Overall API conventions are described in the [API conventions doc](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/docs/devel/api-conventions.md).
The Kubernetes API also serves as the foundation for the declarative configuration schema for the system. The [Kubectl](/docs/user-guide/kubectl/kubectl) command-line tool can be used to create, update, delete, and get API objects.
Kubernetes also stores its serialized state (currently in [etcd](https://coreos.com/docs/distributed-configuration/getting-started-with-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](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/docs/devel/api_changes.md).
Complete API details are documented using [Swagger v1.2](http://swagger.io/). The Kubernetes apiserver (aka "master") exposes an API that can be used to retrieve the Swagger 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.
We also host a version of the [latest API documentation](http://kubernetes.io/docs/api-reference/README/). This is updated with the latest release, so if you are using a different version of Kubernetes you will want to use the spec from your apiserver.
Kubernetes implements an alternative Protobuf based serialization format for the API that is primarily intended for intra-cluster communication, documented in the [design proposal](https://github.com/kubernetes/kubernetes/blob/{{ page.githubbranch }}/docs/proposals/protobuf.md) and the IDL files for each schema are located in the Go packages that define the API objects.
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](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/versioning.md) 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](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/docs/devel/api_changes.md#alpha-beta-and-stable-versions). They are summarized here:
- 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 do 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.
- Stable versions of features will appear in released software for many subsequent versions.
## API groups
To make it easier to extend the Kubernetes API, we are in the process of implementing [*API
groups*](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/proposals/api-group.md). These are simply different interfaces to read and/or modify the
same underlying resources. The API group is specified in a REST path and in the `apiVersion` field
of a serialized object.
Currently there are two API groups in use:
1. the "core" group, which is at REST path `/api/v1` and is not specified as part of the `apiVersion` field, e.g.
`apiVersion: v1`.
1. the "extensions" group, which is at REST path `/apis/extensions/$VERSION`, and which uses
`apiVersion: extensions/$VERSION` (e.g. currently `apiVersion: extensions/v1beta1`).
create their own API groups](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/extending-api.md), and to avoid naming collisions.
