Can someone explain 'patchesStrategicMerge'

This comes in handy you inherit from some base and want to apply partial changes to said base. That way, you can have one source YAML file and perform different customizations based on it, without having to recreate the entire resource. That is one key selling point of kustomize.

The purpose of the Strategic Merge Patch is to be able to patch rich objects partially, instead of replacing them entirely.

Imagine you have a list, of object.

mylist:
 - name: foo
 - name: bar
 - name: baz

How could you change one of the items in the list? With a standard merge patch, you could only replace the entire list. But with a strategic merge patch, you can target one element in the list based on some property, in this case only the name makes sense as it's the only property.

mylist:
 - $patch: delete
   name: foo

In the above example, I used the strategic merge patch to remove the item in the list with the name foo.

Here is another example, suppose I have the following project structure.

sample
├── base
│   ├── kustomization.yaml
│   └── pod.yaml
└── layers
    └── dev
        ├── kustomization.yaml
        └── patch.yaml

In the base, is my full pod definition. While in the layers, I can create multiple layers for different environments, in this case I have only one for dev.

The kustomization.yaml in the base folder looks like this.

resources:
 - pod.yaml

If I execute the base itself with dry run I get this.

kubectl apply -k sample/base --dry-run=client -o yaml

apiVersion: v1
kind: Pod
metadata:
  name: myapp
  namespace: default
spec:
  containers:
  - image: nginx
    name: nginx
  - command:
    - sleep
    - infinity
  - image: busybox
    name: sidecar

The kustomization.yaml in the dev folder looks like this.

bases:
  - ../../base
patchesStrategicMerge:
  - patch.yaml

And the patch looks like this. I want to enable debug logging for the sidecar. Thefore I am using a merge directive to change its arguments without changing the image. I also want to keep the nginx container.

apiVersion: v1
kind: Pod
metadata:
  name: myapp
spec:
  containers:
    - $patch: merge
      name: sidecar
      args: [log-level, debug]

kubectl apply -k sample/layers/dev/ --dry-run=client -o yaml

apiVersion: v1
kind: Pod
metadata:
  name: myapp
  namespace: default
spec:
  containers:
  - args:
    - log-level
    - debug
    command:
    - sleep
    - infinity
    image: busybox
    name: sidecar
  - image: nginx
    name: nginx

Note: The command and args don't make much sense, It's just to illustrate the strategic merge patch.