Modular Kubernetes Programming — Part III

This is a follow up post to the one published yesterday titled “Modular Kubernetes Programming-Part II”. If you aren’t familiar with modular Kubernetes resources, please read the previous article in this series before continuing.

The previous blog post talks about using params keyword to parameterize modules when importing them. This blog post explores the various mechanisms in Koki Short for using an imported array. Namely,

• Substitution
• Array Expansion
• Array Indexing

Indexing into an array to select particular values is a powerful language feature (https://commons.wikimedia.org/wiki/File:CPT-programming-array.svg)

Substitution

In the previous two blog posts, I’ve only used one of the mechanisms available in Koki Short for module usage — substitution.

This operation is performed by simply replacing a variable enclosed in curly braces by the value represented by the variable.

Here’s an example of simple substitution:

params:
  - role: "role of the rethinkdb service (admin|user)"
    default: admin
labels:
  db: rethinkdb
  role: ${role}

In the example above, the value of label.role is substituted by the short tool to be admin.

Array Expansion

The second operator provided by the Koki Short tool is Array Expansion operation. Using this operator, it is possible to expand an imported array and append the values into another list. Let’s understand this with an example.

expose:
  - admin-port: 8080
  - driver-port: 28015
  - cluster-port: 29015

The above yaml document represents an array of exposed ports.

Let’s use this in a pod definition.

imports:
  - ports: ./rethink-expose-module.yaml
pod:
  containers:
  - env:
    - from: metadata.namespace
      key: POD_NAMESPACE
    expose:
    - user-port: 9090
    - ${ports...}  # Expands and Appends list of ports into expose
    image: gcr.io/google_containers/rethinkdb:1.16.0_1
    name: rethinkdb
    volume:
    - mount: /data/rethinkdb_data
      store: rethinkdb-storage
  labels:
    db: rethinkdb
    role: admin
    name: rethinkdb-admin
  version: v1
  volumes:
    rethinkdb-storage: empty_dir

The above syntax of referring the variable in curly braces followed by three dots — ${ports...} represents the list expansion operator. It expands the array that is represented by the variable ports, and adds it to the expose array.

Let’s look at the final result of this array expansion

$ short -k -f rethink-list-expansion.yaml
apiVersion: v1
kind: Pod
metadata:
  labels:
    db: rethinkdb
    name: rethinkdb-admin
    role: admin
spec:
  containers:
  - env:
    - name: POD_NAMESPACE
      valueFrom:
        fieldRef:
          fieldPath: metadata.namespace
    image: gcr.io/google_containers/rethinkdb:1.16.0_1
    name: rethinkdb
    ports:
    - containerPort: 9090
      name: user-port
      protocol: TCP
    - containerPort: 8080
      name: admin-port
      protocol: TCP
    - containerPort: 28015
      name: driver-port
      protocol: TCP
    - containerPort: 29015
      name: cluster-port
      protocol: TCP
    resources: {}
    volumeMounts:
    - mountPath: /data/rethinkdb_data
      mountPropagation: ""
      name: rethinkdb-storage
  volumes:
  - emptyDir: {}
    name: rethinkdb-storage

As you can see, the entire list of ports have been expanded and appended to the expose array in the Kubernetes resource.

Array Indexing

The third operator is the other array operator — Array Indexing operator. This operator can be used to select a particular entry from an imported array. Let’s understand this with an example.

expose:
  - admin-port: 8080
  - driver-port: 28015
  - cluster-port: 29015

Let’s start with the exposed port yaml document again.

Instead of using the entire array in another module as shown in the section above, the indexing operator can be used to select particular entries of the array. Let’s select the admin-port (which is at index 0) in a pod definition

imports:
  - ports: ./rethink-expose-module.yaml
pod:
  containers:
  - env:
    - from: metadata.namespace
      key: POD_NAMESPACE
    expose:
    - user-port: 9090
    - ${ports.0}  # Appends ports[0] into expose
    image: gcr.io/google_containers/rethinkdb:1.16.0_1
    name: rethinkdb
    volume:
    - mount: /data/rethinkdb_data
      store: rethinkdb-storage
  labels:
    db: rethinkdb
    role: admin
    name: rethinkdb-admin
  version: v1
  volumes:
    rethinkdb-storage: empty_dir

Here’s the final result of array indexing

apiVersion: v1
kind: Pod
metadata:
  labels:
    db: rethinkdb
    name: rethinkdb-admin
    role: admin
spec:
  containers:
  - env:
    - name: POD_NAMESPACE
      valueFrom:
        fieldRef:
          fieldPath: metadata.namespace
    image: gcr.io/google_containers/rethinkdb:1.16.0_1
    name: rethinkdb
    ports:
    - containerPort: 9090
      name: user-port
      protocol: TCP
    - containerPort: 8080
      name: admin-port
      protocol: TCP
    resources: {}
    volumeMounts:
    - mountPath: /data/rethinkdb_data
      mountPropagation: ""
      name: rethinkdb-storage
  volumes:
  - emptyDir: {}
    name: rethinkdb-storage

As you can see, admin-port has been indexed and substituted in the Kubernetes array.

Conclusion

This blog post sheds light onto the array specific operators in the Koki Short module system. This powerful feature provide users with the flexibility needed for selecting particular entries from an imported array or expanding an entire imported list into a resource.

You can learn more about Short or download examples to play with. You can also try out the Chrome extension to translate live on your browser!

Stay tuned for more deep dives into Modular Kubernetes programming using Koki Short!