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Custom Resources

Custom resources are extensions of the Kubernetes API. This page discusses when to add a custom resource to your Kubernetes cluster and when to use a standalone service. It describes the two methods for adding custom resources and how to choose between them.

Custom resources

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 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. However, many core Kubernetes functions are now built using custom resources, making Kubernetes more modular.

Custom resources can appear and disappear in a running cluster through dynamic registration, and cluster admins can update custom resources independently of the cluster itself. Once a custom resource is installed, users can create and access its objects using kubectl, just as they do for built-in resources like Pods.

Custom controllers

On their own, custom resources simply let you store and retrieve structured data. When you combine a custom resource with a custom controller, custom resources provide a true declarative API.

A declarative API allows you to declare or specify the desired state of your resource and tries to keep the current state of Kubernetes objects in sync with the desired state. The controller interprets the structured data as a record of the user’s desired state, and continually maintains this state.

You can deploy and update a custom controller on a running cluster, independently of the cluster’s own lifecycle. Custom controllers can work with any kind of resource, but they are especially effective when combined with custom resources. The Operator pattern combines custom resources and custom controllers. You can use custom controllers to encode domain knowledge for specific applications into an extension of the Kubernetes API.

Should I add a custom resource to my Kubernetes Cluster?

When creating a new API, consider whether to aggregate your API with the Kubernetes cluster APIs or let your API stand alone.

Consider API aggregation if: Prefer a stand-alone API if:
Your API is Declarative. Your API does not fit the Declarative model.
You want your new types to be readable and writable using kubectl. kubectl support is not required
You want to view your new types in a Kubernetes UI, such as dashboard, alongside built-in types. Kubernetes UI support is not required.
You are developing a new API. You already have a program that serves your API and works well.
You are willing to accept the format restriction that Kubernetes puts on REST resource paths, such as API Groups and Namespaces. (See the API Overview.) You need to have specific REST paths to be compatible with an already defined REST API.
Your resources are naturally scoped to a cluster or to namespaces of a cluster. Cluster or namespace scoped resources are a poor fit; you need control over the specifics of resource paths.
You want to reuse Kubernetes API support features. You don’t need those features.

Declarative APIs

In a Declarative API, typically:

Imperative APIs are not declarative. Signs that your API might not be declarative include:

Should I use a configMap or a custom resource?

Use a ConfigMap if any of the following apply:

Note: Use a secret for sensitive data, which is similar to a configMap but more secure.

Use a custom resource (CRD or Aggregated API) if most of the following apply:

Adding custom resources

Kubernetes provides two ways to add custom resources to your cluster:

Kubernetes provides these two options to meet the needs of different users, so that neither ease of use nor flexibility is compromised.

Aggregated APIs are subordinate APIServers that sit behind the primary API server, which acts as a proxy. This arrangement is called API Aggregation (AA). To users, it simply appears that the Kubernetes API is extended.

CRDs allow users to create new types of resources without adding another APIserver. You do not need to understand API Aggregation to use CRDs.

Regardless of how they are installed, the new resources are referred to as Custom Resources to distinguish them from built-in Kubernetes resources (like pods).

CustomResourceDefinitions

The CustomResourceDefinition API resource allows you to define custom resources. Defining a CRD object creates a new custom resource with a name and schema that you specify. The Kubernetes API serves and handles the storage of your custom resource.

This frees you from writing your own API server to handle the custom resource, but the generic nature of the implementation means you have less flexibility than with API server aggregation.

Refer to the custom controller example for an example of how to register a new custom resource, work with instances of your new resource type, and use a controller to handle events.

API server aggregation

Usually, each resource in the Kubernetes API requires code that handles REST requests and manages persistent storage of objects. The main Kubernetes API server handles built-in resources like pods and services, and can also handle custom resources in a generic way through CRDs.

The aggregation layer allows you to provide specialized implementations for your custom resources by writing and deploying your own standalone API server. The main API server delegates requests to you for the custom resources that you handle, making them available to all of its clients.

Choosing a method for adding custom resources

CRDs are easier to use. Aggregated APIs are more flexible. Choose the method that best meets your needs.

Typically, CRDs are a good fit if:

Comparing ease of use

CRDs are easier to create than Aggregated APIs.

CRDs Aggregated API
Do not require programming. Users can choose any language for a CRD controller. Requires programming in Go and building binary and image. Users can choose any language for a CRD controller.
No additional service to run; CRs are handled by API Server. An additional service to create and that could fail.
No ongoing support once the CRD is created. Any bug fixes are picked up as part of normal Kubernetes Master upgrades. May need to periodically pickup bug fixes from upstream and rebuild and update the Aggregated APIserver.
No need to handle multiple versions of your API. For example: when you control the client for this resource, you can upgrade it in sync with the API. You need to handle multiple versions of your API, for example: when developing an extension to share with the world.

Advanced features and flexibility

Aggregated APIs offer more advanced API features and customization of other features, for example: the storage layer.

Feature Description CRDs Aggregated API
Validation Help users prevent errors and allow you to evolve your API independently of your clients. These features are most useful when there are many clients who can’t all update at the same time. Yes. Most validation can be specified in the CRD using OpenAPI v3.0 validation. Any other validations supported by addition of a Validating Webhook. Yes, arbitrary validation checks
Defaulting See above Yes, via a Mutating Webhook; Planned, via CRD OpenAPI schema. Yes
Multi-versioning Allows serving the same object through two API versions. Can help ease API changes like renaming fields. Less important if you control your client versions. Yes Yes
Custom Storage If you need storage with a different performance mode (for example, time-series database instead of key-value store) or isolation for security (for example, encryption secrets or different No Yes
Custom Business Logic Perform arbitrary checks or actions when creating, reading, updating or deleting an object Yes, using Webhooks. Yes
Scale Subresource Allows systems like HorizontalPodAutoscaler and PodDisruptionBudget interact with your new resource Yes Yes
Status Subresource
  • Finer-grained access control: user writes spec section, controller writes status section.
  • Allows incrementing object Generation on custom resource data mutation (requires separate spec and status sections in the resource)
 Yes Yes
Other Subresources Add operations other than CRUD, such as “logs” or “exec”. No Yes
strategic-merge-patch The new endpoints support PATCH with Content-Type: application/strategic-merge-patch+json. Useful for updating objects that may be modified both locally, and by the server. For more information, see “Update API Objects in Place Using kubectl patch” No Yes
Protocol Buffers The new resource supports clients that want to use Protocol Buffers No Yes
OpenAPI Schema Is there an OpenAPI (swagger) schema for the types that can be dynamically fetched from the server? Is the user protected from misspelling field names by ensuring only allowed fields are set? Are types enforced (in other words, don’t put an int in a string field?) No, but planned Yes

Common Features

When you create a custom resource, either via a CRDs or an AA, you get many features for your API, compared to implementing it outside the Kubernetes platform:

Feature What it does
CRUD The new endpoints support CRUD basic operations via HTTP and kubectl
Watch The new endpoints support Kubernetes Watch operations via HTTP
Discovery Clients like kubectl and dashboard automatically offer list, display, and field edit operations on your resources
json-patch The new endpoints support PATCH with Content-Type: application/json-patch+json
merge-patch The new endpoints support PATCH with Content-Type: application/merge-patch+json
HTTPS The new endpoints uses HTTPS
Built-in Authentication Access to the extension uses the core apiserver (aggregation layer) for authentication
Built-in Authorization Access to the extension can reuse the authorization used by the core apiserver (e.g. RBAC)
Finalizers Block deletion of extension resources until external cleanup happens.
Admission Webhooks Set default values and validate extension resources during any create/update/delete operation.
UI/CLI Display Kubectl, dashboard can display extension resources.
Unset vs Empty Clients can distinguish unset fields from zero-valued fields.
Client Libraries Generation Kubernetes provides generic client libraries, as well as tools to generate type-specific client libraries.
Labels and annotations Common metadata across objects that tools know how to edit for core and custom resources.

Preparing to install a custom resource

There are several points to be aware of before adding a custom resource to your cluster.

Third party code and new points of failure

While creating a CRD does not automatically add any new points of failure (for example, by causing third party code to run on your API server), packages (for example, Charts) or other installation bundles often include CRDs as well as a Deployment of third-party code that implements the business logic for a new custom resource.

Installing an Aggregated APIserver always involves running a new Deployment.

Storage

Custom resources consume storage space in the same way that ConfigMaps do. Creating too many custom resources may overload your API server’s storage space.

Aggregated API servers may use the same storage as the main API server, in which case the same warning applies.

Authentication, authorization, and auditing

CRDs always use the same authentication, authorization, and audit logging as the built-in resources of your API Server.

If you use RBAC for authorization, most RBAC roles will not grant access to the new resources (except the cluster-admin role or any role created with wildcard rules). You’ll need to explicitly grant access to the new resources. CRDs and Aggregated APIs often come bundled with new role definitions for the types they add.

Aggregated API servers may or may not use the same authentication, authorization, and auditing as the primary API server.

Accessing a custom resource

Kubernetes client libraries can be used to access custom resources. Not all client libraries support custom resources. The go and python client libraries do.

When you add a custom resource, you can access it using:

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