Intro: So… What Powers the Kubernetes Brain?

Everyone loves to show off their YAML-fu and talk about Pods and Deployments, but what’s actually running behind the scenes? What's keeping track of all your services, secrets, and scheduled chaos? Today I'd like to bring your a quick introduction about the Kubernetes control plane components—the brains of the operation. It’s made of several server-side components that work together like an orchestra of background daemons with trust issues and strict roles.

In this post, we’ll demystify each core server running in a Kubernetes control plane:
✅ etcd
✅ kube-apiserver
✅ kube-scheduler
✅ kube-controller-manager
✅ cloud-controller-manager
✅ kubelet
✅ kube-proxy
✅ coredns
✅ Optional players (metrics-server, CRI, CSI, etc.)

Let’s start with the foundation: how these pieces talk.

🧩 Kubernetes Architecture in a Nutshell

Kubernetes has two main types of nodes:

• Control Plane Nodes (aka “masters”): run the brain (scheduler, API, etc.)

• Worker Nodes: run your actual workloads (pods)

Think of the control plane as mission control and the worker nodes as spacecraft. One issues orders, the other executes.

They talk over HTTP/gRPC and communicate securely via TLS.

Now let’s break down the core components—what they are, what they do, and how they fit together.

🔐 1. Kubernetes Control Plane Components - etcd

What it is:
A distributed key-value store used to store all cluster data: objects, state, configs.

Think of it as:
The brain’s hard disk.

What lives inside etcd?

• Pod definitions

• ConfigMaps

• Secrets

• Cluster state

• Node info

• RoleBindings, CRDs, everything

Backed by:
etcd (from CoreOS), written in Go, uses the Raft consensus algorithm for HA and consistency.

Why it matters:
You never talk to etcd directly. The API server does only.

🧭 2. Kubernetes Control Plane Components - kube-apiserver

What it is:
The main RESTful API that all clients (kubectl, controllers, kubelet) talk to.

Think of it as:
The gatekeeper and translator.

Responsibilities:

• Validates incoming requests

• Authenticates + authorizes them

• Talks to etcd to read/write state

• Notifies other components via the Watch API

It’s stateless, and often run behind a load balancer for HA.

⏱ 3. Kubernetes Control Plane Components - kube-scheduler

What it is:
Assigns unscheduled pods to nodes, based on constraints.

Think of it as:
The Tinder for workloads. It matches pods to nodes based on:

• CPU/memory availability

• Node taints/tolerations

• Affinity rules

• Pod priority

• Custom scoring plugins

The flow:

1. A pod is created with no nodeName.

2. API server stores it in etcd.

3. Scheduler sees it, scores possible nodes.

4. It picks a winner and updates the pod with nodeName.

🧙 4. Kubernetes Control Plane Components - kube-controller-manager

What it is:
A daemon that runs many controllers in one binary.

Think of it as:
The cluster babysitter—constantly checking for drift and correcting it.

Runs controllers for:

• Deployments

• Replicasets

• Nodes (watching for crashes)

• Endpoints

• Persistent Volumes

• Certificates

How it works:
Each controller watches a specific object type via the API server and ensures the desired state matches reality. If not, it acts.

☁️ 5. Kubernetes Control Plane Components - cloud-controller-manager

What it is:
An optional component for clusters running on public clouds (AWS, GCP, Azure).

Think of it as:
The bridge between Kubernetes and your cloud infrastructure.

Responsibilities:

• Creating LoadBalancers

• Managing cloud-based node info

• Attaching volumes (via CSI)

• Updating routes

You won’t see this in bare-metal clusters unless you set up external integrations manually.

🧍 6. Kubernetes Control Plane Components - kubelet

What it is:
A daemon that runs on every worker node.

Think of it as:
The node’s supervisor.

What it does:

• Watches for pods assigned to its node

• Pulls container images via the CRI

• Starts/stops containers using container runtimes (containerd, CRI-O)

• Sends status updates back to API server

Important:
kubelet does not manage containers you started outside of Kubernetes.

🌐 7. Kubernetes Control Plane Components - kube-proxy

What it is:
Runs on each node to implement service networking.

Think of it as:
A port-forwarding ninja that handles cluster IP routing rules.

Modes:

• iptables (legacy, still common)

• ipvs (newer, faster, uses Linux’s IPVS)

• eBPF (if you’re fancy and using Cilium)

What it does:

• Maintains network rules to forward traffic to the correct pod behind a Service

• Enables internal DNS (via CoreDNS)

• Helps implement ClusterIP, NodePort, LoadBalancer behavior

🧠 8. Kubernetes Control Plane Components - CoreDNS

What it is:
Default DNS service in Kubernetes.

Think of it as:
Your cluster’s internal phone book.

What it does:

• Resolves pod/service names to cluster IPs

• Works with kube-dns-compatible tools

• Responds to all *.svc.cluster.local domain lookups

Runs as a Deployment + Service, just like any other app—because Kubernetes eats its own dog food.

🛠 Supporting Components

📦 Kubernetes Control Plan Components Diagram

+--------------------------------------------------+
|                  Control Plane                   |
| +-----------------+      +--------------------+  |
| | kube-apiserver  | <--> |  etcd              |  |
| +-----------------+      +--------------------+  |
|        ^                                         |
|        |                                         |
| +----------------------------+                   |
| |  kube-controller-manager   |                   |
| +----------------------------+                   |
| +----------------------------+                   |
| |       kube-scheduler       |                   |
| +----------------------------+                   |
| +----------------------------+                   |
| | cloud-controller-manager   |                   |
| |       (optional)           |                   |
| +----------------------------+                   |
+--------------------------------------------------+

+-------------------------------------------------+
|                Worker Node                      |
| +---------+  +-----------+  +-----------------+ |
| | kubelet |  | kube-proxy|  | containerd/CRI  | |
| +---------+  +-----------+  +-----------------+ |
| +---------------------------------------------+ |
| |                Pods (your app)              | |
| +---------------------------------------------+ |
+-------------------------------------------------+

I really like this simplifed diagram!

🧠 A Quick Reference