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Enhance documentation for the Gardenlet's /healthz endpoint #3359

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Jan 8, 2021
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Enhance documentation for the Gardenlet's /healthz endpoint #3359

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23 changes: 16 additions & 7 deletions docs/concepts/gardenlet.md
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Expand Up @@ -146,23 +146,24 @@ More information: [Example Gardenlet Component Configuration](../../example/20-c

## Heartbeats

Similar to how Kubernetes is meanwhile using `Lease` objects for node heart beats
Similar to how Kubernetes uses `Lease` objects for node heart beats
(see [KEP](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/0009-node-heartbeat.md)),
the gardenlet is using `Lease` objects for heart beats of the seed cluster.
Every two seconds, the gardenlet is checking its connectivity
to the seed cluster and then it renews its lease.
The status is reported in the `GardenletReady` condition in its `Seed` object list.
Every two seconds, the gardenlet checks that the seed cluster's `/healthz`
endpoint returns HTTP status code 200.
If that is the case, the gardenlet renews the lease in the Garden cluster in the `gardener-system-seed-lease` namespace and updates
the `GardenletReady` condition in the `status.conditions` field of the `Seed` resource(s).

Similarly to the `node-lifecycle-controller` inside the `kube-controller-manager`,
the `gardener-controller-manager` features a `seed-lifecycle-controller` that sets
the `GardenletReady` condition to `Unknown` in case the gardenlet stops
sending its heartbeat signals.
the `GardenletReady` condition to `Unknown` in case the gardenlet fails to renew the lease.
As a consequence, the `gardener-scheduler` doesn’t consider this seed cluster for newly created shoot clusters anymore.

### `/healthz` Endpoint

The gardenlet includes an HTTPS server that serves a `/healthz` endpoint.
It’s used as a liveness probe in the `Deployment` of the gardenlet.
If the gardenlet fails trying to renew its lease,
If the gardenlet fails to renew its lease
then the endpoint returns `500 Internal Server Error`, otherwise it returns `200 OK`.

> ⚠️<br> In case the gardenlet is managing multiple seeds
Expand All @@ -171,6 +172,14 @@ then the endpoint returns `500 Internal Server Error`, otherwise it returns `200
> for which it couldn’t renew its lease. Only if it can renew the lease
> for all seeds it reports `200 OK`.

Please note, that the `/healthz` only indicates whether the gardenlet
could successfully probe the Seed's API server and renew the lease with
the Garden cluster.
It does *not* show that the Gardener extension API server (with the Gardener resource groups)
is available.
However, the Gardenlet is designed to withstand such connection outages and
retries until the connection is reestablished.

## Shooted Seeds

Gardener users can use shoot clusters as seed clusters, so-called "shooted seeds",
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