This is an example about how to extend Kubernetes with a Custom Resource Definition.

I have used code-generator to generate types, clients and informers for this CRD. The cmd/main.go starts a shared informer to extend capabilities with your business code.

Run on kubernetes

1. Start minikube
2. Deploy CRD, apps

kubectl apply -f artifacts/crd.yaml
kubectl apply -f artifacts/app.yaml

3. Tail the logs from the pod to see what the application does
4. Create your first item: kubectl apply -f artifacts/todo.yaml

Dependency Management

The project uses go mod but it is requited by code-generator for the project to be in the GOPATH. You should export GO111MODULE=on to be sure that the project uses go mod even if it is inside the GOPATH.

Generate code

Checkout the project in your GOPATH because code-generator still uses GOPATH. And this is the command I used to generate the code inside pkg/client.

~/go/src/k8s.io/code-generator/generate-groups.sh all \
    github.com/gianarb/todo-crd/pkg/client \
    github.com/gianarb/todo-crd/pkg/apis todoexample:v1

Run locally

You can start minkube, register the crd with kubectl apply -f artifacts/crd.yaml and run go run ./cmd/main.go to start the shared informer app:

go run cmd/main.go
{"level":"info","msg":"The todo operator started."}

This the expected log. As you can see in ./cmd/main.go I have just initialized the Shared Informer without implementing any method. When you create a new todo item from ./artifacts/todo.yaml you should see logs from the application:

$ go run cmd/main.go
{"level":"info","msg":"The todo operator started."}
{"level":"info","msg":"new key received","message_key":"default/buy-book"}
{"level":"info","msg":"new key received","message_key":"default/buy-book"}

Credits

Some of the project and articles I read to make this to work:

• https://github.com/kubernetes/sample-controller
• https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/
• https://github.com/kubernetes/client-go/blob/master/INSTALL.md#go-modules