Add documentation for API server health checks

Author: johscheuer

content/en/docs/reference/using-api/health-checks.md

@@ -0,0 +1,93 @@
+---
+title: Kubernetes API health endpoints
+reviewers:
+- logicalhan
+content_type: concept
+weight: 50
+---
+
+<!-- overview -->
+The Kubernetes {{< glossary_tooltip term_id="kube-apiserver" text="API server" >}} provides API endpoints to indicate the current status of the API server.
+This page describes these API endpoints and explains how you can use them.
+
+<!-- body -->
+
+## API endpoints for health
+
+The Kubernetes API server provides 3 API endpoints (`healthz`, `livez` and `readyz`) to indicate the current status of the API server.
+The `healthz` endpoint is deprecated (since Kubernetes v1.16), and you should use the more specific `livez` and `readyz` endpoints instead.
+The `livez` endpoint can be used with the `--livez-grace-period` [flag](/docs/reference/command-line-tools-reference/kube-apiserver) to specify the startup duration.
+For a graceful shutdown you can specify the `--shutdown-delay-duration` [flag](/docs/reference/command-line-tools-reference/kube-apiserver) with the `/readyz` endpoint.
+Machines that check the `health`/`livez`/`readyz` of the API server should rely on the HTTP status code.
+A status code `200` indicates the the API server is `healthy`/`live`/`ready`, depending of the called endpoint.
+The more verbose options shown below are intended to be used by human operators to debug their cluster or specially the state of the API server.
+
+The following examples will show how you can interact with the health API endpoints.
+
+For all endpoints you can use the `verbose` parameter to print out the checks and their status.
+This can be useful for a human operator to debug the current status of the Api server, it is not intended to be consumed by a machine:
+
+    ```shell
+    curl -k https://localhost:6443/livez?verbose
+    ```
+
+The output will look like this:
+
+    [+]ping ok
+    [+]log ok
+    [+]etcd ok
+    [+]poststarthook/start-kube-apiserver-admission-initializer ok
+    [+]poststarthook/generic-apiserver-start-informers ok
+    [+]poststarthook/start-apiextensions-informers ok
+    [+]poststarthook/start-apiextensions-controllers ok
+    [+]poststarthook/crd-informer-synced ok
+    [+]poststarthook/bootstrap-controller ok
+    [+]poststarthook/rbac/bootstrap-roles ok
+    [+]poststarthook/scheduling/bootstrap-system-priority-classes ok
+    [+]poststarthook/start-cluster-authentication-info-controller ok
+    [+]poststarthook/start-kube-aggregator-informers ok
+    [+]poststarthook/apiservice-registration-controller ok
+    [+]poststarthook/apiservice-status-available-controller ok
+    [+]poststarthook/kube-apiserver-autoregistration ok
+    [+]autoregister-completion ok
+    [+]poststarthook/apiservice-openapi-controller ok
+    healthz check passed
+
+The Kubernetes API server also supports to exclude specific checks.
+The query parameters can also be combined like in this example:
+
+    ```shell
+    curl -k 'https://localhost:6443/readyz?verbose&exclude=etcd'
+    ```
+
+The output show that the `etcd` check is excluded:
+
+    [+]ping ok
+    [+]log ok
+    [+]etcd excluded: ok
+    [+]poststarthook/start-kube-apiserver-admission-initializer ok
+    [+]poststarthook/generic-apiserver-start-informers ok
+    [+]poststarthook/start-apiextensions-informers ok
+    [+]poststarthook/start-apiextensions-controllers ok
+    [+]poststarthook/crd-informer-synced ok
+    [+]poststarthook/bootstrap-controller ok
+    [+]poststarthook/rbac/bootstrap-roles ok
+    [+]poststarthook/scheduling/bootstrap-system-priority-classes ok
+    [+]poststarthook/start-cluster-authentication-info-controller ok
+    [+]poststarthook/start-kube-aggregator-informers ok
+    [+]poststarthook/apiservice-registration-controller ok
+    [+]poststarthook/apiservice-status-available-controller ok
+    [+]poststarthook/kube-apiserver-autoregistration ok
+    [+]autoregister-completion ok
+    [+]poststarthook/apiservice-openapi-controller ok
+    [+]shutdown ok
+    healthz check passed
+
+Each individual health check exposes an http endpoint and could can be checked individually.
+The schema for the individual health checks is `/livez/<healthcheck-name>` where `livez` and `readyz` and be used to indicate if you want to check thee liveness or the readiness of the API server.
+The `<healthcheck-name>` path can be discovered using the `verbose` flag from above and take the path between `[+]` and `ok`.
+These individual health checks should not be consumed by machines but can be helpful for a human operator to debug a system:
+
+    ```shell
+    curl -k https://localhost:6443/livez/etcd
+    ```