Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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The Kubernetes API

Overall API conventions are described in the API conventions doc.

API endpoints, resource types and samples are described in API Reference.

Remote access to the API is discussed in the access doc.

The Kubernetes API also serves as the foundation for the declarative configuration schema for the system. The Kubectl command-line tool can be used to create, update, delete, and get API objects.

Kubernetes also stores its serialized state (currently in etcd) in terms of the API resources.

Kubernetes itself is decomposed into multiple components, which interact through its API.

API changes

In our experience, any system that is successful needs to grow and change as new use cases emerge or existing ones change. Therefore, we expect the Kubernetes API to continuously change and grow. However, we intend to not break compatibility with existing clients, for an extended period of time. In general, new API resources and new resource fields can be expected to be added frequently. Elimination of resources or fields will require following a deprecation process. The precise deprecation policy for eliminating features is TBD, but once we reach our 1.0 milestone, there will be a specific policy.

What constitutes a compatible change and how to change the API are detailed by the API change document.

OpenAPI and Swagger definitions

Complete API details are documented using Swagger v1.2 and OpenAPI. The Kubernetes apiserver (aka “master”) exposes an API that can be used to retrieve the Swagger v1.2 Kubernetes API spec located at /swaggerapi. You can also enable a UI to browse the API documentation at /swagger-ui by passing the --enable-swagger-ui=true flag to apiserver.

Starting with Kubernetes 1.4, OpenAPI spec is also available at /swagger.json. While we are transitioning from Swagger v1.2 to OpenAPI (aka Swagger v2.0), some of the tools such as kubectl and swagger-ui are still using v1.2 spec. OpenAPI spec is in Beta as of Kubernetes 1.5.

Kubernetes implements an alternative Protobuf based serialization format for the API that is primarily intended for intra-cluster communication, documented in the design proposal and the IDL files for each schema are located in the Go packages that define the API objects.

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.

We chose to version 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-lifed 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. They are summarized here:

API groups

To make it easier to extend the Kubernetes API, we implemented API groups. 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:

  1. The “core” (oftentimes called “legacy”, due to not having explicit group name) group, which is at REST path /api/v1 and is not specified as part of the apiVersion field, e.g. apiVersion: v1.
  2. The named groups are at REST path /apis/$GROUP_NAME/$VERSION, and use apiVersion: $GROUP_NAME/$VERSION (e.g. 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. They can be enabled or disabled by setting --runtime-config on apiserver. --runtime-config accepts comma separated values. For ex: 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. Other extensions resources can be enabled by setting --runtime-config on apiserver. --runtime-config accepts comma separated values. For ex: to disable deployments and ingress, set --runtime-config=extensions/v1beta1/deployments=false,extensions/v1beta1/ingress=false

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