controller-runtime: Index | Files | Directories

package pkg

import ""

Package pkg provides libraries for building Controllers. Controllers implement Kubernetes APIs and are foundational to building Operators, Workload APIs, Configuration APIs, Autoscalers, and more.


Client provides a Read + Write client for reading and writing Kubernetes objects.


Cache provides a Read client for reading objects from a local cache. A cache may register handlers to respond to events that update the cache.


Manager is required for creating a Controller and provides the Controller shared dependencies such as clients, caches, schemes, etc. Controllers should be Started through the Manager by calling Manager.Start.


Controller implements a Kubernetes API by responding to events (object Create, Update, Delete) and ensuring that the state specified in the Spec of the object matches the state of the system. This is called a reconcile. If they do not match, the Controller will create / update / delete objects as needed to make them match.

Controllers are implemented as worker queues that process reconcile.Requests (requests to reconcile the state for a specific object).

Unlike http handlers, Controllers DO NOT handle events directly, but enqueue Requests to eventually reconcile the object. This means the handling of multiple events may be batched together and the full state of the system must be read for each reconcile.

* Controllers require a Reconciler to be provided to perform the work pulled from the work queue.

* Controllers require Watches to be configured to enqueue reconcile.Requests in response to events.


Admission Webhooks are a mechanism for extending kubernetes APIs. Webhooks can be configured with target event type (object Create, Update, Delete), the API server will send AdmissionRequests to them when certain events happen. The webhooks may mutate and (or) validate the object embedded in the AdmissionReview requests and send back the response to the API server.

There are 2 types of admission webhook: mutating and validating admission webhook. Mutating webhook is used to mutate a core API object or a CRD instance before the API server admits it. Validating webhook is used to validate if an object meets certain requirements.

* Admission Webhooks require Handler(s) to be provided to process the received AdmissionReview requests.


Reconciler is a function provided to a Controller that may be called at anytime with the Name and Namespace of an object. When called, the Reconciler will ensure that the state of the system matches what is specified in the object at the time the Reconciler is called.

Example: Reconciler invoked for a ReplicaSet object. The ReplicaSet specifies 5 replicas but only 3 Pods exist in the system. The Reconciler creates 2 more Pods and sets their OwnerReference to point at the ReplicaSet with controller=true.

* Reconciler contains all of the business logic of a Controller.

* Reconciler typically works on a single object type. - e.g. it will only reconcile ReplicaSets. For separate types use separate Controllers. If you wish to trigger reconciles from other objects, you can provide a mapping (e.g. owner references) that maps the object that triggers the reconcile to the object being reconciled.

* Reconciler is provided the Name / Namespace of the object to reconcile.

* Reconciler does not care about the event contents or event type responsible for triggering the reconcile. - e.g. it doesn't matter whether a ReplicaSet was created or updated, Reconciler will always compare the number of Pods in the system against what is specified in the object at the time it is called.


resource.Source is an argument to Controller.Watch that provides a stream of events. Events typically come from watching Kubernetes APIs (e.g. Pod Create, Update, Delete).

Example: source.Kind uses the Kubernetes API Watch endpoint for a GroupVersionKind to provide Create, Update, Delete events.

* Source provides a stream of events (e.g. object Create, Update, Delete) for Kubernetes objects typically through the Watch API.

* Users SHOULD only use the provided Source implementations instead of implementing their own for nearly all cases.


handler.EventHandler is a argument to Controller.Watch that enqueues reconcile.Requests in response to events.

Example: a Pod Create event from a Source is provided to the eventhandler.EnqueueHandler, which enqueues a reconcile.Request containing the name / Namespace of the Pod.

* EventHandlers handle events by enqueueing reconcile.Requests for one or more objects.

* EventHandlers MAY map an event for an object to a reconcile.Request for an object of the same type.

* EventHandlers MAY map an event for an object to a reconcile.Request for an object of a different type - e.g. map a Pod event to a reconcile.Request for the owning ReplicaSet.

* EventHandlers MAY map an event for an object to multiple reconcile.Requests for objects of the same or a different type - e.g. map a Node event to objects that respond to cluster resize events.

* Users SHOULD only use the provided EventHandler implementations instead of implementing their own for almost all cases.


predicate.Predicate is an optional argument to Controller.Watch that filters events. This allows common filters to be reused and composed.

* Predicate takes an event and returns a bool (true to enqueue)

* Predicates are optional arguments

* Users SHOULD use the provided Predicate implementations, but MAY implement additional Predicates e.g. generation changed, label selectors changed etc.

PodController Diagram

Source provides event:

* &source.KindSource{&v1.Pod{}} -> (Pod foo/bar Create Event)

EventHandler enqueues Request:

* &handler.EnqueueRequestForObject{} -> (reconcile.Request{types.NamespaceName{Name: "foo", Namespace: "bar"}})

Reconciler is called with the Request:

* Reconciler(reconcile.Request{types.NamespaceName{Name: "foo", Namespace: "bar"}})


The following example shows creating a new Controller program which Reconciles ReplicaSet objects in response to Pod or ReplicaSet events. The Reconciler function simply adds a label to the ReplicaSet.

See the example/main.go for a usage example.

Controller Example

1. Watch ReplicaSet and Pods Sources

1.1 ReplicaSet -> handler.EnqueueRequestForObject - enqueue a Request with the ReplicaSet Namespace and Name.

1.2 Pod (created by ReplicaSet) -> handler.EnqueueRequestForOwnerHandler - enqueue a Request with the Owning ReplicaSet Namespace and Name.

2. reconcile ReplicaSet in response to an event

2.1 ReplicaSet object created -> Read ReplicaSet, try to read Pods -> if is missing create Pods.

2.2 Reconciler triggered by creation of Pods -> Read ReplicaSet and Pods, do nothing.

2.3 Reconciler triggered by deletion of Pods from some other actor -> Read ReplicaSet and Pods, create replacement Pods.

Watching and EventHandling

Controllers may Watch multiple Kinds of objects (e.g. Pods, ReplicaSets and Deployments), but they reconcile only a single Type. When one Type of object must be updated in response to changes in another Type of object, an EnqueueRequestFromMapFunc may be used to map events from one type to another. e.g. Respond to a cluster resize event (add / delete Node) by re-reconciling all instances of some API.

A Deployment Controller might use an EnqueueRequestForObject and EnqueueRequestForOwner to:

* Watch for Deployment Events - enqueue the Namespace and Name of the Deployment.

* Watch for ReplicaSet Events - enqueue the Namespace and Name of the Deployment that created the ReplicaSet (e.g the Owner)

Note: reconcile.Requests are deduplicated when they are enqueued. Many Pod Events for the same ReplicaSet may trigger only 1 reconcile invocation as each Event results in the Handler trying to enqueue the same reconcile.Request for the ReplicaSet.

Controller Writing Tips

Reconciler Runtime Complexity:

* It is better to write Controllers to perform an O(1) reconcile N times (e.g. on N different objects) instead of performing an O(N) reconcile 1 time (e.g. on a single object which manages N other objects).

* Example: If you need to update all Services in response to a Node being added - reconcile Services but Watch Nodes (transformed to Service object name / Namespaces) instead of Reconciling Nodes and updating Services

Event Multiplexing:

* reconcile.Requests for the same Name / Namespace are batched and deduplicated when they are enqueued. This allows Controllers to gracefully handle a high volume of events for a single object. Multiplexing multiple event Sources to a single object Type will batch requests across events for different object types.

* Example: Pod events for a ReplicaSet are transformed to a ReplicaSet Name / Namespace, so the ReplicaSet will be Reconciled only 1 time for multiple events from multiple Pods.


Package Files



builderPackage builder provides wraps other controller-runtime libraries and exposes simple patterns for building common Controllers.
cachePackage cache provides object caches that act as caching client.Reader instances and help drive Kubernetes-object-based event handlers.
clientPackage client contains functionality for interacting with Kubernetes API servers.
client/apiutilPackage apiutil contains utilities for working with raw Kubernetes API machinery, such as creating RESTMappers and raw REST clients, and extracting the GVK of an object.
client/configPackage config contains libraries for initializing REST configs for talking to the Kubernetes API
client/fakePackage fake provides a fake client for testing.
controllerPackage controller provides types and functions for building Controllers.
controller/controllertestPackage controllertest contains fake informers for testing controllers When in doubt, it's almost always better to test against a real API server using envtest.Environment.
controller/controllerutilPackage controllerutil contains utility functions for working with and implementing Controllers.
conversionPackage conversion provides interface definitions that an API Type needs to implement for it to be supported by the generic conversion webhook handler defined under pkg/webhook/conversion.
envtestPackage envtest provides libraries for integration testing by starting a local control plane
envtest/printerPackage printer contains setup for a friendlier Ginkgo printer that's easier to parse by test automation.
eventPackage event contains the definitions for the Event types produced by source.Sources and transformed into reconcile.Requests by handler.EventHandler.
handlerPackage handler defines EventHandlers that enqueue reconcile.Requests in response to Create, Update, Deletion Events observed from Watching Kubernetes APIs.
internal/logPackage log contains utilities for fetching a new logger when one is not already available.
leaderelectionPackage leaderelection contains a constructors for a leader election resource lock.
leaderelection/fakePackage fake mocks a resource lock for testing purposes.
logPackage log contains utilities for fetching a new logger when one is not already available.
log/zapPackage zap contains helpers for setting up a new logr.Logger instance using the Zap logging framework.
managerPackage manager is required to create Controllers and provides shared dependencies such as clients, caches, schemes, etc.
manager/signalsPackage signals contains libraries for handling signals to gracefully shutdown the manager in combination with Kubernetes pod graceful termination policy.
metricsPackage metrics contains controller related metrics utilities
predicatePackage predicate defines Predicates used by Controllers to filter Events before they are provided to EventHandlers.
reconcilePackage reconcile defines the Reconciler interface to implement Kubernetes APIs.
recorderPackage recorder defines interfaces for working with Kubernetes event recorders.
runtimePackage runtime contains not-quite-internal mechanisms for controller-runtime, plus some deprecated exports of functionality moved elsewhere.
runtime/injectPackage inject defines interfaces and functions for propagating dependencies from a ControllerManager to the components registered with it.
runtime/logPackage log contains (deprecated) utilities for fetching a new logger when one is not already available.
runtime/schemePackage scheme contains utilities for gradually building Schemes, which contain information associating Go types with Kubernetes groups, versions, and kinds.
runtime/signalsPackage signals contains libraries for handling signals to gracefully shutdown the manager in combination with Kubernetes pod graceful termination policy.
schemePackage scheme contains utilities for gradually building Schemes, which contain information associating Go types with Kubernetes groups, versions, and kinds.
sourcePackage source provides event streams to hook up to Controllers with Controller.Watch.
webhookPackage webhook provides methods to build and bootstrap a webhook server.
webhook/admissionPackage admission provides implementation for admission webhook and methods to implement admission webhook handlers.
webhook/conversionPackage conversion provides implementation for CRD conversion webhook that implements handler for version conversion requests for types that are convertible.

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