CAEP: machine deletion phase hooks

Defines a set of annotations that can be applied to a machine which affect the
linear progress of a machine’s lifecycle after a machine has been marked for
deletion.  These annotations are optional and may be applied during machine
creation, sometime after machine creation by a user, or sometime after machine
creation by another controller or application.

docs/proposals/20200602-machine-deletion-phase-hooks.md

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+---
+title: Machine Deletion Phase Hooks
+authors:
+  - "@michaelgugino"
+reviewers:
+  - "@enxebre"
+  - "@vincepri"
+  - "@detiber"
+  - "@ncdc"
+creation-date: 2020-06-02
+last-updated: 2020-06-02
+status: implementable
+---
+
+# Machine Deletion Phase Hooks
+
+## Table of Contents
+<!-- toc -->
+- [Machine Deletion Phase Hooks](#machine-deletion-phase-hooks)
+  - [Table of Contents](#table-of-contents)
+  - [Glossary](#glossary)
+    - [lifecycle hook](#lifecycle-hook)
+    - [deletion phase](#deletion-phase)
+  - [Summary](#summary)
+  - [Motivation](#motivation)
+    - [Goals](#goals)
+    - [Non-Goals/Future Work](#non-goalsfuture-work)
+  - [Proposal](#proposal)
+    - [User Stories](#user-stories)
+      - [Story 1](#story-1)
+      - [Story 2](#story-2)
+      - [Story 3](#story-3)
+    - [Implementation Details/Notes/Constraints](#implementation-detailsnotesconstraints)
+      - [Lifecycle Points](#lifecycle-points)
+        - [pre-drain](#pre-drain)
+        - [pre-terminate](#pre-terminate)
+      - [Annotation Form](#annotation-form)
+        - [lifecycle-point](#lifecycle-point)
+        - [hook-name](#hook-name)
+        - [owner (Optional)](#owner-optional)
+        - [Annotation Examples](#annotation-examples)
+      - [Changes to machine-controller](#changes-to-machine-controller)
+        - [Reconciliation](#reconciliation)
+        - [Hook failure](#hook-failure)
+        - [Hook ordering](#hook-ordering)
+      - [Owning Controller Design](#owning-controller-design)
+        - [Owning Controllers must](#owning-controllers-must)
+        - [Owning Controllers may](#owning-controllers-may)
+      - [Determining when to take action](#determining-when-to-take-action)
+        - [Failure Mode](#failure-mode)
+    - [Risks and Mitigations](#risks-and-mitigations)
+  - [Alternatives](#alternatives)
+    - [Custom Machine Controller](#custom-machine-controller)
+    - [Finalizers](#finalizers)
+    - [Status Field](#status-field)
+    - [Spec Field](#spec-field)
+    - [CRDs](#crds)
+  - [Upgrade Strategy](#upgrade-strategy)
+  - [Additional Details](#additional-details)
+<!-- /toc -->
+
+## Glossary
+
+Refer to the [Cluster API Book Glossary](https://cluster-api.sigs.k8s.io/reference/glossary.html).
+
+### lifecycle hook
+A specific point in a machine's reconciliation lifecycle where execution of
+normal machine-controller behavior is paused or modified.
+
+### deletion phase
+Describes when a machine has been marked for deletion but is still present
+in the API.  Various actions happen during this phase, such as draining a node,
+deleting an instance from a cloud provider, and deleting a node object.
+
+### Hook Implementing Controller (HIC)
+The Hook Implementing Controller describes a controller, other than the
+machine-controller, that adds, removes, and/or responds to a particular
+lifecycle hook.  Each lifecycle hook should have a single HIC, but an HIC
+can optionally manage one or more hooks.
+
+## Summary
+
+Defines a set of annotations that can be applied to a machine which affect the
+linear progress of a machine’s lifecycle after a machine has been marked for
+deletion.  These annotations are optional and may be applied during machine
+creation, sometime after machine creation by a user, or sometime after machine
+creation by another controller or application.
+
+## Motivation
+
+Allow custom and 3rd party components to easily interact with a machine or
+related resources while that machine's reconciliation is temporarily paused.
+This pause in reconciliation will allow these custom components to take action
+after a machine has been marked for deletion, but prior to the machine being
+drained and/or associated instance terminated.
+
+### Goals
+
+- Define an initial set of hook points for the deletion phase.
+- Define an initial set and form of related annotations.
+- Define basic expectations for a controller or process that responds to a
+lifecycle hook.
+
+
+### Non-Goals/Future Work
+
+- Create an exhaustive list of hooks; we can add more over time.
+- Create new machine phases.
+- Create a mechanism to signal what lifecycle point a machine is at currently.
+- Dictate implementation of controllers that respond to the hooks.
+- Implement ordering in the machine-controller.
+- Require anyone use these hooks for normal machine operations, these are
+strictly optional and for custom integrations only.
+
+
+## Proposal
+
+- Utilize annotations to implement lifecycle hooks.
+- Each lifecycle point can have 0 or more hooks.
+- Hooks do not enforce ordering.
+- Hooks found during machine reconciliation effectively pause reconciliation
+until all hooks for that lifecycle point are removed from a machine's annotations.
+
+
+### User Stories
+#### Story 1
+(pre-terminate) As an operator, I would like to have the ability to perform
+different actions between the time a machine is marked deleted in the api and
+the time the machine is deleted from the cloud.
+
+For example, when replacing a control plane machine, ensure a new control
+plane machine has been successfully created and joined to the cluster before
+removing the instance of the deleted machine. This might be useful in case
+there are disruptions during replacement and we need the disk of the existing
+instance to perform some disaster recovery operation.  This will also prevent
+prolonged periods of having one fewer control plane host in the event the
+replacement instance does not come up in a timely manner.
+
+#### Story 2
+(pre-drain) As an operator, I want the ability to utilize my own draining
+controller instead of the logic built into the machine-controller.  This will
+allow me better flexibility and control over the lifecycle of workloads on each
+node.
+
+#### Story 3
+(pre-drain) As an operator, when I am deleting a control plane machine for
+maintenance reasons, I want to ensure my existing control plane machine is not
+drained until after my replacement comes online.  This will prevent protracted
+periods of a missing control plane member if the replacement machine cannot be
+created in a timely manner.
+
+
+### Implementation Details/Notes/Constraints
+
+For each defined lifecycle point, one or more hooks may be applied as an annotation to the machine object.  These annotations will pause reconciliation of a machine object until all hooks are resolved for that lifecycle point.  The hooks should be managed by an Hook Implementing Controler or other external application, or
+manually created and removed by an administrator.
+
+#### Lifecycle Points
+##### pre-drain
+`pre-drain.delete.hook.machine.cluster.x-k8s.io`
+
+Hooks defined at this point will prevent the machine-controller from draining a node after the machine-object has been marked for deletion until the hooks are removed.
+##### pre-terminate
+`pre-terminate.delete.hook.machine.cluster.x-k8s.io`
+
+Hooks defined at this point will prevent the machine-controller from
+removing/terminating the instance in the cloud provider until the hooks are
+removed.
+
+"pre-terminate" has been chosen over "pre-delete" because "terminate" is more
+easily associated with an instance being removed from the cloud or
+infrastructure, whereas "delete" is ambiguous as to the actual state of the
+machine in its lifecycle.
+
+
+#### Annotation Form
+```
+<lifecycle-point>.delete.hook.machine.cluster-api.x-k8s.io/<hook-name>: <owner/creator>
+```
+
+##### lifecycle-point
+This is the point in the lifecycle of reconciling a machine the annotation will have effect and pause the machine-controller.
+
+##### hook-name
+Each hook should have a unique and descriptive name that describes in 1-3 words what the intent/reason for the hook is.  Each hook name should be unique and managed by a single entity.
+
+##### owner (Optional)
+Some information about who created or is otherwise in charge of managing the annotation.  This might be a controller or a username to indicate an administrator applied the hook directly.
+
+##### Annotation Examples
+
+These examples are all hypothetical to illustrate what form annotations should
+take.  The names of of each hook and the respective controllers are fictional.
+
+pre-drain.hook.machine.cluster-api.x-k8s.io/migrate-important-app: my-app-migration-controller
+
+pre-terminate.hook.machine.cluster-api.x-k8s.io/backup-files: my-backup-controller
+
+pre-terminate.hook.machine.cluster-api.x-k8s.io/wait-for-storage-detach: my-custom-storage-detach-controller
+
+#### Changes to machine-controller
+The machine-controller should check for the existence of 1 or more hooks at
+specific points (lifecycle-points) during reconciliation.  If a hook matching
+the lifecycle-point is discovered, the machine-controller should stop
+reconciling the machine.
+
+An example of where the pre-drain lifecycle-point might be implemented:
+https://github.com/kubernetes-sigs/cluster-api/blob/30c377c0964efc789ab2f3f7361eb323003a7759/controllers/machine_controller.go#L270
+
+##### Reconciliation
+When a Hook Implementing Controller updates the machine, reconciliation will be
+triggered, and the machine will continue reconciling as normal, unless another
+hook is still present; there is no need to 'fail' the reconciliation to
+enforce requeuing.
+
+When all hooks for a given lifecycle-point are removed, reconciliation
+will continue as normal.
+
+##### Hook failure
+The machine-controller should not timeout or otherwise consider the lifecycle
+hook as 'failed.'  Only the Hook Implementing Controller may decide to remove a
+particular lifecycle hook to allow the machine-controller to progress past the
+corresponding lifecycle-point.
+
+##### Hook ordering
+The machine-controller will not attempt to enforce any ordering of hooks.
+
+Hook Implementing Controllers may devise a dependency-based enforcement mechanism
+via whatever means HICs determine.  Examples could be
+using CRDs external to the machine-api, gRPC communications, or
+additional annotations on the machine or other objects.
+
+#### Hook Implementing Controller Design
+Hook Implementing Controller is the component that manages a particular
+lifecycle hook.
+
+##### Hook Implementing Controllers must
+* Watch machine objects and determine when an appropriate action must be taken.
+* After completing the desired hook action, remove the hook annotation.
+##### Hook Implementing Controllers may
+* Watch machine objects and add a hook annotation as desired by the cluster
+administrator.
+* After completing a hook, set an addition annotation that indicates this
+controller has finished for dependency ordering, or perform some other action
+to signal other Hook Implementing Controllers that they may proceed.
+
+#### Determining when to take action
+
+An Hook Implementing Controller should watch machines and determine when is the
+best time to take action.
+
+For example, if an HIC manages a lifecycle hook at the pre-drain lifecycle-point,
+then that controller should take action immediately after a machine has a
+DeletionTimestamp or enters the "Deleting" phase.
+
+Fine-tuned coordination is not possible at this time; eg, it's not
+possible to execute a pre-terminate hook only after a node has been drained.
+This is reserved for future work.
+
+##### Failure Mode
+It is entirely up to the Hook Implementing Controller to determine when it is
+prudent to remove a particular lifecycle hook. Some controllers may want to
+'give up' after a certain time period, and others may want to block indefinitely.
+Cluster operators should consider the characteristics of each controller before
+utilizing them in their clusters.
+
+
+### Risks and Mitigations
+
+* Annotation keys must conform to length limits: https://kubernetes.io/docs/concepts/overview/working-with-objects/annotations/#syntax-and-character-set
+* Requires well-behaved controllers and admins to keep things running
+smoothly.  Would be easy to disrupt machines with poor configuration.
+* Troubleshooting problems may increase in complexity, but this is
+mitigated mostly by the fact that these hooks are opt-in.  Operators
+will or should know they are consuming these hooks, but a future proliferation
+of the cluster-api could result in these components being bundled as a
+complete solution that operators just consume.  To this end, we should
+update any troubleshooting guides to check these hook points where possible.
+
+
+## Alternatives
+
+### Custom Machine Controller
+Require advanced users to fork and customize.  This can already be done if someone chooses, so not much of a solution.
+
+### Finalizers
+We define additional finalizers, but this really only implies the deletion lifecycle point.  A misbehaving controller that
+accidentally removes finalizers could have undesireable
+effects.
+
+### Status Field
+Harder for users to modify or set hooks during machine creation.  How would a user remove a hook if a controller that is supposed to remove it is misbehaving?  We’d probably need an annotation like ‘skip-hook-xyz’ or similar and that seems redundant to just using annotations in the first place
+
+### Spec Field
+We probably don’t want other controllers dynamically adding and removing spec fields on an object.  It’s not very declarative to utilize spec fields in that way.
+
+### CRDs
+Seems like we’d need to sync information to and from a CR.  There are different approaches to CRDs (1-to-1 mapping machine to CR, match labels, present/absent vs status fields) that each have their own drawbacks and are more complex to define and configure.
+
+
+## Upgrade Strategy
+
+Nothing defined here should directly impact upgrades other than defining hooks that impact creation/deletion of a machine, generally.
+
+## Additional Details
+
+Fine-tuned timing of hooks is not possible at this time.
+
+In the future, it is possible to implement this timing via additional
+machine phases, or possible "sub-phases" or some other mechanism
+that might be appropriate.  As stated in the non-goals, that is
+not in scope at this time, and could be future work.
+
+<!-- Links -->
+[community meeting]: https://docs.google.com/document/d/1Ys-DOR5UsgbMEeciuG0HOgDQc8kZsaWIWJeKJ1-UfbY