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Customization approaches can be broadly divided into *configuration*, which only involves changing flags, local configuration files, or API resources; and *extensions*, which involve running additional programs or services. This document is primarily about extensions.
## Configuration
*Configuration files* and *flags* are documented in the Reference section of the online documentation, under each binary:
Flags 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 administrator. 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](/docs/concepts/policy/resource-quotas/), [PodSecurityPolicies](/docs/concepts/policy/pod-security-policy/), [NetworkPolicy](/docs/concepts/services-networking/network-policies/) and Role-based Access Control ([RBAC](/docs/reference/access-authn-authz/rbac/)), are built-in Kubernetes APIs. APIs are typically used with hosted Kubernetes services and with managed Kubernetes installations. They are declarative and use the same conventions as other Kubernetes resources like pods, so new cluster configuration can be repeatable and be managed the same way as applications. And, where they are stable, they enjoy a [defined support policy](/docs/reference/deprecation-policy/) like other Kubernetes APIs. For these reasons, they are preferred over *configuration files* and *flags* where suitable.
1. Users often interact with the Kubernetes API using `kubectl`. [Kubectl plugins](/docs/tasks/extend-kubectl/kubectl-plugins/) extend the kubectl binary. They only affect the individual user's local environment, and so cannot enforce site-wide policies.
2. The apiserver handles all requests. Several types of extension points in the apiserver allow authenticating requests, or blocking them based on their content, editing content, and handling deletion. These are described in the [API Access Extensions](/docs/concepts/overview/extending#api-access-extensions) section.
3. The apiserver serves various kinds of *resources*. *Built-in resource kinds*, like `pods`, are defined by the Kubernetes project and can't be changed. You can also add resources that you define, or that other projects have defined, called *Custom Resources*, as explained in the [Custom Resources](/docs/concepts/overview/extending#user-defined-types) section. Custom Resources are often used with API Access Extensions.
4. The Kubernetes scheduler decides which nodes to place pods on. There are several ways to extend scheduling. These are described in the [Scheduler Extensions](/docs/concepts/overview/extending#scheduler-extensions) section.
5. Much of the behavior of Kubernetes is implemented by programs called Controllers which are clients of the API-Server. Controllers are often used in conjunction with Custom Resources.
6. The kubelet runs on servers, and helps pods appear like virtual servers with their own IPs on the cluster network. [Network Plugins](/docs/concepts/overview/extending#network-plugins) allow for different implementations of pod networking.
7. The kubelet also mounts and unmounts volumes for containers. New types of storage can be supported via [Storage Plugins](/docs/concepts/overview/extending#storage-plugins).
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`.
Do not use a Custom Resource as data storage for application, user, or monitoring data.
Often, when you add a new API, you also add a control loop that reads and/or writes the new APIs. When the combination of a Custom API and a control loop is used to manage a specific, usually stateful, application, this is called the *Operator* pattern. Custom APIs and control loops can also be used to control other resources, such as storage, policies, and so on.
### 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 (e.g. Pods), but API Access Extensions do.
When a request reaches the Kubernetes API Server, it is first Authenticated, then Authorized, then subject to various types of Admission Control. See [Controlling Access to the Kubernetes API](/docs/reference/access-authn-authz/controlling-access/)] for more on this flow.
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 (a webhook). All of these methods are covered in the [Authentication documentation](/docs/reference/access-authn-authz/authentication/).
[Authentication](/docs/reference/access-authn-authz/authentication/) maps headers or certificates in all requests to a username for the client making the request.
Kubernetes provides several built-in authentication methods, and an [Authentication webhook](/docs/reference/access-authn-authz/authentication/#webhook-token-authentication) method if those don't meet your needs.
[Authorization](/docs/reference/access-authn-authz/webhook/) determines whether specific users can read, write, and do other operations on API resources. It just 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, and [Authorization webhook](/docs/reference/access-authn-authz/webhook/) allows calling out to user-provided code to make an authorization decision.
After a request is authorized, if it is a write operation, it also goes through [Admission Control](/docs/reference/access-authn-authz/admission-controllers/) steps. In addition to the built-in steps, there are several extensions:
* The [Image Policy webhook](/docs/reference/access-authn-authz/admission-controllers/#imagepolicywebhook) restricts what images can be run in containers.
* To make arbitrary admission control decisions, a general [Admission webhook](/docs/reference/access-authn-authz/extensible-admission-controllers/#admission-webhooks) can be used. Admission Webhooks can reject creations or updates.
* [Initializers](/docs/reference/access-authn-authz/extensible-admission-controllers/#initializers) are controllers that can modify objects before they are created. Initializers can modify initial object creations but cannot affect updates to objects. Initializers can also reject objects.
