applyconfigurations

Documentation

Overview

Package applyconfigurations provides typesafe go representations of the apply configurations that are used to constructs Server-side Apply requests.

Basics

The Apply functions in the typed client (see the k8s.io/client-go/kubernetes/typed packages) offer a direct and typesafe way of calling Server-side Apply. Each Apply function takes an "apply configuration" type as an argument, which is a structured representation of an Apply request. For example:

import (
     ...
     v1ac "k8s.io/client-go/applyconfigurations/autoscaling/v1"
)
hpaApplyConfig := v1ac.HorizontalPodAutoscaler(autoscalerName, ns).
     WithSpec(v1ac.HorizontalPodAutoscalerSpec().
              WithMinReplicas(0)
     )
return hpav1client.Apply(ctx, hpaApplyConfig, metav1.ApplyOptions{FieldManager: "mycontroller", Force: true})

Note in this example that HorizontalPodAutoscaler is imported from an "applyconfigurations" package. Each "apply configuration" type represents the same Kubernetes object kind as the corresponding go struct, but where all fields are pointers to make them optional, allowing apply requests to be accurately represented. For example, this when the apply configuration in the above example is marshalled to YAML, it produces:

apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
    name: myHPA
    namespace: myNamespace
spec:
    minReplicas: 0

To understand why this is needed, the above YAML cannot be produced by the v1.HorizontalPodAutoscaler go struct. Take for example:

hpa := v1.HorizontalPodAutoscaler{
     TypeMeta: metav1.TypeMeta{
              APIVersion: "autoscaling/v1",
              Kind:       "HorizontalPodAutoscaler",
     },
     ObjectMeta: ObjectMeta{
              Namespace: ns,
              Name:      autoscalerName,
     },
     Spec: v1.HorizontalPodAutoscalerSpec{
              MinReplicas: pointer.Int32Ptr(0),
     },
}

The above code attempts to declare the same apply configuration as shown in the previous examples, but when marshalled to YAML, produces:

kind: HorizontalPodAutoscaler
apiVersion: autoscaling/v1
metadata:
  name: myHPA
  namespace: myNamespace
  creationTimestamp: null
spec:
  scaleTargetRef:
    kind: ""
    name: ""
  minReplicas: 0
  maxReplicas: 0

Which, among other things, contains spec.maxReplicas set to 0. This is almost certainly not what the caller intended (the intended apply configuration says nothing about the maxReplicas field), and could have serious consequences on a production system: it directs the autoscaler to downscale to zero pods. The problem here originates from the fact that the go structs contain required fields that are zero valued if not set explicitly. The go structs work as intended for create and update operations, but are fundamentally incompatible with apply, which is why we have introduced the generated "apply configuration" types.

The "apply configurations" also have convenience With<FieldName> functions that make it easier to build apply requests. This allows developers to set fields without having to deal with the fact that all the fields in the "apply configuration" types are pointers, and are inconvenient to set using go. For example "MinReplicas: &0" is not legal go code, so without the With functions, developers would work around this problem by using a library, .e.g. "MinReplicas: pointer.Int32Ptr(0)", but string enumerations like corev1.Protocol are still a problem since they cannot be supported by a general purpose library. In addition to the convenience, the With functions also isolate developers from the underlying representation, which makes it safer for the underlying representation to be changed to support additional features in the future.

Controller Support

The new client-go support makes it much easier to use Server-side Apply in controllers, by either of two mechanisms.

Mechanism 1:

When authoring new controllers to use Server-side Apply, a good approach is to have the controller recreate the apply configuration for an object each time it reconciles that object. This ensures that the controller fully reconciles all the fields that it is responsible for. Controllers typically should unconditionally set all the fields they own by setting "Force: true" in the ApplyOptions. Controllers must also provide a FieldManager name that is unique to the reconciliation loop that apply is called from.

When upgrading existing controllers to use Server-side Apply the same approach often works well--migrate the controllers to recreate the apply configuration each time it reconciles any object. For cases where this does not work well, see Mechanism 2.

Mechanism 2:

When upgrading existing controllers to use Server-side Apply, the controller might have multiple code paths that update different parts of an object depending on various conditions. Migrating a controller like this to Server-side Apply can be risky because if the controller forgets to include any fields in an apply configuration that is included in a previous apply request, a field can be accidentally deleted. For such cases, an alternative to mechanism 1 is to replace any controller reconciliation code that performs a "read/modify-in-place/update" (or patch) workflow with a "extract/modify-in-place/apply" workflow. Here's an example of the new workflow:

    fieldMgr := "my-field-manager"
    deploymentClient := clientset.AppsV1().Deployments("default")
    // read, could also be read from a shared informer
    deployment, err := deploymentClient.Get(ctx, "example-deployment", metav1.GetOptions{})
    if err != nil {
      // handle error
    }
    // extract
    deploymentApplyConfig, err := appsv1ac.ExtractDeployment(deployment, fieldMgr)
    if err != nil {
      // handle error
    }
    // modify-in-place
    deploymentApplyConfig.Spec.Template.Spec.WithContainers(corev1ac.Container().
	WithName("modify-slice").
	WithImage("nginx:1.14.2"),
    )
    // apply
    applied, err := deploymentClient.Apply(ctx, extractedDeployment, metav1.ApplyOptions{FieldManager: fieldMgr})

Index

• func ForKind(kind schema.GroupVersionKind) interface{}

Functions

func ForKind

func ForKind(kind schema.GroupVersionKind) interface{}

ForKind returns an apply configuration type for the given GroupVersionKind, or nil if no apply configuration type exists for the given GroupVersionKind.