reconcile.go

/*
Copyright 2018 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package reconcile

import (
	"time"

	"k8s.io/apimachinery/pkg/types"
)

// Result contains the result of a Reconciler invocation.
type Result struct {
	// Requeue tells the Controller to requeue the reconcile key.  Defaults to false.
	Requeue bool

	// RequeueAfter if greater than 0, tells the Controller to requeue the reconcile key after the Duration.
	// Implies that Requeue is true, there is no need to set Requeue to true at the same time as RequeueAfter.
	RequeueAfter time.Duration
}

// IsZero returns true if this result is empty.
func (r *Result) IsZero() bool {
	if r == nil {
		return true
	}
	return *r == Result{}
}

// Request contains the information necessary to reconcile a Kubernetes object.  This includes the
// information to uniquely identify the object - its Name and Namespace.  It does NOT contain information about
// any specific Event or the object contents itself.
type Request struct {
	// NamespacedName is the name and namespace of the object to reconcile.
	types.NamespacedName
}

/*
Reconciler implements a Kubernetes API for a specific Resource by Creating, Updating or Deleting Kubernetes
objects, or by making changes to systems external to the cluster (e.g. cloudproviders, github, etc).

reconcile implementations compare the state specified in an object by a user against the actual cluster state,
and then perform operations to make the actual cluster state reflect the state specified by the user.

Typically, reconcile is triggered by a Controller in response to cluster Events (e.g. Creating, Updating,
Deleting Kubernetes objects) or external Events (GitHub Webhooks, polling external sources, etc).

Example reconcile Logic:

	* Read an object and all the Pods it owns.
	* Observe that the object spec specifies 5 replicas but actual cluster contains only 1 Pod replica.
	* Create 4 Pods and set their OwnerReferences to the object.

reconcile may be implemented as either a type:

	type reconcile struct {}

	func (reconcile) reconcile(controller.Request) (controller.Result, error) {
		// Implement business logic of reading and writing objects here
		return controller.Result{}, nil
	}

Or as a function:

	controller.Func(func(o controller.Request) (controller.Result, error) {
		// Implement business logic of reading and writing objects here
		return controller.Result{}, nil
	})

Reconciliation is level-based, meaning action isn't driven off changes in individual Events, but instead is
driven by actual cluster state read from the apiserver or a local cache.
For example if responding to a Pod Delete Event, the Request won't contain that a Pod was deleted,
instead the reconcile function observes this when reading the cluster state and seeing the Pod as missing.
*/
type Reconciler interface {
	// Reconciler performs a full reconciliation for the object referred to by the Request.
	// The Controller will requeue the Request to be processed again if an error is non-nil or
	// Result.Requeue is true, otherwise upon completion it will remove the work from the queue.
	Reconcile(Request) (Result, error)
}

// Func is a function that implements the reconcile interface.
type Func func(Request) (Result, error)

var _ Reconciler = Func(nil)

// Reconcile implements Reconciler.
func (r Func) Reconcile(o Request) (Result, error) { return r(o) }