posts: Mutate a K8S object

Signed-off-by: Edward Haas <[email protected]>

_posts/2022-08-09-mutating-the-k8s-object.md

@@ -0,0 +1,85 @@
+---
+title: "Mutating the Kubernetes object"
+date: 2022-08-09
+categories:
+  - blog
+tags:
+  - kubernetes
+  - go
+---
+
+## Overview
+
+[Kubernetes](https://kubernetes.io/) exposes API/s to modify its objects,
+each with special behavior and characteristics. 
+
+The existence of multiple methods that mutate an object is sometimes
+confusing, potentially causing suboptimal usage.
+
+We will start by covering the existing methods and continue focusing on
+usages that fit them.
+
+## Operations on Objects
+
+The K8S API server supports several operations which can be used on
+an object.
+
+These operations are exposed at different layers of the stack, from the
+low level HTTP, through the clientset and up to the application (e.g. kubectl).
+
+The following diagram illustrates the relation.
+
+![k8s-oper-on-object](/assets/images/k8s-oper-on-object.svg) 
+
+
+### Mutable Operations
+
+This section focuses on mutable operations.
+
+> **_Note:_** The Create and Delete operations are excluded.
+
+There are 3 mutable operations that can be performed on an object:
+- Update
+- Patch
+- [Apply](https://kubernetes.io/docs/reference/using-api/server-side-apply/) 
+
+> **_Note:_** `Apply` has also a client-side implementation, which uses
+the server-side `Patch`. It is a pattern used by `kubectl` and can be
+mimic by other application (e.g. controllers).
+
+#### Update
+
+The update operation replaces an existing object content with the new
+content. In case some fields have not been included in the update,
+they would be removed on the server-side.
+
+The operation is using the opportunistic lock method using the `resourceVersion` field.
+
+#### Patch
+
+The patch operation has 3 variations:
+- [JSON Patch](https://datatracker.ietf.org/doc/html/rfc6902/)
+- [Merge Patch](https://datatracker.ietf.org/doc/html/rfc7386)
+- [Strategic Merge Patch](https://kubernetes.io/docs/tasks/manage-kubernetes-objects/update-api-object-kubectl-patch/#notes-on-the-strategic-merge-patch)
+
+#### Server Side Apply
+
+[SSA](https://kubernetes.io/docs/reference/using-api/server-side-apply/) is the most recent
+method introduced by Kubernetes to modify and own specific fields in the object manifest.
+
+## Usages
+
+There are various scenarios and cases to modify an object.
+
+The following list is probably incomplete, but still should provide
+enough usage context to allow evaluating which method fits better.
+
+- Client that needs to update multiple fields based on other fields content.
+  - When the client is the owner.
+  - When the client is not the owner.
+- Multiple clients, that each own specific fields.
+- Client/s need to update individual fields:
+  - When the new values dependent on existing fields content.
+  - When the new values do not depend on other fields.
+- Client/s are interested in modifying the object in a declarative manner.
+