Kubernetes components

A Kubernetes cluster has several master components used to control the cluster, as well as node components that run on each cluster node. Let's get to know all these components and how they work together.

Master components

The master components typically run on one node, but in a highly available or very large cluster, they may be spread across multiple nodes.

API server

The kube API server exposes the Kubernetes REST API. It can easily scale horizontally as it is stateless and stores all the data in the etcd cluster. The API server is the embodiment of the Kubernetes control plane.

Etcd

Etcd is a highly reliable distributed data store. Kubernetes uses it to store the entire cluster state. In small, transient cluster a single instance of etcd can run on the same node with all the other master components. But, for more substantial clusters it is typical to have a 3-node or even 5-node etcd cluster for redundancy and high availability.

Controller manager

The controller manager is a collection of various managers rolled up into one binary. It contains the replication controller, the pod controller, the services controller, the endpoints controller, and others. All these managers watch over the state of the cluster via the API and their job is to steer the cluster into the desired state.

Scheduler

The kube-scheduler is responsible for scheduling pods into nodes. This is a very complicated task as it needs to consider multiple interacting factors, such as the following:

• Resource requirements
• Service requirements
• Hardware/software policy constraints
• Affinity and anti-affinity specifications
• Data locality
• Deadlines

DNS

Starting with Kubernetes 1.3, a DNS service is part of the standard Kubernetes cluster. It is scheduled as a regular pod. Every service (except headless services) receives a DNS name. Pods can receive a DNS name too. This is very useful for automatic discovery.

Node components

Nodes in the cluster need a couple of components to interact with the cluster master components, receive workloads to execute, and update the cluster on their status.

Proxy

The kube proxy does low-level network housekeeping on each node. It reflects the Kubernetes services locally and can do TCP and UDP forwarding. It finds cluster IPs via environment variables or DNS.

Kubelet

The kubelet is the Kubernetes representative on the node. It oversees communicating with the master components and manage the running pods. That includes the following:

• Download pod secrets from the API server
• Mount volumes
• Run the pod's container (Docker or Rkt)
• Report the status of the node and each pod
• Run container liveness probes

In this section, we dug into the guts of Kubernetes and explored its architecture from a very high level of vision and supported design patterns, through its APIs and the components used to control and manage the cluster. In the next section, we will take a quick look at the various runtimes that Kubernetes supports.