kubebuilder vs operator-sdk


kubebuilder and operator-sdk are two major tools used to set up a controller/operator code base for kubernetes CRDs. Both of them under the hood using controller-runtime to adding a manager, api scheme and creates an structs implements a reconciler interface, thus the major difference are around the tools for integration, testing and etc.


Kubernetes APIs provide consistent and well defined endpoints for objects adhering to a consistent and rich structure. A resource is an endpoint in the Kubernetes API that stores a collection of API objects of a certain kind. For example, the built-in pods resource contains a collection of Pod objects. A custom resource (CR) is an extension of the Kubernetes API that is not necessarily available in a default Kubernetes installation. It represents a customization of a particular Kubernetes installation. A CR is dynamically registered by Custom Resource Definition (CRD).

Building services as Kubernetes APIs provides many advantages to plain old REST, including:

Developers may build and publish their own Kubernetes APIs for installation into running Kubernetes clusters.

In order to build and publish Kubernetes APIs in Go, there are two SDKs that provides abstractions and scaffolds to reduce developers’ time spent on setting up project, so the developers could focus on features velocity. These two SDKs are kubebuilder and operator-sdk.


kubebuilder is one of the kubernetes-sigs (Special Interest Groups), part of the API Machinery group, both owners are from google, most commits from google, redhat.

kubebuilder v1.0.0 was released at July 19, 2018, the latest release is v1.0.8, the document could be found at On github there are 1129 stars, 205 forks, 61 contributors at the time this is written.


operator-sdk is part of operator-framework, part of CNCF landscape (, owners are from redhat, most commits from redhat.

operator-sdk first release v0.1.0 was at Oct 31, 2018, the latest release is v0.7.0, the document could be found at On github there are 1606 stars, 353 forks, 75 contributors at the time this is written.


By comparing the backgrouds of two projects, they are pretty similar, we call it even on this round.


Installing kubebuilder and operator-sdk are pretty straight-forward, belows are steps for each of them.

Install kubebuilder

To install kubebuilder, there is a script from kubebuilder repo that helps to install the tools quickly.


Meanwhile, since kubebuilder using kustomize for testing, we also need to install it, we will look into it later.

# on OSX
brew install kustomize

Since there is no go dependency on kubebuilder in the controller artefact, we don’t need to use a fixed version of kubebuilder, in another word changing kubebuilder version only impacts the scaffold codes.

Install operator-sdk

To install operator-sdk, user can easily run

brew install operator-sdk

Of course in order to run testing with helm, tools like helm needs to be installed.

operator-sdk is imported as part of the artefact it builds (though only a tiny portion), a fixed version that matches vendored go package is preferred.


By comparing the installing process, both of these two SDKs provide simple way to install. Though the kubebuilder decouples the tool setting up the scaffolds and the actual codes, wins a little.


Development workflow for both tools are well-documented, the following sections compare them in steps.

Create new project

kubebuilder creates a project by run kubebuilder init --domain --license apache2 --owner "The Kubernetes Authors", after it completes, the project will look like below

├── Dockerfile
├── Gopkg.lock
├── Gopkg.toml
├── Makefile
├── bin
├── cmd
│   └── manager
├── config
│   ├── crds
│   ├── default
│   ├── manager
│   ├── rbac
│   └── samples
├── hack
├── pkg
│   ├── apis
│   ├── controller
│   └── webhook
└── vendor

operator-sdk creates a project by run operator-sdk new app-operator, after it completes, the project will look like below

├── Gopkg.lock
├── Gopkg.toml
├── build
│   ├── Dockerfile
│   └── bin
├── cmd
│   └── manager
├── deploy
│   ├── operator.yaml
│   ├── role.yaml
│   ├── role_binding.yaml
│   └── service_account.yaml
├── pkg
│   ├── apis
│   └── controller
├── vendor
└── version

Create New API (Resource)

kubebuilder creates a new API by run kubebuilder create api --group mygroup --version v1beta1 --kind MyKind. It will create a resource type under apis/mygroup/v1beta1/ directory and a controller (by answer y when it asks if need to create controller) under controler/mykind directory.

operator-sdk creates a new API by run operator-sdk add api --kind=AppService and creates a controller for it by run operator-sdk add controller --kind=AppService. The first step will create an API under apis/app/v1alpha1/ and the second will create a controller under controller/appservice/

Difference on created resources:

Other than above, there is really no much difference between the codes generated by the SDKs. All the codes generated for API are an implemetation of runtime.Object interface and all the codes for controller are an implemetation of reconcile.Reconciler.

Generate manifests

In the project set up by kubebuilder, user could run make generate and make manifests to generate the codes and manifests. It uses go generate and go build hooks to generate the codes and manifests (includes CRDs). Whenever user makes a change on the API spec, add a validation for a field, managing subresources (e.g. status, scale), printcolumn, they can just update the codes and re-run make manifests, the CRDs and role-binding will be generated with proper configuration.


# in code
type IPAddressSpec struct {
    // IPAllocatorName is the IPAllocator's name used to allocate this ip.
    // +kubebuilder:validation:MinLength=1
    IPAllocatorName string `json:"ipallocatorName"`

# in CRD yaml
  description: IPAllocatorName is the IPAllocator's name used to allocate
    this ip.
  minLength: 1
  type: string
# in code
// IPAllocator is the Schema for the ipallocators API
// +genclient
// +kubebuilder:printcolumn:name="source",type="string",JSONPath=".spec.source"
// +kubebuilder:printcolumn:name="state",type="string",JSONPath=".status.state"
// +kubebuilder:printcolumn:name="free",type="string",JSONPath=""
// +kubebuilder:subresource:status
// +k8s:openapi-gen=true
type IPAllocator struct { ... }

# in CRD yaml
  - JSONPath: .spec.source
    name: source
    type: string
  - JSONPath: .status.state
    name: state
    type: string
  - JSONPath:
    name: free
    type: string
    status: {}
func (r *ReconcileIPAllocator) Reconcile(request reconcile.Request) (reconcile.Result, error) { ... }

In the project setup by operator-sdk, user could run operator-sdk generate k8s to generate codes, but it doesn’t provides updating of CRD and roles through codes.


kubebuilder will create testing scaffolds for all API and controller it creates, named as _test.go, it uses gomega as testing framework, and uses controller-runtime/pkg/envtest as to bring up a testing environment. Since kubebuilder is part of kubernetes-sigs, it has better set up to work with the existing kubernetes-sigs projects. During kubebuilder installation, it will put the kubebuilder binary with a etcd, kube-apiserver and kubectl under /usr/local/kubebuilder/bin/, which is the default binary path for the envtest. During testing of controller, it will bring up an actual kube-apiserver and etcd (done by envtest), and doing the testing. To run tests, there is a make target for it, user could run make test to start testing.

On the other hand, operator-sdk doesn’t generates testing scaffolds, instead it only provdes a document. User follows the document could run operator-sdk test local to test the controller.

Deploy & Run

To deploy CRDs, kubebuilder provides a make target to deploy it easiy, while operator-sdk is a manual process (write your own makefile).

# kubebuilder
make install

# operator-sdk
kubectl create -f deploy/crds/

Both kubebuilder and operator-sdk provide two options to run the artefact, outside and inside a k8s cluster.

With kubebuilder, to run the controller outside k8s cluster as binary, run make run, to run the controller within a cluster, need to run make docker-build deploy which builds out the docker container, and generates deploy YAML files by kustomize, then get applied on a k8s cluster.

With operator-sdk, to run the controller outside k8s cluster as bianry, run operator-sdk up local. to run the controller within a cluster as a pod, after run operator-sdk build to build the docker image, then deploy it either using its helm watcher, or manually with YAML files. Meanwhile, operator-sdk provides a watcher helpers for helm and ansible, that automatically reload containers/configs when there is change made.


kubebuilder provides a webhook service, metric service and upstream is working on document generating for a project.

operator-sdk provides metric service and integration with helm and ansible.


Looking through the developing steps for both tools, the kubebuilder is the winner, the best part of it is it generates CRDs and roles from codes, so we don’t have to manually manages it by editing YAML files. The operator-sdk also has some extra features like helm watcher that helps developing.


Both kubebuilder and operator-sdk are good tools to start writing k8s controllers, they are well documented, having good communities and provide helper commands to start a new project or adding new controller. The operator-sdk has better integration with helm, and having better integration with the operator-framework ecosystem, while the kuebuilder integrates with existing kubernetes-sigs projects naturally. This post goes through background , installing and development process for both tools, and I think the kuebuilder could provides better management over codes, so I would suggest to use kuebuilder to start a new projects.

Wei T. 2019-04-10