Kubernetes API Aggregation Layer [page]deterministic
The aggregation layer allows Kubernetes to be extended with additional APIs, beyond what is offered by the core Kubernetes APIs. The additional APIs can either be ready-made solutions such as a [metrics server](https://github.com/kubernetes-sigs/metrics-server), or APIs that you develop yourself.
The aggregation layer is different from [Custom Resource Definitions](/docs/concepts/extend-kubernetes/api-extension/custom-resources/), which are a way to make the [kube-apiserver](#gloss:kube-apiserver) recognise new kinds of object.
## Aggregation layer
The aggregation layer runs in-process with the kube-apiserver. Until an extension resource is registered, the aggregation layer will do nothing. To register an API, you add an _APIService_ object, which "claims" the URL path in the Kubernetes API. At that point, the aggregation layer will proxy anything sent to that API path (e.g. `/apis/myextension.mycompany.io/v1/…`) to the registered APIService.
The most common way to implement the APIService is to run an *extension API server* in Pod(s) that run in your cluster. If you're using the extension API server to manage resources in your cluster, the extension API server (also written as "extension-apiserver") is typically paired with one or more [controllers](#gloss:controller). The apiserver-builder library provides a skeleton for both extension API servers and the associated controller(s).
### Response latency
Extension API servers should have low latency networking to and from the kube-apiserver. Discovery requests are required to round-trip from the kube-apiserver in five seconds or less.
If your extension API server cannot achieve that latency requirement, consider making changes that let you meet it.
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* To get the aggregator working in your environment, [configure the aggregation layer](/docs/tasks/extend-kubernetes/configure-aggregation-layer/). * Then, [setup an extension api-server](/docs/tasks/extend-kubernetes/setup-extension-api-server/) to work with the aggregation layer. * Read about [APIService](/docs/reference/kubernetes-api/cluster-resources/api-service-v1/) in the API reference * Learn about [Declarative Validation Concepts](/docs/reference/using-api/declarative-validation/), an internal mechanism for defining validation rules that in the future will help support validation for extension API server development.
Alternatively: learn how to extend the Kubernetes API using [Custom Resource Definitions](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/).