetes-coverage-unit
+
+This can inform certain test coverage improvements that we want to do before
+extending the production code to implement this enhancement.
+-->
+
+##### Integration tests
+
+Test that currentRevision, updatedRevision, and collisionCount work as intended.
+
+Test that worker pod is recreated with the correct pod template if restarted during rolling update.
+
+<!--
+Integration tests are contained in k8s.io/kubernetes/test/integration.
+Integration tests allow control of the configuration parameters used to start the binaries under test.
+This is different from e2e tests which do not allow configuration of parameters.
+Doing this allows testing non-default options and multiple different and potentially conflicting command line options.
+-->
+
+<!--
+This question should be filled when targeting a release.
+For Alpha, describe what tests will be added to ensure proper quality of the enhancement.
+
+For Beta and GA, add links to added tests together with links to k8s-triage for those tests:
+https://storage.googleapis.com/k8s-triage/index.html
+-->
+
+
+##### e2e tests
+
+<!--
+This question should be filled when targeting a release.
+For Alpha, describe what tests will be added to ensure proper quality of the enhancement.
+
+For Beta and GA, add links to added tests together with links to k8s-triage for those tests:
+https://storage.googleapis.com/k8s-triage/index.html
+
+We expect no non-infra related flakes in the last month as a GA graduation criteria.
+-->
+
+
+### Graduation Criteria
+
+<!--
+
+Clearly define what it means for the feature to be implemented and
+considered stable.
+
+If the feature you are introducing has high complexity, consider adding graduation
+milestones with these graduation criteria:
+- [Maturity levels (`alpha`, `beta`, `stable`)][maturity-levels]
+- [Feature gate][feature gate] lifecycle
+- [Deprecation policy][deprecation-policy]
+
+[feature gate]: https://git.k8s.io/community/contributors/devel/sig-architecture/feature-gates.md
+[maturity-levels]: https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#alpha-beta-and-stable-versions
+[deprecation-policy]: https://kubernetes.io/docs/reference/using-api/deprecation-policy/
+-->
+
+## Implementation History
+
+<!--
+Major milestones in the lifecycle of a KEP should be tracked in this section.
+Major milestones might include:
+- the `Summary` and `Motivation` sections being merged, signaling SIG acceptance
+- the `Proposal` section being merged, signaling agreement on a proposed design
+- the date implementation started
+- the first Kubernetes release where an initial version of the KEP was available
+- the version of Kubernetes where the KEP graduated to general availability
+- when the KEP was retired or superseded
+-->
+
+## Drawbacks
+
+<!--
+Why should this KEP _not_ be implemented?
+-->
+
+## Alternatives
+An implementation similar to the one done for StatefulSet was considered. In this implementation, we would create
+a label to store what controller revision was used to generate the pods, and replacing the label `leaderworkerset.sigs.k8s.io/template-revision-hash`.
+However, this requires updating the value of those labels (or adding a new label alltogether). Doing this, triggers a rolling update when upgrading 
+from an LWS version that does not have controller revision. 
+
+Moreover, there are features that need to trigger a rolling update (e.g NetworkConfig). In order to replicate this behavior when using 
+a label to store controller revision, NetworkConfig must be part of the controller revision patch. However, this would mean that NetworkConfig would 
+also be rolled back.
+
+<!--
+What other approaches did you consider, and why did you rule them out? These do
+not need to be as detailed as the proposal, but should include enough
+information to express the idea and why it was not acceptable.
+-->