Create a Controller

This tutorial walks through a simple example of creating a controller in Python with Metacontroller.


  • Kubernetes v1.9+
  • You should have kubectl available and configured to talk to the desired cluster.
  • You should have already installed Metacontroller.

Hello, World!

In this example, we'll create a useless controller that runs a single Pod that prints a greeting to its standard output. Once you're familiar with the general process, you can look through the examples page to find concepts that actually do something useful.

To make cleanup easier, first create a new Namespace called hello:

kubectl create namespace hello

We'll put all our Namespace-scoped objects there by adding -n hello to the kubectl commands.

Define a custom resource

Our example controller will implement the behavior for a new API represented as a custom resource.

First, let's use the built-in CustomResourceDefinition API to set up a storage location (a helloworlds resource) for objects of our custom type (HelloWorld).

Save the following to a file called crd.yaml:

kind: CustomResourceDefinition
    kind: HelloWorld
    plural: helloworlds
    singular: helloworld
  scope: Namespaced
  - name: v1
    served: true
    storage: true
        type: object
            type: object
                type: string
     status: {}

Then apply it to your cluster:

kubectl apply -f crd.yaml

Define a custom controller

For each HelloWorld object, we're going to create a Pod as a child object, so we'll use the CompositeController API to implement a controller that defines this parent-child relationship.

Save the following to a file called controller.yaml:

kind: CompositeController
  name: hello-controller
  generateSelector: true
    resource: helloworlds
  - apiVersion: v1
    resource: pods
      method: Recreate
        url: http://hello-controller.hello/sync

Then apply it to your cluster:

kubectl apply -f controller.yaml

This tells Metacontroller to start a reconciling control loop for you, running inside the Metacontroller server. The parameters under spec: let you tune the behavior of the controller declaratively.

In this case:

  • We set generateSelector to true to mimic the built-in Job API since we're running a Pod to completion and don't want to share Pods across invocations.
  • The parentResource is our custom resource called helloworlds.
  • The idea of CompositeController is that the parent resource represents objects that are composed of other objects. A HelloWorld is composed of just a Pod, so we have only one entry in the childResources list.
  • For each child resource, we can optionally set an updateStrategy to specify what to do if a child object needs to be updated. Since Pods are effectively immutable, we use the Recreate method, which means, "delete the outdated object and create a new one".
  • Finally, we tell Metacontroller how to invoke the sync webhook, which is where we'll define the business logic of our controller. The example relies on in-cluster DNS to resolve the address of the hello-controller Service (which we'll define below) within the hello Namespace.

Write a webhook

Metacontroller will handle the controllery bits for us, but we still need to tell it what our controller actually does.

To define our business logic, we write a webhook that generates child objects based on the parent spec, which is provided as JSON in the webhook request. The sync hook request contains additional information as well, but the parent spec is all we need for this example.

You can write Metacontroller hooks in any language, but Python is particularly nice because its dictionary type is convenient for programmatically building JSON objects (like the Pod object below).

If you have a preferred Functions-as-a-Service framework, you can use that to write your webhook, but we'll keep this example self-contained by relying on the basic HTTP server module in the Python standard library. The do_POST() method handles decoding and encoding the request and response as JSON.

The real hook logic is in the sync() method, and consists primarily of building a Pod object. Because Metacontroller uses apply semantics, you can simply return the Pod object as if you were creating it, every time. If the Pod already exists, Metacontroller will take care of updates according to your update strategy.

In this case, we set the update method to Recreate, so an existing Pod would be deleted and replaced if it doesn't match the desired state returned by your hook. Notice, however, that the hook code below doesn't need to mention any of that because it's only responsible for computing the desired state; the Metacontroller server takes care of reconciling with the observed state.

Save the following to a file called

from http.server import BaseHTTPRequestHandler, HTTPServer
import json

class Controller(BaseHTTPRequestHandler):
  def sync(self, parent, children):
    # Compute status based on observed state.
    desired_status = {
      "pods": len(children["Pod.v1"])

    # Generate the desired child object(s).
    who = parent.get("spec", {}).get("who", "World")
    desired_pods = [
        "apiVersion": "v1",
        "kind": "Pod",
        "metadata": {
          "name": parent["metadata"]["name"]
        "spec": {
          "restartPolicy": "OnFailure",
          "containers": [
              "name": "hello",
              "image": "busybox",
              "command": ["echo", "Hello, %s!" % who]

    return {"status": desired_status, "children": desired_pods}

  def do_POST(self):
    # Serve the sync() function as a JSON webhook.
    observed = json.loads("content-length"))))
    desired = self.sync(observed["parent"], observed["children"])

    self.send_header("Content-type", "application/json")

HTTPServer(("", 80), Controller).serve_forever()

Then load it into your cluster as a ConfigMap:

kubectl -n hello create configmap hello-controller

Note: The -n hello flag is important to put the ConfigMap in the hello namespace we created for the tutorial.

Deploy the webhook

Finally, since we wrote our hook as a self-contained Python web server, we need to deploy it somewhere that Metacontroller can reach. Luckily, we have this thing called Kubernetes which is great at hosting stateless web services.

Since our hook consists of only a small Python script, we'll use a generic Python container image and mount the script from the ConfigMap we created.

Save the following to a file called webhook.yaml:

apiVersion: apps/v1
kind: Deployment
  name: hello-controller
  replicas: 1
      app: hello-controller
        app: hello-controller
      - name: controller
        image: python:3
        command: ["python3", "/hooks/"]
        - name: hooks
          mountPath: /hooks
      - name: hooks
          name: hello-controller
apiVersion: v1
kind: Service
  name: hello-controller
    app: hello-controller
  - port: 80

Then apply it to your cluster:

kubectl -n hello apply -f webhook.yaml

Try it out

Now we can create HelloWorld objects and see what they do.

Save the following to a file called hello.yaml:

kind: HelloWorld
  name: your-name
  who: Your Name

Then apply it to your cluster:

kubectl -n hello apply -f hello.yaml

Our controller should see this and create a Pod that prints a greeting and then exits.

kubectl -n hello get pods

You should see something like this:

NAME                                READY     STATUS      RESTARTS   AGE
hello-controller-746fc7c4dc-rzslh   1/1       Running     0          2m
your-name                           0/1       Completed   0          15s

Then you can check the logs on the Completed Pod:

kubectl -n hello logs your-name

Which should look like this:

Hello, Your Name!

Now let's look at what happens when you update the parent object, for example to change the name:

kubectl -n hello patch helloworld your-name --type=merge -p '{"spec":{"who":"My Name"}}'

If you now check the Pod logs again:

kubectl -n hello logs your-name

You should see that the Pod was updated (actually deleted and recreated) to print a greeting to the new name, even though the hook code doesn't mention anything about updates.

Hello, My Name!

Clean up

Another thing Metacontroller does for you by default is set up links so that child objects are removed by the garbage collector when the parent goes away (assuming your cluster is version 1.8+).

You can check this by deleting the parent:

kubectl -n hello delete helloworld your-name

And then checking for the child Pod:

kubectl -n hello get pods

You should see that the child Pod was cleaned up automatically, so only the webhook Pod remains:

NAME                                READY     STATUS      RESTARTS   AGE
hello-controller-746fc7c4dc-rzslh   1/1       Running     0          3m

When you're done with the tutorial, you should remove the controller, CRD, and Namespace as follows:

kubectl delete compositecontroller hello-controller
kubectl delete crd
kubectl delete ns hello

Next Steps