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https://github.com/ahmedwaleedmalik/pwatcher

Example Kubernetes Controller built with Kubebuilder
https://github.com/ahmedwaleedmalik/pwatcher

controller helm kubernetes operator

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Example Kubernetes Controller built with Kubebuilder

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# pwatcher



Example Kubernetes Controller built with the `kubebuilder` to showcase different use-cases in a controller.



`pwatcher` is a Kubernetes controller which:



- Listens for new pods

- Annotates those pods with a timestamp

- Logs the pod and timestamp to stdout

- Only respond to pods with a particular annotation (e.g. observable=true)

- Only respond to pods in namespaces with a particular annotation (e.g. observable=true)

- Implement leader election



## Use Cases



### Listens for new pods



To listen for only new pods, we used [predicate](https://stuartleeks.com/posts/kubebuilder-event-filters-part-1-delete/) to supress `Update, Delete, Generic` events.



This filtering only leaves us with `Create` event. Requests against this event will now be passed on to the `reconciler`.



### Annotates those pods with a timestamp



Annotate pod with current timestamp if the annotation doesn't already exist



### Logs the pod and timestamp to stdout



Log the namespaced name of the pod along with the timestamp.



### Only respond to pods with a particular annotation (e.g. observable=true)



To facilitate this, we added a new environment variable `POD_FILTER_KEY`. By using this environment variable we can specify the `key` for filtering. Pod should have the given key in it's annotations.



[predicate](https://stuartleeks.com/posts/kubebuilder-event-filters-part-1-delete/) was used to add this filter. We added the logic to filter out pods against `Create` event in the `predicate`.



### Only respond to pods in namespaces with a particular annotation (e.g. observable=true)



To facilitate this, we added a new environment variable `NAMESPACE_FILTER_KEY`. By using this environment variable we can specify the `key` for filtering. Namespace containing the pod should have the given key in it's annotations.



[predicate](https://stuartleeks.com/posts/kubebuilder-event-filters-part-1-delete/) was used to add this filter. We added the logic to drop namespace, containing the pod, that doesn't have the required key in annotations. `Create` event in the `predicate` was used for this.



#### Alternate Approach



An alternate approach for this would have been to:



1. Compute list of namespaces that contain the annotation

2. Update manager options and specify namespaces like:



```go

// Options are the arguments to specify manager configuration

options := ctrl.Options{}



// watchNamespace contains comma separated list of namespaces (e.g ns1,ns2)

options.Namespace = ""

options.NewCache = cache.MultiNamespacedCacheBuilder(strings.Split(watchNamespace, ","))

```



This idea was dropped because the biggest downside of this is that this list is populated when the controller is initialized. We cannot change this list at the runtime. So any new namespaces that will have this annotation will be ignored.



## Installation



### Helm Chart



For `helm` view [chart](charts/pwatcher/README.md)



### Kustomize



To install using `kustomize`



- Ensure that [kustomize](https://kubernetes-sigs.github.io/kustomize/installation/) is installed

- Run `VERSION=v0.0.x make deploy` from the root directory to deploy controller with an image tag `v0.0.x`



## Local Development



### Requirements



- golang 1.16

- [kubebuilder](https://book-v1.book.kubebuilder.io/getting_started/installation_and_setup.html)

- [kustomize](https://kubernetes-sigs.github.io/kustomize/installation/)



### Execution



- Ensure that a valid kubeconfig is loaded

- Run `make run` from the root directory to build and execute the binary



#### Test for filters



- Run `NAMESPACE_FILTER_KEY=pwatcher-test POD_FILTER_KEY=pwatcher-test make run` to run the controller

- Run `kubectl apply -f examples/manifest.yaml` to create resource that complies with the filters

- `kubectl get pod pwatcher-test -n pwatcher-test -o jsonpath='{.metadata.annotations}'` to check that the `pwatcher.io/timestamp` annotation has been added