JSON Patching

A kustomization file supports customizing resources via JSON patches.

Make a place to work:

DEMO_HOME=$(mktemp -d)

We'll be editting an Ingress object:

cat <<EOF >$DEMO_HOME/ingress.yaml
apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: my-ingress
spec:
  rules:
  - host: foo.bar.com
    http:
      paths:
      - path: /
        backend:
          serviceName: homepage
          servicePort: 8888
      - path: /api
        backend:
          serviceName: my-api
          servicePort: 7701
      - path: /test
        backend:
          serviceName: hello
          servicePort: 7702
EOF

The edits we want to make are:

change the value of host to foo.bar.io
change the port for '/' from 8888 to 80
insert an entirely new serving path /healthz at a particular point in the paths list, rather than at the end or the beginning.

Here's the patch file to do that:

cat <<EOF >$DEMO_HOME/ingress_patch.json
[
  {"op": "replace",
   "path": "/spec/rules/0/host",
   "value": "foo.bar.io"},

  {"op": "replace",
   "path": "/spec/rules/0/http/paths/0/backend/servicePort",
   "value": 80},

  {"op": "add",
   "path": "/spec/rules/0/http/paths/1",
   "value": { "path": "/healthz", "backend": {"servicePort":7700} }}
]
EOF

We'll of course need a kustomization file referring to the Ingress:

cat <<EOF >$DEMO_HOME/kustomization.yaml
resources:
- ingress.yaml
EOF

To this same kustomization file, add a patches field refering to the patch file we just made and target it to the Ingress object:

cat <<EOF >>$DEMO_HOME/kustomization.yaml
patches:
- path: ingress_patch.json
  target:
    group: networking.k8s.io
    version: v1beta1
    kind: Ingress
    name: my-ingress
EOF

Define the expected output:

cat <<EOF >$DEMO_HOME/out_expected.yaml
apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
  name: my-ingress
spec:
  rules:
  - host: foo.bar.io
    http:
      paths:
      - backend:
          serviceName: homepage
          servicePort: 80
        path: /
      - backend:
          servicePort: 7700
        path: /healthz
      - backend:
          serviceName: my-api
          servicePort: 7701
        path: /api
      - backend:
          serviceName: hello
          servicePort: 7702
        path: /test
EOF

Run the build:

kustomize build $DEMO_HOME >$DEMO_HOME/out_actual.yaml

Confirm they match:

diff $DEMO_HOME/out_actual.yaml $DEMO_HOME/out_expected.yaml

If you prefer YAML to JSON, the patch can be expressed in YAML format (nevertheless following JSON patch rules):

cat <<EOF >$DEMO_HOME/ingress_patch.yaml
- op: add
  path: /spec/rules/0/http/paths/-
  value:
    path: '/canada'
    backend:
      serviceName: hoser
      servicePort: 7703
EOF

Now add this to the list of patches in the kustomization file:

cat <<EOF >>$DEMO_HOME/kustomization.yaml
- path: ingress_patch.yaml
  target:
    group: networking.k8s.io
    version: v1beta1
    kind: Ingress
    name: my-ingress
EOF

We expect the following at the end of the output:

cat <<EOF >$DEMO_HOME/out_expected.yaml
      - backend:
          serviceName: hello
          servicePort: 7702
        path: /test
      - backend:
          serviceName: hoser
          servicePort: 7703
        path: /canada
EOF

Try it:

kustomize build $DEMO_HOME | tail -n 8 |\
    diff  $DEMO_HOME/out_expected.yaml -

To see how to apply one JSON patch to many resources, see the multi-patch demo.