Implementing Runtime Extensions


As a developer building systems on top of Cluster API, if you want to hook into the Cluster’s lifecycle via a Runtime Hook, you have to implement a Runtime Extension handling requests according to the OpenAPI specification for the Runtime Hook you are interested in.

Runtime Extensions by design are very powerful and flexible, however given that with great power comes great responsibility, a few key consideration should always be kept in mind (more details in the following sections):

  • Runtime Extensions are components that should be designed, written and deployed with great caution given that they can affect the proper functioning of the Cluster API runtime.
  • Cluster administrators should carefully vet any Runtime Extension registration, thus preventing malicious components from being added to the system.

Please note that following similar practices is already commonly accepted in the Kubernetes ecosystem for Kubernetes API server admission webhooks. Runtime Extensions share the same foundation and most of the same considerations/concerns apply.


As mentioned above as a developer building systems on top of Cluster API, if you want to hook in the Cluster’s lifecycle via a Runtime Extension, you have to implement an HTTPS server handling a discovery request and a set of additional requests according to the OpenAPI specification for the Runtime Hook you are interested in.

The following shows a minimal example of a Runtime Extension server implementation:

package main

import (

	cliflag ""
	logsv1 ""
	ctrl ""

	runtimecatalog ""
	runtimehooksv1 ""

var (
	// catalog contains all information about RuntimeHooks.
	catalog = runtimecatalog.New()

	// Flags.
	profilerAddress string
	webhookPort     int
	webhookCertDir  string
	logOptions      = logs.NewOptions()

func init() {
	// Adds to the catalog all the RuntimeHooks defined in cluster API.
	_ = runtimehooksv1.AddToCatalog(catalog)

// InitFlags initializes the flags.
func InitFlags(fs *pflag.FlagSet) {
	// Initialize logs flags using Kubernetes component-base machinery.
	logsv1.AddFlags(logOptions, fs)

	// Add test-extension specific flags
	fs.StringVar(&profilerAddress, "profiler-address", "",
		"Bind address to expose the pprof profiler (e.g. localhost:6060)")

	fs.IntVar(&webhookPort, "webhook-port", 9443,
		"Webhook Server port")

	fs.StringVar(&webhookCertDir, "webhook-cert-dir", "/tmp/k8s-webhook-server/serving-certs/",
		"Webhook cert dir.")

func main() {
	// Creates a logger to be used during the main func.
	setupLog := ctrl.Log.WithName("setup")

	// Initialize and parse command line flags.
	// Set log level 2 as default.
	if err := pflag.CommandLine.Set("v", "2"); err != nil {
		setupLog.Error(err, "failed to set default log level")

	// Validates logs flags using Kubernetes component-base machinery and applies them
	if err := logsv1.ValidateAndApply(logOptions, nil); err != nil {
		setupLog.Error(err, "unable to start extension")

	// Add the klog logger in the context.

	// Initialize the golang profiler server, if required.
	if profilerAddress != "" {
		klog.Infof("Profiler listening for requests at %s", profilerAddress)
		go func() {
			klog.Info(http.ListenAndServe(profilerAddress, nil))

	// Create a http server for serving runtime extensions
	webhookServer, err := server.New(server.Options{
		Catalog: catalog,
		Port:    webhookPort,
		CertDir: webhookCertDir,
	if err != nil {
		setupLog.Error(err, "error creating webhook server")

	// Register extension handlers.
	if err := webhookServer.AddExtensionHandler(server.ExtensionHandler{
		Hook:        runtimehooksv1.BeforeClusterCreate,
		Name:        "before-cluster-create",
		HandlerFunc: DoBeforeClusterCreate,
	}); err != nil {
		setupLog.Error(err, "error adding handler")
	if err := webhookServer.AddExtensionHandler(server.ExtensionHandler{
		Hook:        runtimehooksv1.BeforeClusterUpgrade,
		Name:        "before-cluster-upgrade",
		HandlerFunc: DoBeforeClusterUpgrade,
	}); err != nil {
		setupLog.Error(err, "error adding handler")

	// Setup a context listening for SIGINT.
	ctx := ctrl.SetupSignalHandler()

	// Start the https server.
	setupLog.Info("Starting Runtime Extension server")
	if err := webhookServer.Start(ctx); err != nil {
		setupLog.Error(err, "error running webhook server")

func DoBeforeClusterCreate(ctx context.Context, request *runtimehooksv1.BeforeClusterCreateRequest, response *runtimehooksv1.BeforeClusterCreateResponse) {
	log := ctrl.LoggerFrom(ctx)
	log.Info("BeforeClusterCreate is called")
	// Your implementation

func DoBeforeClusterUpgrade(ctx context.Context, request *runtimehooksv1.BeforeClusterUpgradeRequest, response *runtimehooksv1.BeforeClusterUpgradeResponse) {
	log := ctrl.LoggerFrom(ctx)
	log.Info("BeforeClusterUpgrade is called")
	// Your implementation

For a full example see our test extension.

Please note that a Runtime Extension server can serve multiple Runtime Hooks (in the example above BeforeClusterCreate and BeforeClusterUpgrade) at the same time. Each of them are handled at a different path, like the Kubernetes API server does for different API resources. The exact format of those paths is handled by the server automatically in accordance to the OpenAPI specification of the Runtime Hooks.

There is an additional Discovery endpoint which is automatically served by the Server. The Discovery endpoint returns a list of extension handlers to inform Cluster API which Runtime Hooks are implemented by this Runtime Extension server.

Please note that Cluster API is only able to enforce the correct request and response types as defined by a Runtime Hook version. Developers are fully responsible for all other elements of the design of a Runtime Extension implementation, including:

  • To choose which programming language to use; please note that Golang is the language of choice, and we are not planning to test or provide tooling and libraries for other languages. Nevertheless, given that we rely on Open API and plain HTTPS calls, other languages should just work but support will be provided at best effort.
  • To choose if a dedicated or a shared HTTPS Server is used for the Runtime Extension (it can be e.g. also used to serve a metric endpoint).

When using Golang the Runtime Extension developer can benefit from the following packages (provided by the module) as shown in the example above:

  • exp/runtime/hooks/api/v1alpha1 contains the Runtime Hook Golang API types, which are also used to generate the OpenAPI specification.
  • exp/runtime/catalog provides the Catalog object to register Runtime Hook definitions. The Catalog is then used by the server package to handle requests. Catalog is similar to the runtime.Scheme of the package, but it is designed to store Runtime Hook registrations.
  • exp/runtime/server provides a Server object which makes it easy to implement a Runtime Extension server. The Server will automatically handle tasks like Marshalling/Unmarshalling requests and responses. A Runtime Extension developer only has to implement a strongly typed function that contains the actual logic.


While writing a Runtime Extension the following important guidelines must be considered:


Runtime Extension processing adds to reconcile durations of Cluster API controllers. They should respond to requests as quickly as possible, typically in milliseconds. Runtime Extension developers can decide how long the Cluster API Runtime should wait for a Runtime Extension to respond before treating the call as a failure (max is 30s) by returning the timeout during discovery. Of course a Runtime Extension can trigger long-running tasks in the background, but they shouldn’t block synchronously.


Runtime Extension failure could result in errors in handling the workload clusters lifecycle, and so the implementation should be robust, have proper error handling, avoid panics, etc. Failure policies can be set up to mitigate the negative impact of a Runtime Extension on the Cluster API Runtime, but this option can’t be used in all cases (see Error Management).

Blocking Hooks

A Runtime Hook can be defined as “blocking” - e.g. the BeforeClusterUpgrade hook allows a Runtime Extension to prevent the upgrade from starting. A Runtime Extension registered for the BeforeClusterUpgrade hook can block by returning a non-zero retryAfterSeconds value. Following consideration apply:

  • The system might decide to retry the same Runtime Extension even before the retryAfterSeconds period expires, e.g. due to other changes in the Cluster, so retryAfterSeconds should be considered as an approximate maximum time before the next reconcile.
  • If there is more than one Runtime Extension registered for the same Runtime Hook and more than one returns retryAfterSeconds, the shortest non-zero value will be used.
  • If there is more than one Runtime Extension registered for the same Runtime Hook and at least one returns retryAfterSeconds, all Runtime Extensions will be called again.

Detailed description of what “blocking” means for each specific Runtime Hooks is documented case by case in the hook-specific implementation documentation (e.g. Implementing Lifecycle Hook Runtime Extensions).

Side Effects

It is recommended that Runtime Extensions should avoid side effects if possible, which means they should operate only on the content of the request sent to them, and not make out-of-band changes. If side effects are required, rules defined in the following sections apply.


An idempotent Runtime Extension is able to succeed even in case it has already been completed before (the Runtime Extension checks current state and changes it only if necessary). This is necessary because a Runtime Extension may be called many times after it already succeeded because other Runtime Extensions for the same hook may not succeed in the same reconcile.

A practical example that explains why idempotence is relevant is the fact that extensions could be called more than once for the same lifecycle transition, e.g.

  • Two Runtime Extensions are registered for the BeforeClusterUpgrade hook.
  • Before a Cluster upgrade is started both extensions are called, but one of them temporarily blocks the operation by asking to retry after 30 seconds.
  • After 30 seconds the system retries the lifecycle transition, and both extensions are called again to re-evaluate if it is now possible to proceed with the Cluster upgrade.

Avoid dependencies

Each Runtime Extension should accomplish its task without depending on other Runtime Extensions. Introducing dependencies across Runtime Extensions makes the system fragile, and it is probably a consequence of poor “Separation of Concerns” between extensions.

Deterministic result

A deterministic Runtime Extension is implemented in such a way that given the same input it will always return the same output.

Some Runtime Hooks, e.g. like external patches, might explicitly request for corresponding Runtime Extensions to support this property. But we encourage developers to follow this pattern more generally given that it fits well with practices like unit testing and generally makes the entire system more predictable and easier to troubleshoot.

Error messages

RuntimeExtension authors should be aware that error messages are surfaced as a conditions in Kubernetes resources and recorded in Cluster API controller’s logs. As a consequence:

  • Error message must not contain any sensitive information.
  • Error message must be deterministic, and must avoid to including timestamps or values changing at every call.
  • Error message must not contain external errors when it’s not clear if those errors are deterministic (e.g. errors return from cloud APIs).


To register your runtime extension apply the ExtensionConfig resource in the management cluster, including your CA certs, ClusterIP service associated with the app and namespace, and the target namespace for the given extension. Once created, the extension will detect the associated service and discover the associated Hooks. For clarification, you can check the status of the ExtensionConfig. Below is an example of ExtensionConfig -

kind: ExtensionConfig
  annotations: default/test-runtime-sdk-svc-cert
  name: test-runtime-sdk-extensionconfig
      name: test-runtime-sdk-svc
      namespace: default # Note: this assumes the test extension get deployed in the default namespace
      port: 443
      - key:
        operator: In
          - default # Note: this assumes the test extension is used by Cluster in the default namespace only


Settings can be added to the ExtensionConfig object in the form of a map with string keys and values. These settings are sent with each request to hooks registered by that ExtensionConfig. Extension developers can implement behavior in their extensions to alter behavior based on these settings. Settings should be well documented by extension developers so that ClusterClass authors can understand usage and expected behaviour.

Settings can be provided for individual external patches by providing them in the ClusterClass .spec.patches[*].external.settings. This can be used to overwrite settings at the ExtensionConfig level for that patch.

Error management

In case a Runtime Extension returns an error, the error will be handled according to the corresponding failure policy defined in the response of the Discovery call.

If the failure policy is Ignore the error is going to be recorded in the controller’s logs, but the processing will continue. However we recognize that this failure policy cannot be used in most of the use cases because Runtime Extension implementers want to ensure that the task implemented by an extension is completed before continuing with the cluster’s lifecycle.

If instead the failure policy is Fail the system will retry the operation until it passes. The following general considerations apply:

  • It is the responsibility of Cluster API components to surface Runtime Extension errors using conditions.
  • Operations will be retried with an exponential backoff or whenever the state of a Cluster changes (we are going to rely on controller runtime exponential backoff/watches).
  • If there is more than one Runtime Extension registered for the same Runtime Hook and at least one of them fails, all the registered Runtime Extension will be retried. See Idempotence

Additional considerations about errors that apply only to a specific Runtime Hook will be documented in the hook-specific implementation documentation.

Tips & tricks

After you implemented and deployed a Runtime Extension you can manually test it by sending HTTP requests. This can be for example done via kubectl:

Via kubectl create --raw:

# Send a Discovery Request to the webhook-service in namespace default with protocol https on port 443:
kubectl create --raw '/api/v1/namespaces/default/services/https:webhook-service:443/proxy/' \
  -f <(echo '{"apiVersion":"","kind":"DiscoveryRequest"}') | jq

Via kubectl proxy and curl:

# Open a proxy with kubectl and then use curl to send the request
## First terminal:
kubectl proxy
## Second terminal:
curl -X 'POST' '' \
  -d '{"apiVersion":"","kind":"DiscoveryRequest"}' | jq

For more details about the API of the Runtime Extensions please see . For more details on proxy support please see Proxies in Kubernetes.