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Extending Kubernetes

Different ways to change the behavior of your Kubernetes cluster.

Kubernetes is highly configurable and extensible. As a result, there is rarely a need to fork or submit patches to the Kubernetes project code.

This guide describes the options for customizing a Kubernetes cluster. It is aimed at cluster operators who want to understand how to adapt their Kubernetes cluster to the needs of their work environment. Developers who are prospective Platform Developers or Kubernetes Project Contributors will also find it useful as an introduction to what extension points and patterns exist, and their trade-offs and limitations.

Customization approaches can be broadly divided into configuration, which only involves changing command line arguments, local configuration files, or API resources; and extensions, which involve running additional programs, additional network services, or both. This document is primarily about extensions.


Configuration files and command arguments are documented in the Reference section of the online documentation, with a page for each binary:

Command arguments and configuration files may not always be changeable in a hosted Kubernetes service or a distribution with managed installation. When they are changeable, they are usually only changeable by the cluster operator. Also, they are subject to change in future Kubernetes versions, and setting them may require restarting processes. For those reasons, they should be used only when there are no other options.

Built-in policy APIs, such as ResourceQuota, NetworkPolicy and Role-based Access Control (RBAC), are built-in Kubernetes APIs that provide declaratively configured policy settings. APIs are typically usable even with hosted Kubernetes services and with managed Kubernetes installations. The built-in policy APIs follow the same conventions as other Kubernetes resources such as Pods. When you use a policy APIs that is stable, you benefit from a defined support policy like other Kubernetes APIs. For these reasons, policy APIs are recommended over configuration files and command arguments where suitable.


Extensions are software components that extend and deeply integrate with Kubernetes. They adapt it to support new types and new kinds of hardware.

Many cluster administrators use a hosted or distribution instance of Kubernetes. These clusters come with extensions pre-installed. As a result, most Kubernetes users will not need to install extensions and even fewer users will need to author new ones.

Extension patterns

Kubernetes is designed to be automated by writing client programs. Any program that reads and/or writes to the Kubernetes API can provide useful automation. Automation can run on the cluster or off it. By following the guidance in this doc you can write highly available and robust automation. Automation generally works with any Kubernetes cluster, including hosted clusters and managed installations.

There is a specific pattern for writing client programs that work well with Kubernetes called the controller pattern. Controllers typically read an object's .spec, possibly do things, and then update the object's .status.

A controller is a client of the Kubernetes API. When Kubernetes is the client and calls out to a remote service, Kubernetes calls this a webhook. The remote service is called a webhook backend. As with custom controllers, webhooks do add a point of failure.

In the webhook model, Kubernetes makes a network request to a remote service. With the alternative binary Plugin model, Kubernetes executes a binary (program). Binary plugins are used by the kubelet (for example, CSI storage plugins and CNI network plugins), and by kubectl (see Extend kubectl with plugins).

Extension points

This diagram shows the extension points in a Kubernetes cluster and the clients that access it.

Symbolic representation of seven numbered extension points for Kubernetes

Kubernetes extension points

Key to the figure

  1. Users often interact with the Kubernetes API using kubectl. Plugins customise the behaviour of clients. There are generic extensions that can apply to different clients, as well as specific ways to extend kubectl.

  2. The API server handles all requests. Several types of extension points in the API server allow authenticating requests, or blocking them based on their content, editing content, and handling deletion. These are described in the API Access Extensions section.

  3. The API server serves various kinds of resources. Built-in resource kinds, such as pods, are defined by the Kubernetes project and can't be changed. Read API extensions to learn about extending the Kubernetes API.

  4. The Kubernetes scheduler decides which nodes to place pods on. There are several ways to extend scheduling, which are described in the Scheduling extensions section.

  5. Much of the behavior of Kubernetes is implemented by programs called controllers, that are clients of the API server. Controllers are often used in conjunction with custom resources. Read combining new APIs with automation and Changing built-in resources to learn more.

  6. The kubelet runs on servers (nodes), and helps pods appear like virtual servers with their own IPs on the cluster network. Network Plugins allow for different implementations of pod networking.

  7. You can use Device Plugins to integrate custom hardware or other special node-local facilities, and make these available to Pods running in your cluster. The kubelet includes support for working with device plugins.

    The kubelet also mounts and unmounts volume for pods and their containers. You can use Storage Plugins to add support for new kinds of storage and other volume types.

Extension point choice flowchart

If you are unsure where to start, this flowchart can help. Note that some solutions may involve several types of extensions.

Flowchart with questions about use cases and guidance for implementers. Green circles indicate yes; red circles indicate no.

Flowchart guide to select an extension approach

Client extensions

Plugins for kubectl are separate binaries that add or replace the behavior of specific subcommands. The kubectl tool can also integrate with credential plugins These extensions only affect a individual user's local environment, and so cannot enforce site-wide policies.

If you want to extend the kubectl tool, read Extend kubectl with plugins.

API extensions

Custom resource definitions

Consider adding a Custom Resource to Kubernetes if you want to define new controllers, application configuration objects or other declarative APIs, and to manage them using Kubernetes tools, such as kubectl.

For more about Custom Resources, see the Custom Resources concept guide.

API aggregation layer

You can use Kubernetes' API Aggregation Layer to integrate the Kubernetes API with additional services such as for metrics.

Combining new APIs with automation

A combination of a custom resource API and a control loop is called the controllers pattern. If your controller takes the place of a human operator deploying infrastructure based on a desired state, then the controller may also be following the operator pattern. The Operator pattern is used to manage specific applications; usually, these are applications that maintain state and require care in how they are managed.

You can also make your own custom APIs and control loops that manage other resources, such as storage, or to define policies (such as an access control restriction).

Changing built-in resources

When you extend the Kubernetes API by adding custom resources, the added resources always fall into a new API Groups. You cannot replace or change existing API groups. Adding an API does not directly let you affect the behavior of existing APIs (such as Pods), whereas API Access Extensions do.

API access extensions

When a request reaches the Kubernetes API Server, it is first authenticated, then authorized, and is then subject to various types of admission control (some requests are in fact not authenticated, and get special treatment). See Controlling Access to the Kubernetes API for more on this flow.

Each of the steps in the Kubernetes authentication / authorization flow offers extension points.


Authentication maps headers or certificates in all requests to a username for the client making the request.

Kubernetes has several built-in authentication methods that it supports. It can also sit behind an authenticating proxy, and it can send a token from an Authorization: header to a remote service for verification (an authentication webhook) if those don't meet your needs.


Authorization determines whether specific users can read, write, and do other operations on API resources. It works at the level of whole resources -- it doesn't discriminate based on arbitrary object fields.

If the built-in authorization options don't meet your needs, an authorization webhook allows calling out to custom code that makes an authorization decision.

Dynamic admission control

After a request is authorized, if it is a write operation, it also goes through Admission Control steps. In addition to the built-in steps, there are several extensions:

  • The Image Policy webhook restricts what images can be run in containers.
  • To make arbitrary admission control decisions, a general Admission webhook can be used. Admission webhooks can reject creations or updates. Some admission webhooks modify the incoming request data before it is handled further by Kubernetes.

Infrastructure extensions

Device plugins

Device plugins allow a node to discover new Node resources (in addition to the builtin ones like cpu and memory) via a Device Plugin.

Storage plugins

Container Storage Interface (CSI) plugins provide a way to extend Kubernetes with supports for new kinds of volumes. The volumes can be backed by durable external storage, or provide ephemeral storage, or they might offer a read-only interface to information using a filesystem paradigm.

Kubernetes also includes support for FlexVolume plugins, which are deprecated since Kubernetes v1.23 (in favour of CSI).

FlexVolume plugins allow users to mount volume types that aren't natively supported by Kubernetes. When you run a Pod that relies on FlexVolume storage, the kubelet calls a binary plugin to mount the volume. The archived FlexVolume design proposal has more detail on this approach.

The Kubernetes Volume Plugin FAQ for Storage Vendors includes general information on storage plugins.

Network plugins

Your Kubernetes cluster needs a network plugin in order to have a working Pod network and to support other aspects of the Kubernetes network model.

Network Plugins allow Kubernetes to work with different networking topologies and technologies.

Kubelet image credential provider plugins

FEATURE STATE: Kubernetes v1.26 [stable]
Kubelet image credential providers are plugins for the kubelet to dynamically retrieve image registry credentials. The credentials are then used when pulling images from container image registries that match the configuration.

The plugins can communicate with external services or use local files to obtain credentials. This way, the kubelet does not need to have static credentials for each registry, and can support various authentication methods and protocols.

For plugin configuration details, see Configure a kubelet image credential provider.

Scheduling extensions

The scheduler is a special type of controller that watches pods, and assigns pods to nodes. The default scheduler can be replaced entirely, while continuing to use other Kubernetes components, or multiple schedulers can run at the same time.

This is a significant undertaking, and almost all Kubernetes users find they do not need to modify the scheduler.

You can control which scheduling plugins are active, or associate sets of plugins with different named scheduler profiles. You can also write your own plugin that integrates with one or more of the kube-scheduler's extension points.

Finally, the built in kube-scheduler component supports a webhook that permits a remote HTTP backend (scheduler extension) to filter and / or prioritize the nodes that the kube-scheduler chooses for a pod.

What's next

1 - Compute, Storage, and Networking Extensions

This section covers extensions to your cluster that do not come as part as Kubernetes itself. You can use these extensions to enhance the nodes in your cluster, or to provide the network fabric that links Pods together.

  • CSI and FlexVolume storage plugins

    Container Storage Interface (CSI) plugins provide a way to extend Kubernetes with supports for new kinds of volumes. The volumes can be backed by durable external storage, or provide ephemeral storage, or they might offer a read-only interface to information using a filesystem paradigm.

    Kubernetes also includes support for FlexVolume plugins, which are deprecated since Kubernetes v1.23 (in favour of CSI).

    FlexVolume plugins allow users to mount volume types that aren't natively supported by Kubernetes. When you run a Pod that relies on FlexVolume storage, the kubelet calls a binary plugin to mount the volume. The archived FlexVolume design proposal has more detail on this approach.

    The Kubernetes Volume Plugin FAQ for Storage Vendors includes general information on storage plugins.

  • Device plugins

    Device plugins allow a node to discover new Node facilities (in addition to the built-in node resources such as cpu and memory), and provide these custom node-local facilities to Pods that request them.

  • Network plugins

    A network plugin allow Kubernetes to work with different networking topologies and technologies. Your Kubernetes cluster needs a network plugin in order to have a working Pod network and to support other aspects of the Kubernetes network model.

    Kubernetes 1.30 is compatible with CNI network plugins.

1.1 - Network Plugins

Kubernetes 1.30 supports Container Network Interface (CNI) plugins for cluster networking. You must use a CNI plugin that is compatible with your cluster and that suits your needs. Different plugins are available (both open- and closed- source) in the wider Kubernetes ecosystem.

A CNI plugin is required to implement the Kubernetes network model.

You must use a CNI plugin that is compatible with the v0.4.0 or later releases of the CNI specification. The Kubernetes project recommends using a plugin that is compatible with the v1.0.0 CNI specification (plugins can be compatible with multiple spec versions).


A Container Runtime, in the networking context, is a daemon on a node configured to provide CRI Services for kubelet. In particular, the Container Runtime must be configured to load the CNI plugins required to implement the Kubernetes network model.

For specific information about how a Container Runtime manages the CNI plugins, see the documentation for that Container Runtime, for example:

For specific information about how to install and manage a CNI plugin, see the documentation for that plugin or networking provider.

Network Plugin Requirements

Loopback CNI

In addition to the CNI plugin installed on the nodes for implementing the Kubernetes network model, Kubernetes also requires the container runtimes to provide a loopback interface lo, which is used for each sandbox (pod sandboxes, vm sandboxes, ...). Implementing the loopback interface can be accomplished by re-using the CNI loopback plugin. or by developing your own code to achieve this (see this example from CRI-O).

Support hostPort

The CNI networking plugin supports hostPort. You can use the official portmap plugin offered by the CNI plugin team or use your own plugin with portMapping functionality.

If you want to enable hostPort support, you must specify portMappings capability in your cni-conf-dir. For example:

  "name": "k8s-pod-network",
  "cniVersion": "0.4.0",
  "plugins": [
      "type": "calico",
      "log_level": "info",
      "datastore_type": "kubernetes",
      "nodename": "",
      "ipam": {
        "type": "host-local",
        "subnet": "usePodCidr"
      "policy": {
        "type": "k8s"
      "kubernetes": {
        "kubeconfig": "/etc/cni/net.d/calico-kubeconfig"
      "type": "portmap",
      "capabilities": {"portMappings": true},
      "externalSetMarkChain": "KUBE-MARK-MASQ"

Support traffic shaping

Experimental Feature

The CNI networking plugin also supports pod ingress and egress traffic shaping. You can use the official bandwidth plugin offered by the CNI plugin team or use your own plugin with bandwidth control functionality.

If you want to enable traffic shaping support, you must add the bandwidth plugin to your CNI configuration file (default /etc/cni/net.d) and ensure that the binary is included in your CNI bin dir (default /opt/cni/bin).

  "name": "k8s-pod-network",
  "cniVersion": "0.4.0",
  "plugins": [
      "type": "calico",
      "log_level": "info",
      "datastore_type": "kubernetes",
      "nodename": "",
      "ipam": {
        "type": "host-local",
        "subnet": "usePodCidr"
      "policy": {
        "type": "k8s"
      "kubernetes": {
        "kubeconfig": "/etc/cni/net.d/calico-kubeconfig"
      "type": "bandwidth",
      "capabilities": {"bandwidth": true}

Now you can add the and annotations to your Pod. For example:

apiVersion: v1
kind: Pod
  annotations: 1M 1M

What's next

1.2 - Device Plugins

Device plugins let you configure your cluster with support for devices or resources that require vendor-specific setup, such as GPUs, NICs, FPGAs, or non-volatile main memory.
FEATURE STATE: Kubernetes v1.26 [stable]

Kubernetes provides a device plugin framework that you can use to advertise system hardware resources to the Kubelet.

Instead of customizing the code for Kubernetes itself, vendors can implement a device plugin that you deploy either manually or as a DaemonSet. The targeted devices include GPUs, high-performance NICs, FPGAs, InfiniBand adapters, and other similar computing resources that may require vendor specific initialization and setup.

Device plugin registration

The kubelet exports a Registration gRPC service:

service Registration {
	rpc Register(RegisterRequest) returns (Empty) {}

A device plugin can register itself with the kubelet through this gRPC service. During the registration, the device plugin needs to send:

  • The name of its Unix socket.
  • The Device Plugin API version against which it was built.
  • The ResourceName it wants to advertise. Here ResourceName needs to follow the extended resource naming scheme as vendor-domain/resourcetype. (For example, an NVIDIA GPU is advertised as

Following a successful registration, the device plugin sends the kubelet the list of devices it manages, and the kubelet is then in charge of advertising those resources to the API server as part of the kubelet node status update. For example, after a device plugin registers hardware-vendor.example/foo with the kubelet and reports two healthy devices on a node, the node status is updated to advertise that the node has 2 "Foo" devices installed and available.

Then, users can request devices as part of a Pod specification (see container). Requesting extended resources is similar to how you manage requests and limits for other resources, with the following differences:

  • Extended resources are only supported as integer resources and cannot be overcommitted.
  • Devices cannot be shared between containers.


Suppose a Kubernetes cluster is running a device plugin that advertises resource hardware-vendor.example/foo on certain nodes. Here is an example of a pod requesting this resource to run a demo workload:

apiVersion: v1
kind: Pod
  name: demo-pod
    - name: demo-container-1
          hardware-vendor.example/foo: 2
# This Pod needs 2 of the hardware-vendor.example/foo devices
# and can only schedule onto a Node that's able to satisfy
# that need.
# If the Node has more than 2 of those devices available, the
# remainder would be available for other Pods to use.

Device plugin implementation

The general workflow of a device plugin includes the following steps:

  1. Initialization. During this phase, the device plugin performs vendor-specific initialization and setup to make sure the devices are in a ready state.

  2. The plugin starts a gRPC service, with a Unix socket under the host path /var/lib/kubelet/device-plugins/, that implements the following interfaces:

    service DevicePlugin {
          // GetDevicePluginOptions returns options to be communicated with Device Manager.
          rpc GetDevicePluginOptions(Empty) returns (DevicePluginOptions) {}
          // ListAndWatch returns a stream of List of Devices
          // Whenever a Device state change or a Device disappears, ListAndWatch
          // returns the new list
          rpc ListAndWatch(Empty) returns (stream ListAndWatchResponse) {}
          // Allocate is called during container creation so that the Device
          // Plugin can run device specific operations and instruct Kubelet
          // of the steps to make the Device available in the container
          rpc Allocate(AllocateRequest) returns (AllocateResponse) {}
          // GetPreferredAllocation returns a preferred set of devices to allocate
          // from a list of available ones. The resulting preferred allocation is not
          // guaranteed to be the allocation ultimately performed by the
          // devicemanager. It is only designed to help the devicemanager make a more
          // informed allocation decision when possible.
          rpc GetPreferredAllocation(PreferredAllocationRequest) returns (PreferredAllocationResponse) {}
          // PreStartContainer is called, if indicated by Device Plugin during registration phase,
          // before each container start. Device plugin can run device specific operations
          // such as resetting the device before making devices available to the container.
          rpc PreStartContainer(PreStartContainerRequest) returns (PreStartContainerResponse) {}
  3. The plugin registers itself with the kubelet through the Unix socket at host path /var/lib/kubelet/device-plugins/kubelet.sock.

  4. After successfully registering itself, the device plugin runs in serving mode, during which it keeps monitoring device health and reports back to the kubelet upon any device state changes. It is also responsible for serving Allocate gRPC requests. During Allocate, the device plugin may do device-specific preparation; for example, GPU cleanup or QRNG initialization. If the operations succeed, the device plugin returns an AllocateResponse that contains container runtime configurations for accessing the allocated devices. The kubelet passes this information to the container runtime.

    An AllocateResponse contains zero or more ContainerAllocateResponse objects. In these, the device plugin defines modifications that must be made to a container's definition to provide access to the device. These modifications include:

    • Annotations
    • device nodes
    • environment variables
    • mounts
    • fully-qualified CDI device names

Handling kubelet restarts

A device plugin is expected to detect kubelet restarts and re-register itself with the new kubelet instance. A new kubelet instance deletes all the existing Unix sockets under /var/lib/kubelet/device-plugins when it starts. A device plugin can monitor the deletion of its Unix socket and re-register itself upon such an event.

Device plugin deployment

You can deploy a device plugin as a DaemonSet, as a package for your node's operating system, or manually.

The canonical directory /var/lib/kubelet/device-plugins requires privileged access, so a device plugin must run in a privileged security context. If you're deploying a device plugin as a DaemonSet, /var/lib/kubelet/device-plugins must be mounted as a Volume in the plugin's PodSpec.

If you choose the DaemonSet approach you can rely on Kubernetes to: place the device plugin's Pod onto Nodes, to restart the daemon Pod after failure, and to help automate upgrades.

API compatibility

Previously, the versioning scheme required the Device Plugin's API version to match exactly the Kubelet's version. Since the graduation of this feature to Beta in v1.12 this is no longer a hard requirement. The API is versioned and has been stable since Beta graduation of this feature. Because of this, kubelet upgrades should be seamless but there still may be changes in the API before stabilization making upgrades not guaranteed to be non-breaking.

As a project, Kubernetes recommends that device plugin developers:

  • Watch for Device Plugin API changes in the future releases.
  • Support multiple versions of the device plugin API for backward/forward compatibility.

To run device plugins on nodes that need to be upgraded to a Kubernetes release with a newer device plugin API version, upgrade your device plugins to support both versions before upgrading these nodes. Taking that approach will ensure the continuous functioning of the device allocations during the upgrade.

Monitoring device plugin resources

FEATURE STATE: Kubernetes v1.28 [stable]

In order to monitor resources provided by device plugins, monitoring agents need to be able to discover the set of devices that are in-use on the node and obtain metadata to describe which container the metric should be associated with. Prometheus metrics exposed by device monitoring agents should follow the Kubernetes Instrumentation Guidelines, identifying containers using pod, namespace, and container prometheus labels.

The kubelet provides a gRPC service to enable discovery of in-use devices, and to provide metadata for these devices:

// PodResourcesLister is a service provided by the kubelet that provides information about the
// node resources consumed by pods and containers on the node
service PodResourcesLister {
    rpc List(ListPodResourcesRequest) returns (ListPodResourcesResponse) {}
    rpc GetAllocatableResources(AllocatableResourcesRequest) returns (AllocatableResourcesResponse) {}
    rpc Get(GetPodResourcesRequest) returns (GetPodResourcesResponse) {}

List gRPC endpoint

The List endpoint provides information on resources of running pods, with details such as the id of exclusively allocated CPUs, device id as it was reported by device plugins and id of the NUMA node where these devices are allocated. Also, for NUMA-based machines, it contains the information about memory and hugepages reserved for a container.

Starting from Kubernetes v1.27, the List endpoint can provide information on resources of running pods allocated in ResourceClaims by the DynamicResourceAllocation API. To enable this feature kubelet must be started with the following flags:

// ListPodResourcesResponse is the response returned by List function
message ListPodResourcesResponse {
    repeated PodResources pod_resources = 1;

// PodResources contains information about the node resources assigned to a pod
message PodResources {
    string name = 1;
    string namespace = 2;
    repeated ContainerResources containers = 3;

// ContainerResources contains information about the resources assigned to a container
message ContainerResources {
    string name = 1;
    repeated ContainerDevices devices = 2;
    repeated int64 cpu_ids = 3;
    repeated ContainerMemory memory = 4;
    repeated DynamicResource dynamic_resources = 5;

// ContainerMemory contains information about memory and hugepages assigned to a container
message ContainerMemory {
    string memory_type = 1;
    uint64 size = 2;
    TopologyInfo topology = 3;

// Topology describes hardware topology of the resource
message TopologyInfo {
        repeated NUMANode nodes = 1;

// NUMA representation of NUMA node
message NUMANode {
        int64 ID = 1;

// ContainerDevices contains information about the devices assigned to a container
message ContainerDevices {
    string resource_name = 1;
    repeated string device_ids = 2;
    TopologyInfo topology = 3;

// DynamicResource contains information about the devices assigned to a container by Dynamic Resource Allocation
message DynamicResource {
    string class_name = 1;
    string claim_name = 2;
    string claim_namespace = 3;
    repeated ClaimResource claim_resources = 4;

// ClaimResource contains per-plugin resource information
message ClaimResource {
    repeated CDIDevice cdi_devices = 1 [(gogoproto.customname) = "CDIDevices"];

// CDIDevice specifies a CDI device information
message CDIDevice {
    // Fully qualified CDI device name
    // for example:
    // see more details in the CDI specification:
    string name = 1;

GetAllocatableResources gRPC endpoint

FEATURE STATE: Kubernetes v1.28 [stable]

GetAllocatableResources provides information on resources initially available on the worker node. It provides more information than kubelet exports to APIServer.

// AllocatableResourcesResponses contains information about all the devices known by the kubelet
message AllocatableResourcesResponse {
    repeated ContainerDevices devices = 1;
    repeated int64 cpu_ids = 2;
    repeated ContainerMemory memory = 3;

ContainerDevices do expose the topology information declaring to which NUMA cells the device is affine. The NUMA cells are identified using a opaque integer ID, which value is consistent to what device plugins report when they register themselves to the kubelet.

The gRPC service is served over a unix socket at /var/lib/kubelet/pod-resources/kubelet.sock. Monitoring agents for device plugin resources can be deployed as a daemon, or as a DaemonSet. The canonical directory /var/lib/kubelet/pod-resources requires privileged access, so monitoring agents must run in a privileged security context. If a device monitoring agent is running as a DaemonSet, /var/lib/kubelet/pod-resources must be mounted as a Volume in the device monitoring agent's PodSpec.

Get gRPC endpoint

FEATURE STATE: Kubernetes v1.27 [alpha]

The Get endpoint provides information on resources of a running Pod. It exposes information similar to those described in the List endpoint. The Get endpoint requires PodName and PodNamespace of the running Pod.

// GetPodResourcesRequest contains information about the pod
message GetPodResourcesRequest {
    string pod_name = 1;
    string pod_namespace = 2;

To enable this feature, you must start your kubelet services with the following flag:


The Get endpoint can provide Pod information related to dynamic resources allocated by the dynamic resource allocation API. To enable this feature, you must ensure your kubelet services are started with the following flags:


Device plugin integration with the Topology Manager

FEATURE STATE: Kubernetes v1.27 [stable]

The Topology Manager is a Kubelet component that allows resources to be co-ordinated in a Topology aligned manner. In order to do this, the Device Plugin API was extended to include a TopologyInfo struct.

message TopologyInfo {
    repeated NUMANode nodes = 1;

message NUMANode {
    int64 ID = 1;

Device Plugins that wish to leverage the Topology Manager can send back a populated TopologyInfo struct as part of the device registration, along with the device IDs and the health of the device. The device manager will then use this information to consult with the Topology Manager and make resource assignment decisions.

TopologyInfo supports setting a nodes field to either nil or a list of NUMA nodes. This allows the Device Plugin to advertise a device that spans multiple NUMA nodes.

Setting TopologyInfo to nil or providing an empty list of NUMA nodes for a given device indicates that the Device Plugin does not have a NUMA affinity preference for that device.

An example TopologyInfo struct populated for a device by a Device Plugin:

pluginapi.Device{ID: "25102017", Health: pluginapi.Healthy, Topology:&pluginapi.TopologyInfo{Nodes: []*pluginapi.NUMANode{&pluginapi.NUMANode{ID: 0,},}}}

Device plugin examples

Here are some examples of device plugin implementations:

What's next

2 - Extending the Kubernetes API

Custom resources are extensions of the Kubernetes API. Kubernetes provides two ways to add custom resources to your cluster:

  • The CustomResourceDefinition (CRD) mechanism allows you to declaratively define a new custom API with an API group, kind, and schema that you specify. The Kubernetes control plane serves and handles the storage of your custom resource. CRDs allow you to create new types of resources for your cluster without writing and running a custom API server.
  • The aggregation layer sits behind the primary API server, which acts as a proxy. This arrangement is called API Aggregation (AA), which allows you to provide specialized implementations for your custom resources by writing and deploying your own API server. The main API server delegates requests to your API server for the custom APIs that you specify, making them available to all of its clients.

2.1 - 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 let you store and retrieve structured data. When you combine a custom resource with a custom controller, custom resources provide a true declarative API.

The Kubernetes declarative API enforces a separation of responsibilities. You declare the desired state of your resource. The Kubernetes controller keeps the current state of Kubernetes objects in sync with your declared desired state. This is in contrast to an imperative API, where you instruct a server what to do.

You can deploy and update a custom controller on a running cluster, independently of the cluster's 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 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:

  • Your API consists of a relatively small number of relatively small objects (resources).
  • The objects define configuration of applications or infrastructure.
  • The objects are updated relatively infrequently.
  • Humans often need to read and write the objects.
  • The main operations on the objects are CRUD-y (creating, reading, updating and deleting).
  • Transactions across objects are not required: the API represents a desired state, not an exact state.

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

  • The client says "do this", and then gets a synchronous response back when it is done.
  • The client says "do this", and then gets an operation ID back, and has to check a separate Operation object to determine completion of the request.
  • You talk about Remote Procedure Calls (RPCs).
  • Directly storing large amounts of data; for example, > a few kB per object, or > 1000s of objects.
  • High bandwidth access (10s of requests per second sustained) needed.
  • Store end-user data (such as images, PII, etc.) or other large-scale data processed by applications.
  • The natural operations on the objects are not CRUD-y.
  • The API is not easily modeled as objects.
  • You chose to represent pending operations with an operation ID or an operation object.

Should I use a ConfigMap or a custom resource?

Use a ConfigMap if any of the following apply:

  • There is an existing, well-documented configuration file format, such as a mysql.cnf or pom.xml.
  • You want to put the entire configuration into one key of a ConfigMap.
  • The main use of the configuration file is for a program running in a Pod on your cluster to consume the file to configure itself.
  • Consumers of the file prefer to consume via file in a Pod or environment variable in a pod, rather than the Kubernetes API.
  • You want to perform rolling updates via Deployment, etc., when the file is updated.

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

  • You want to use Kubernetes client libraries and CLIs to create and update the new resource.
  • You want top-level support from kubectl; for example, kubectl get my-object object-name.
  • You want to build new automation that watches for updates on the new object, and then CRUD other objects, or vice versa.
  • You want to write automation that handles updates to the object.
  • You want to use Kubernetes API conventions like .spec, .status, and .metadata.
  • You want the object to be an abstraction over a collection of controlled resources, or a summarization of other resources.

Adding custom resources

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

  • CRDs are simple and can be created without any programming.
  • API Aggregation requires programming, but allows more control over API behaviors like how data is stored and conversion between API versions.

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 API servers that sit behind the primary API server, which acts as a proxy. This arrangement is called API Aggregation(AA). To users, the Kubernetes API appears extended.

CRDs allow users to create new types of resources without adding another API server. 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).


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. The name of a CRD object must be a valid DNS subdomain name.

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 generically handle custom resources through CRDs.

The aggregation layer allows you to provide specialized implementations for your custom resources by writing and deploying your own API server. The main API server delegates requests to your API server 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:

  • You have a handful of fields
  • You are using the resource within your company, or as part of a small open-source project (as opposed to a commercial product)

Comparing ease of use

CRDs are easier to create than Aggregated APIs.

CRDsAggregated API
Do not require programming. Users can choose any language for a CRD controller.Requires programming and building binary and image.
No additional service to run; CRDs 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 API server.
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.

FeatureDescriptionCRDsAggregated API
ValidationHelp 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. CRDValidationRatcheting feature gate allows failing validations specified using OpenAPI also can be ignored if the failing part of the resource was unchanged. Any other validations supported by addition of a Validating Webhook.Yes, arbitrary validation checks
DefaultingSee aboveYes, either via OpenAPI v3.0 validation default keyword (GA in 1.17), or via a Mutating Webhook (though this will not be run when reading from etcd for old objects).Yes
Multi-versioningAllows serving the same object through two API versions. Can help ease API changes like renaming fields. Less important if you control your client versions.YesYes
Custom StorageIf you need storage with a different performance mode (for example, a time-series database instead of key-value store) or isolation for security (for example, encryption of sensitive information, etc.)NoYes
Custom Business LogicPerform arbitrary checks or actions when creating, reading, updating or deleting an objectYes, using Webhooks.Yes
Scale SubresourceAllows systems like HorizontalPodAutoscaler and PodDisruptionBudget interact with your new resourceYesYes
Status SubresourceAllows fine-grained access control where user writes the spec section and the controller writes the status section. Allows incrementing object Generation on custom resource data mutation (requires separate spec and status sections in the resource)YesYes
Other SubresourcesAdd operations other than CRUD, such as "logs" or "exec".NoYes
strategic-merge-patchThe 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"NoYes
Protocol BuffersThe new resource supports clients that want to use Protocol BuffersNoYes
OpenAPI SchemaIs 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?)Yes, based on the OpenAPI v3.0 validation schema (GA in 1.16).Yes

Common Features

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

FeatureWhat it does
CRUDThe new endpoints support CRUD basic operations via HTTP and kubectl
WatchThe new endpoints support Kubernetes Watch operations via HTTP
DiscoveryClients like kubectl and dashboard automatically offer list, display, and field edit operations on your resources
json-patchThe new endpoints support PATCH with Content-Type: application/json-patch+json
merge-patchThe new endpoints support PATCH with Content-Type: application/merge-patch+json
HTTPSThe new endpoints uses HTTPS
Built-in AuthenticationAccess to the extension uses the core API server (aggregation layer) for authentication
Built-in AuthorizationAccess to the extension can reuse the authorization used by the core API server; for example, RBAC.
FinalizersBlock deletion of extension resources until external cleanup happens.
Admission WebhooksSet default values and validate extension resources during any create/update/delete operation.
UI/CLI DisplayKubectl, dashboard can display extension resources.
Unset versus EmptyClients can distinguish unset fields from zero-valued fields.
Client Libraries GenerationKubernetes provides generic client libraries, as well as tools to generate type-specific client libraries.
Labels and annotationsCommon 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 API server always involves running a new Deployment.


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:

  • kubectl
  • The Kubernetes dynamic client.
  • A REST client that you write.
  • A client generated using Kubernetes client generation tools (generating one is an advanced undertaking, but some projects may provide a client along with the CRD or AA).

Custom resource field selectors

Field Selectors let clients select custom resources based on the value of one or more resource fields.

All custom resources support the and metadata.namespace field selectors.

Fields declared in a CustomResourceDefinition may also be used with field selectors when included in the spec.versions[*].selectableFields field of the CustomResourceDefinition.

Selectable fields for custom resources

FEATURE STATE: Kubernetes v1.30 [alpha]

You need to enable the CustomResourceFieldSelectors feature gate to use this behavior, which then applies to all CustomResourceDefinitions in your cluster.

The spec.versions[*].selectableFields field of a CustomResourceDefinition may be used to declare which other fields in a custom resource may be used in field selectors. The following example adds the .spec.color and .spec.size fields as selectable fields.

kind: CustomResourceDefinition
  scope: Namespaced
    plural: shirts
    singular: shirt
    kind: Shirt
  - name: v1
    served: true
    storage: true
        type: object
            type: object
                type: string
                type: string
    - jsonPath: .spec.color
    - jsonPath: .spec.size
    - jsonPath: .spec.color
      name: Color
      type: string
    - jsonPath: .spec.size
      name: Size
      type: string

Field selectors can then be used to get only resources with with a color of blue:

kubectl get --field-selector spec.color=blue

The output should be:

example1   blue   S
example2   blue   M

What's next

2.2 - Kubernetes API Aggregation Layer

The aggregation layer allows Kubernetes to be extended with additional APIs, beyond what is offered by the core Kubernetes APIs. The additional APIs can either be ready-made solutions such as a metrics server, or APIs that you develop yourself.

The aggregation layer is different from Custom Resources, which are a way to make the kube-apiserver recognise new kinds of object.

Aggregation layer

The aggregation layer runs in-process with the kube-apiserver. Until an extension resource is registered, the aggregation layer will do nothing. To register an API, you add an APIService object, which "claims" the URL path in the Kubernetes API. At that point, the aggregation layer will proxy anything sent to that API path (e.g. /apis/…) to the registered APIService.

The most common way to implement the APIService is to run an extension API server in Pod(s) that run in your cluster. If you're using the extension API server to manage resources in your cluster, the extension API server (also written as "extension-apiserver") is typically paired with one or more controllers. The apiserver-builder library provides a skeleton for both extension API servers and the associated controller(s).

Response latency

Extension API servers should have low latency networking to and from the kube-apiserver. Discovery requests are required to round-trip from the kube-apiserver in five seconds or less.

If your extension API server cannot achieve that latency requirement, consider making changes that let you meet it.

What's next

Alternatively: learn how to extend the Kubernetes API using Custom Resource Definitions.

3 - Operator pattern

Operators are software extensions to Kubernetes that make use of custom resources to manage applications and their components. Operators follow Kubernetes principles, notably the control loop.


The operator pattern aims to capture the key aim of a human operator who is managing a service or set of services. Human operators who look after specific applications and services have deep knowledge of how the system ought to behave, how to deploy it, and how to react if there are problems.

People who run workloads on Kubernetes often like to use automation to take care of repeatable tasks. The operator pattern captures how you can write code to automate a task beyond what Kubernetes itself provides.

Operators in Kubernetes

Kubernetes is designed for automation. Out of the box, you get lots of built-in automation from the core of Kubernetes. You can use Kubernetes to automate deploying and running workloads, and you can automate how Kubernetes does that.

Kubernetes' operator pattern concept lets you extend the cluster's behaviour without modifying the code of Kubernetes itself by linking controllers to one or more custom resources. Operators are clients of the Kubernetes API that act as controllers for a Custom Resource.

An example operator

Some of the things that you can use an operator to automate include:

  • deploying an application on demand
  • taking and restoring backups of that application's state
  • handling upgrades of the application code alongside related changes such as database schemas or extra configuration settings
  • publishing a Service to applications that don't support Kubernetes APIs to discover them
  • simulating failure in all or part of your cluster to test its resilience
  • choosing a leader for a distributed application without an internal member election process

What might an operator look like in more detail? Here's an example:

  1. A custom resource named SampleDB, that you can configure into the cluster.
  2. A Deployment that makes sure a Pod is running that contains the controller part of the operator.
  3. A container image of the operator code.
  4. Controller code that queries the control plane to find out what SampleDB resources are configured.
  5. The core of the operator is code to tell the API server how to make reality match the configured resources.
    • If you add a new SampleDB, the operator sets up PersistentVolumeClaims to provide durable database storage, a StatefulSet to run SampleDB and a Job to handle initial configuration.
    • If you delete it, the operator takes a snapshot, then makes sure that the StatefulSet and Volumes are also removed.
  6. The operator also manages regular database backups. For each SampleDB resource, the operator determines when to create a Pod that can connect to the database and take backups. These Pods would rely on a ConfigMap and / or a Secret that has database connection details and credentials.
  7. Because the operator aims to provide robust automation for the resource it manages, there would be additional supporting code. For this example, code checks to see if the database is running an old version and, if so, creates Job objects that upgrade it for you.

Deploying operators

The most common way to deploy an operator is to add the Custom Resource Definition and its associated Controller to your cluster. The Controller will normally run outside of the control plane, much as you would run any containerized application. For example, you can run the controller in your cluster as a Deployment.

Using an operator

Once you have an operator deployed, you'd use it by adding, modifying or deleting the kind of resource that the operator uses. Following the above example, you would set up a Deployment for the operator itself, and then:

kubectl get SampleDB                   # find configured databases

kubectl edit SampleDB/example-database # manually change some settings

…and that's it! The operator will take care of applying the changes as well as keeping the existing service in good shape.

Writing your own operator

If there isn't an operator in the ecosystem that implements the behavior you want, you can code your own.

You also implement an operator (that is, a Controller) using any language / runtime that can act as a client for the Kubernetes API.

Following are a few libraries and tools you can use to write your own cloud native operator.

What's next