docs: Add server-side apply proposal

docs/proposals/server-side-apply.md

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+---
+title: Server-Side Apply
+
+authors:
+- "@leoluz"
+
+sponsors:
+- TBD
+
+reviewers:
+- TBD
+
+approvers:
+- TBD
+
+creation-date: 2022-03-17
+
+---
+
+# Server-Side Apply support for ArgoCD
+
+[Server-Side Apply (SSA)][1] allows calculating the final patch to update
+resources in Kubernetes in the server instead of the client. This proposal
+describes how ArgoCD can leverage SSA during syncs.
+
+* [Open Questions](#open-questions)
+    * [[Q-1] How to handle conflicts?](#q-1-how-to-handle-conflicts)
+    * [[Q-2] Should we support multiple managers?](#q-2-should-we-support-multiple-managers)
+* [ Summary ](#summary)
+* [Motivation](#motivation)
+    * [Better interoperability with Admission Controllers](#better-interoperability-with-admission-controllers)
+    * [Better resource conflict management](#better-resource-conflict-management)
+    * [Better CRD support](#better-crd-support)
+* [Goals](#goals)
+* [Non-Goals](#non-goals)
+* [Proposal](#proposal)
+    * [Non-Functional Requirements](#non-functional-requirements)
+    * [Use cases](#use-cases)
+        * [[UC-1]: enable SSA at the controller level](#uc-1-as-a-user-i-would-like-enable-ssa-at-the-controller-level-so-all-application-are-applied-server-side)
+        * [[UC-2]: enable SSA at the Application level](#uc-2-as-a-user-i-would-like-enable-ssa-at-the-application-level-so-all-resources-are-applied-server-side)
+        * [[UC-3]: enable SSA at the resource level](#uc-3-as-a-user-i-would-like-enable-ssa-at-the-resource-level-so-only-a-single-manifest-is-applied-server-side)
+    * [Security Considerations](#security-considerations)
+    * [Risks and Mitigations](#risks-and-mitigations)
+    * [Upgrade / Downgrade](#upgrade--downgrade)
+* [Drawbacks](#drawbacks)
+
+---
+
+## Open Questions
+
+### [Q-1] How to handle conflicts?
+When SSA is enabled, the server may return field conflicts with other managers.
+What ArgoCD controller should do in case of conflict? Just force the sync and
+log warnings (like some other controllers do?)
+
+### [Q-2] Should we support multiple managers?
+Should Server-Side Apply support in ArgoCD be implemented allowing multiple
+managers for the same controller? ([more details][10])
+
+## Summary
+
+ArgoCD can benefit from [Server-Side Apply][1] during syncs. A few
+improvements to consider:
+
+- More reliable dry-runs (as admission controller is executed) ([ISSUE-804][5])
+- [Syncs always run][2] mutating webhooks (even without diff)
+- [Fix big CRD][3] sync issues
+- Better interoperability with different controllers
+
+## Motivation
+
+ArgoCD uses kubectl library while syncing resources in the cluster. Kubectl uses
+by default a 3-way-merge logic between the live state (in k8s), desired state
+(in git) and the previous state (`last-applied-configuration` annotation) to
+calculate diffs and patch resources in the cluster. This logic is executed in
+the client (ArgoCD) and once the patch is calculated it is then sent to the
+server.
+
+This strategy works well in the majority of the use cases. However, there are
+some scenarios where calculating patches in the client side can cause problems.
+
+Some of the known problems about using client-side approach:
+
+### Better interoperability with Admission Controllers 
+
+More and more users are adopting and configuring [Admission Controllers][4] in
+Kubernetes with different flavors of Validating Webhooks and Mutating Webhooks.
+Admission Controllers will only execute in server-side. In cases where users
+rely on dry-run executions to decide if they should proceed with a deployment,
+having the patch calculated at the client side might provide undesired results.
+Having SSA enabled syncs also guaranties that Admission Controllers are always
+executed, even when there is no diff in the resource.
+
+### Better resource conflict management
+
+Server-Side Apply will better handle and identify conflicts during syncs by
+analyzing the `managedFields` metadata available in all Kubernetes resources
+(since kubernetes 1.18). 
+
+### Better CRD support
+
+By not having to rely on the `last-applied-configuration` annotation, SSA would
+help with failing syncs caused by exceeded annotation size limit. This is a
+common issue when syncing CRDs with large schemas.
+
+## Goals
+
+All following goals should be achieve in order to conclude this proposal:
+
+- Provide the ability for users to define if they want to use SSA during syncs
+  ([ISSUE-2267][6])
+  - Users should be able to enable SSA at the controller level (via binary flag)
+    (see [UC-1](#uc-1-as-a-user-i-would-like-enable-ssa-at-the-controller-level-so-all-application-are-applied-server-side))
+  - Users should be able to enable SSA for a given Application (via syncOptions)
+    (see [UC-2](#uc-2-as-a-user-i-would-like-enable-ssa-at-the-application-level-so-all-resources-are-applied-server-side))
+  - Users should be able to enable SSA at resource level (via annotation) (see
+    [UC-3](#uc-3-as-a-user-i-would-like-enable-ssa-at-the-resource-level-so-only-a-single-manifest-is-applied-server-side)
+- Diffing needs to support strategic merge patch (see [ISSUE-2268][7])
+- Allow Admission Controllers to execute even when there is no diff for a
+  particular resource. (Needs investigation) ([more details][2])
+- ArgoCD should respect field ownership and provide a configuration to allow
+  users to define the behavior in case of conflicts (see [Q-1](#q-1-how-to-handle-conflicts) outcome)
+- ArgoCD should register itself with a proper manager (see [non-functional
+  requirements](#non-functional-requirements))
+
+## Non-Goals
+
+TBD
+
+## Proposal
+
+Change ArgoCD controller to accept new parameter to enable Server-Side Apply
+during syncs. Changes are necessary in ArgoCD as well as in
+gitops-engine library.
+
+### Non-Functional Requirements
+
+- ArgoCD must register itself with a pre-defined manager (suggestion:
+  `argocd-controller`). It shouldn't rely on the default value defined in the
+  kubectl code. ([more details][11])
+
+### Use cases
+
+The following use cases should be implemented:
+
+#### [UC-1]: As a user, I would like enable SSA at the controller level so all Application are applied server-side
+
+Implement a binary flag to configure ArgoCD to run all syncs using SSA.
+(suggestion: `--server-side-apply=true`). Default value should be `false`.
+
+#### [UC-2]: As a user, I would like enable SSA at the Application level so all resources are applied server-side
+
+Implement a new syncOption to allow users to enable SSA at the application
+level (Suggestion `ServerSideApply=true`). UI needs to be updated to support
+this new sync option. If not informed, the controller should keep the current
+behaviour (client-side).
+
+#### [UC-3]: As a user, I would like enable SSA at the resource level so only a single manifest is applied server-side
+
+Leverage the existing `argocd.argoproj.io/sync-options` annotation allowing the
+`ServerSideApply=true` to be informed at the resource level. Must not impact
+other sync-options informed in the annotation (make sure this annotation
+supports providing multiple options).
+
+### Security Considerations
+
+TBD
+
+### Risks and Mitigations
+ArgoCD must check if the target Kubernetes cluster has full support for SSA. The
+feature turned [GA in Kubernetes 1.22][8]. Full support for managed fields was
+introduced as [beta in Kubernetes 1.18][9]. The implementation must check that
+the target kubernetes cluster is running at least version 1.18. If SSA is
+enabled and target cluster version < 1.18 ArgoCD should log warning and fallback
+to client sync.
+
+### Upgrade / Downgrade
+No CRD update necessary as `syncOption` field in Application resource is non-typed
+(string array). Upgrade will only require ArgoCD controller update.
+
+## Drawbacks
+Slight increase in ArgoCD code base complexity.
+
+[1]: https://kubernetes.io/docs/reference/using-api/server-side-apply/
+[2]: https://github.com/argoproj/argo-cd/issues/2267#issuecomment-920445236
+[3]: https://github.com/prometheus-community/helm-charts/issues/1500#issuecomment-1017961377
+[4]: https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/
+[5]: https://github.com/argoproj/argo-cd/issues/804
+[6]: https://github.com/argoproj/argo-cd/issues/2267
+[7]: https://github.com/argoproj/argo-cd/issues/2268
+[8]: https://kubernetes.io/blog/2021/08/06/server-side-apply-ga/
+[9]: https://kubernetes.io/blog/2020/04/01/kubernetes-1.18-feature-server-side-apply-beta-2/
+[10]: https://github.com/argoproj/gitops-engine/pull/363#issuecomment-1013641708
+[11]: https://github.com/argoproj/gitops-engine/pull/363#issuecomment-1013289982