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Controller

This example demonstrates a simple Controller which evaluates ConfigMaps and restart associated pods if such a ConfigMap changes.

This controller is been inspired from configmap-controller but for more limited.

This controller watches all ConfigMap in the namespace in which this controller is deployed and reacts on the annotation k8spatterns.io/podDeleteSelector. If this annotation is given on a ConfigMap that has changed, then the value of this annotation is taken as a label selector to find Pods to kill. Supposed that these pods are managed by a backend controller like Deployment, these Pods will be respawned again, eventually picking up the changed configuration.

For a full explanation of how this controller works, please refer to the "Controller" pattern in our book.

Warning
 Note, that this is example is meant for educational purposes only and is not suitable for general purpose usage.

The easiest way to try this example is by using minikube with an ingress controller enabled:

# Start minikube and enable the ingress addon
minikube start
minikube addons enable ingress

The controller script itself is stored in a ConfigMap and can be easily edited later on:

# Create a configmap holding the controller shell script:
kubectl create configmap config-watcher-controller --from-file=./config-watcher-controller.sh

In order to deploy the controller a Deployment creates a pod with two containers:

  • One Kubernetes API proxy which exposes the Kubernetes API on localhost with port 8001. The image for k8spatterns/kubeapi-proxy is defined in this Dockerfile.

  • The main container which executes the script from the configmap. It is based on a single Alpine base image with curl and jq included. The Dockerfile for this image k8spattern/curl-jq can be found here.

Both images, k8spatterns/kubeapi-proxy and k8spatterns/curl-jq are available from Docker Hub.

To create this deployment in the current namespace, call:

# Create the controller Deployment
kubectl create -f config-watcher-controller.yml

You might want to have a look at the descriptor file; it contains some useful comments along the way and also the security setup to allow the controller to watch resources and to restart Pods.

That’s all. Now you have a nice little Controller which watches on ConfigMap and restart Pods in case of updates.

To try it out, we are reusing the super simple web application which just exposes an environment variable as content. This image uses nc to deliver the content and can be found on Docker Hub as k8spatterns/mini-http-server.

Before we deploy this app, we should tail on the log of our controller (e.g. kubectl logs -f config-watcher-controller-…​.) to see the events received by the controller as they come in, e.g. with

controller_pod=$(kubectl get pod -o name | grep config-watcher-controller | sed -e "s/^pods\///")
kubectl logs -f $controller_pod config-watcher

Then create the web application itself:

# Create a sample web application with an 'expose' annotation:
kubectl create -f web-app.yml

If you look into this descriptor, you will find a Deployment using our dumb HTTP server which references the content environment variable via a ConfigMap. The ConfigMap itself is annotated with a pod selector k8spatterns.io/podDeleteSelector: "app=webapp" which directly select the webapp Pod.

This resource file also includes the definition of a Service and an Ingress object so that we can access the server from the outside.

You can access our web app directly

# Access webapp via the included Ingress obect
curl -sk https://webapp.$(minikube ip).nip.io

Now change the content of ConfigMap and watch the log of your controller:

# Patch config-map to update the web content
kubectl patch configmap webapp-config -p '{"data":{"message":"Take this update!"}}'

and then finally call the URL again to check that the content has been updated to

# Access webapp via the included Ingress obect
curl -sk https://webapp.$(minikube ip).nip.io

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