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Kubernetes API Overview

This page contains an overview of the Kubernetes API.

• API versioning
• API groups
• Enabling API groups
• Enabling resources in the groups

The REST API is the fundamental fabric of Kubernetes. All operations and communications between components are REST API calls handled by the API Server, including external user commands. Consequently, everything in the Kubernetes platform is treated as an API object and has a corresponding entry in the API.

Most operations can be performed through the kubectl command-line interface or other command-line tools, such as kubeadm, which in turn use the API. However, the API can also be accessed directly using REST calls.

Consider using one of the client libraries if you are writing an application using the Kubernetes API.

API versioning

To make it easier to eliminate fields or restructure resource representations, Kubernetes supports multiple API versions, each at a different API path, such as /api/v1 or /apis/extensions/v1beta1.

The version is set at the API level rather than at the resource or field level to ensure that the API presents a clear, consistent view of system resources and behavior, and to enable controlling access to end-of-life and/or experimental APIs. The JSON and Protobuf serialization schemas follow the same guidelines for schema changes; all descriptions below cover both formats.

Note that API versioning and software versioning are only indirectly related. The API and release versioning proposal describes the relationship between API versioning and software versioning.

Different API versions imply different levels of stability and support. The criteria for each level are described in more detail in the API Changes documentation.

The criteria are summarized here:

• Alpha level:
  • The version names contain alpha (for example, v1alpha1).
  • The software may contain bugs. Enabling a feature may expose bugs. A feature may be disabled by default.
  • The support for a feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • The software is recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.
• Beta level:
  • The version names contain beta (for example, v2beta3).
  • The software is well tested. Enabling a feature is considered safe. Features are enabled by default.
  • The support for a feature will not be dropped, though the details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, migration instructions will be provided. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • The software is recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters which can be upgraded independently, you may be able to relax this restriction.
  • Please try our beta features and give feedback on them! Once they exit beta, it may not be practical for us to make more changes.
• Stable level:
  • The version name is vX where X is an integer.
  • The stable versions of features will appear in released software for many subsequent versions.

API groups

API groups make it easier to extend the Kubernetes API. The API group is specified in a REST path and in the apiVersion field of a serialized object.

Currently, there are several API groups in use:

• The core (also called legacy) group, which is at REST path /api/v1 and is not specified as part of the apiVersion field, for example, apiVersion: v1.
• The named groups are at REST path /apis/$GROUP_NAME/$VERSION, and use apiVersion: $GROUP_NAME/$VERSION (for example, apiVersion: batch/v1). Full list of supported API groups can be seen in Kubernetes API reference.

There are two supported paths to extending the API with custom resources:

1. CustomResourceDefinition is for users with very basic CRUD needs.
2. Coming soon: users needing the full set of Kubernetes API semantics can implement their own apiserver and use the aggregator to make it seamless for clients.

Enabling API groups

Certain resources and API groups are enabled by default. You can enable or disable them by setting --runtime-config on apiserver. --runtime-config accepts comma separated values. For example, to disable batch/v1, set --runtime-config=batch/v1=false, to enable batch/v2alpha1, set --runtime-config=batch/v2alpha1. The flag accepts comma separated set of key=value pairs describing runtime configuration of the apiserver.

IMPORTANT: Enabling or disabling groups or resources requires restarting apiserver and controller-manager to pick up the --runtime-config changes.

Enabling resources in the groups

DaemonSets, Deployments, HorizontalPodAutoscalers, Ingress, Jobs and ReplicaSets are enabled by default. You can enable other extensions resources by setting --runtime-config on apiserver. --runtime-config accepts comma separated values. For example, to disable deployments and jobs, set --runtime-config=extensions/v1beta1/deployments=false,extensions/v1beta1/jobs=false