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Merge pull request #23866 from tengqm/refactor-api-concept

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Remove duplicated content in concepts and reference
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Kubernetes Prow Robot
2020-09-21 01:18:31 -07:00
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content/en/docs/concepts/overview/kubernetes-api.md
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@@ -16,30 +16,23 @@ card:
The core of Kubernetes' {{< glossary_tooltip text="control plane" term_id="control-plane" >}}
is the {{< glossary_tooltip text="API server" term_id="kube-apiserver" >}}. The API server
exposes an HTTP API that lets end users, different parts of your cluster, and external components
communicate with one another.
exposes an HTTP API that lets end users, different parts of your cluster, and
external components communicate with one another.
The Kubernetes API lets you query and manipulate the state of objects in the Kubernetes API
(for example: Pods, Namespaces, ConfigMaps, and Events).
API endpoints, resource types and samples are described in the [API Reference](/docs/reference/kubernetes-api/).
Most operations can be performed through the
[kubectl](/docs/reference/kubectl/overview/) command-line interface or other
command-line tools, such as
[kubeadm](/docs/reference/setup-tools/kubeadm/kubeadm/), which in turn use the
API. However, you can also access the API directly using REST calls.
Consider using one of the [client libraries](/docs/reference/using-api/client-libraries/)
if you are writing an application using the Kubernetes API.
<!-- body -->
## API changes
Any system that is successful needs to grow and change as new use cases emerge or existing ones change.
Therefore, Kubernetes has design features to allow the Kubernetes API to continuously change and grow.
The Kubernetes project aims to _not_ break compatibility with existing clients, and to maintain that
compatibility for a length of time so that other projects have an opportunity to adapt.
In general, new API resources and new resource fields can be added often and frequently.
Elimination of resources or fields requires following the
[API deprecation policy](/docs/reference/using-api/deprecation-policy/).
What constitutes a compatible change, and how to change the API, are detailed in
[API changes](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#readme).
## OpenAPI specification {#api-specification}
Complete API details are documented using [OpenAPI](https://www.openapis.org/).
@@ -78,95 +71,58 @@ You can request the response format using request headers as follows:
<caption>Valid request header values for OpenAPI v2 queries</caption>
</table>
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/community/blob/master/contributors/design-proposals/api-machinery/protobuf.md) and the IDL files for each schema are located in the Go packages that define the API objects.
Kubernetes implements an alternative Protobuf based serialization format that
is primarily intended for intra-cluster communication. For more information
about this format, see the [Kubernetes Protobuf serialization](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/api-machinery/protobuf.md) design proposal and the
Interface Definition Language (IDL) files for each schema located in the Go
packages that define the API objects.
## API versioning
## API changes
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/rbac.authorization.k8s.io/v1alpha1`.
Any system that is successful needs to grow and change as new use cases emerge or existing ones change.
Therefore, Kubernetes has designed its features to allow the Kubernetes API to continuously change and grow.
The Kubernetes project aims to _not_ break compatibility with existing clients, and to maintain that
compatibility for a length of time so that other projects have an opportunity to adapt.
Versioning is done 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.
In general, new API resources and new resource fields can be added often and frequently.
Elimination of resources or fields requires following the
[API deprecation policy](/docs/reference/using-api/deprecation-policy/).
The JSON and Protobuf serialization schemas follow the same guidelines for schema changes - all descriptions below cover both formats.
What constitutes a compatible change, and how to change the API, are detailed in
[API changes](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#readme).
Note that API versioning and Software versioning are only indirectly related. The
[Kubernetes Release Versioning](https://git.k8s.io/community/contributors/design-proposals/release/versioning.md)
proposal describes the relationship between API versioning and software versioning.
## API groups and 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](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#alpha-beta-and-stable-versions)
documentation. They are summarized here:
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/rbac.authorization.k8s.io/v1alpha1`.
- Alpha level:
- The version names contain `alpha` (e.g. `v1alpha1`).
- May be buggy. Enabling the feature may expose bugs. Disabled by default.
- Support for feature may be dropped at any time without notice.
- The API may change in incompatible ways in a later software release without notice.
- 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` (e.g. `v2beta3`).
- Code is well tested. Enabling the feature is considered safe. Enabled by default.
- Support for the overall feature will not be dropped, though 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,
we will provide instructions for migrating to the next version. 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.
- 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.
Versioning is done 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.
## API groups
Refer to [API versions reference](/docs/reference/using-api/api-overview/#api-versioning)
for more details on the API version level definitions.
To make it easier to extend its API, Kubernetes implements [*API groups*](https://git.k8s.io/community/contributors/design-proposals/api-machinery/api-group.md).
The API group is specified in a REST path and in the `apiVersion` field of a serialized object.
To make it easier to evolve and to extend its API, Kubernetes implements
[API groups](/docs/reference/using-api/api-overview/#api-groups) that can be
[enabled or disabled](/docs/reference/using-api/api-overview/#enabling-or-disabling).
There are several API groups in a cluster:
## API Extension
1. The *core* group, also referred to as the *legacy* group, is at the REST path `/api/v1` and uses `apiVersion: v1`.
1. *Named* groups are at REST path `/apis/$GROUP_NAME/$VERSION`, and use `apiVersion: $GROUP_NAME/$VERSION`
(e.g. `apiVersion: batch/v1`). The Kubernetes [API reference](/docs/reference/kubernetes-api/) has a
full list of available API groups.
There are two paths to extending the API with [custom resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/):
1. [CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/)
lets you declaratively define how the API server should provide your chosen resource API.
1. You can also [implement your own extension API server](/docs/tasks/extend-kubernetes/setup-extension-api-server/)
and use the [aggregator](/docs/tasks/extend-kubernetes/configure-aggregation-layer/)
to make it seamless for clients.
## Enabling or disabling API groups
Certain resources and API groups are enabled by default. They can be enabled or disabled by setting `--runtime-config`
as a command line option to the kube-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 API server.
{{< note >}}Enabling or disabling groups or resources requires restarting the kube-apiserver and the
kube-controller-manager to pick up the `--runtime-config` changes.{{< /note >}}
## Persistence
Kubernetes stores its serialized state in terms of the API resources by writing them into
{{< glossary_tooltip term_id="etcd" >}}.
The Kubernetes API can be extended in one of two ways:
1. [Custom resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/)
let you declaratively define how the API server should provide your chosen resource API.
1. You can also extend the Kubernetes API by implementing an
[aggregation layer](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/).
## {{% heading "whatsnext" %}}
[Controlling API Access](/docs/reference/access-authn-authz/controlling-access/) describes
how the cluster manages authentication and authorization for API access.
Overall API conventions are described in the
[API conventions](https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#api-conventions)
document.
API endpoints, resource types and samples are described in the [API Reference](/docs/reference/kubernetes-api/).
- Learn how to extend the Kubernetes API by adding your own
[CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/).
- [Controlling API Access](/docs/reference/access-authn-authz/controlling-access/) describes
how the cluster manages authentication and authorization for API access.
- Learn about API endpoints, resource types and samples by reading
[API Reference](/docs/reference/kubernetes-api/).