# Cluster Internals

This article describes several of the service elements that constitute a notable portion of the Platform9 components. In addition to the pf9-hostagent, pf9-comms, and pf9-nodelet services, Platform9 also installs other services that comprise a Kubernetes Cluster. The [Host Internals](https://platform9.com/docs/v5.3/PES/host-internals) page describes these elements in additional detail.

For further details on nodelet, please review the [Nodelet](https://platform9.com/docs/kubernetes/nodelet) page filed under the PMK documentation. The following diagram illustrates a high-level view of the different services, containers, and processes involved in creating the cluster. In the diagram below, note the hierarchy of various components.

* **pf9-hostagent:** This is the prominent service that is responsible for initiating nodeletd.
* **nodeletd:** The nodeletd functions on the worker and master differ slightly. For example, the following diagram shows a master configuration. It starts these three primary services:
  * kubelet
  * dockerd
  * keepalived

![](https://content.gitbook.com/content/tZryvDiHvZIhOZofYOWU/blobs/11MZhQmnM0PvFacf8LIe/1633651672.png)

* **dockerd:** This process creates containers via the docker daemon.
* **keepalived:** Bare metal systems primarily use keepalived to provide highly available IP addresses between the different Kubernetes API servers.
* **kubelet:** This Kubernetes component runs as needed as a systemd unit file. It initiates various containers like the k8s-master, apiserver, controller, scheduler, etc. It also starts the addon-operator, which in turn launches other critical system addons.
* **kube-proxy, etcd:** Both of these components run as docker containers.

### Master - Worker Connections

The following diagram illustrates the different networking connections between masters and workers.

* **etcd:** etcd creates clusters and allows them to talk, if it exists in a mesh network.
* **apiserver:** The apiserver on each master talks to the other corresponding etcd processes on each localhost.
* **VIP:** A virtual IP (or VIP) is created and is maintained by keepalived using the VRRP protocol. Workers use the VIP to communicate with the masters. If a node fails, the VIP will move between the master nodes.
* **kubelet:** The kubelet on each worker node will connect to the VIP (and hence, each of the servers).

![](https://content.gitbook.com/content/tZryvDiHvZIhOZofYOWU/blobs/2s8X6CuDd64DFzbTgq3T/1633652193.png)