Architecture
Control Plane - Data Plane Design Pattern
On a very high level, Kubernetes architecture can be divided into two planes: the control plane and the data plane. This design pattern used in distributed systems as a way for separation of concerns. To put it simply,
The control plane makes decisions and the data plane carries out those decisions.
Keeping this in mind, let's dive into the architecture of Kubernetes.
The Kubernetes Control Plane
The Kubernetes control plane has the following components:
- API Server
- etcd
- Scheduler
- Controller Manager
- Cloud Controller Manager
1. API Server
The API Server is how you interact with Kubernetes. In the following chapters, you will use a command-line tool called kubectl. This tool communicates with the API Server and directs the cluster to do what you want.
2. etcd
etcd is a key-value store. The d in etcd stands for distributed.
Kubernetes uses etcd to store data for all of it's resources, such as pods, services, deployments, and more. Kubernetes uses etcd because it's a reliable way to store data for distributed systems, as you can have multiple instances of etcd running at the same time, synchronizing data between them.
Remind me to use etcd as an example for distributed systems in the future.
3. Scheduler
It's job is straightforward: it schedules pods to run on nodes.
When you create a pod, you don't have to worry about where it runs. The Scheduler monitors the cluster and assigns pods to nodes. The scheduler interacts with the etcd through the API Server to get information about the cluster and make decisions about where to run pods.
4. Controller Manager
It is a collection of controllers. Each controller is responsible for managing a specific resource in the cluster.
For example, the ReplicaSet controller is responsible for managing ReplicaSets. The Deployment controller is responsible for managing Deployments.
5. Cloud Controller Manager
This component is helps you run your Kubernetes cluster on a cloud provider. It's a way to abstract the cloud provider's APIs from the core Kubernetes code.
For example, if you're running Kubernetes on AWS, the Cloud Controller Manager will help you interact with AWS APIs. If you're running Kubernetes on GCP, the Cloud Controller Manager will help you interact with GCP APIs.
Now that we have seen the control plane, let's move on to the data plane.
The Kubernetes Data Plane
The data plane is where the actual work happens. It mainly consists of nodes which run the following components:
- Kubelet
- Kube Proxy
- Container Runtime
1. Kubelet
The Kubelet runs as a linux service on each node. This is responsible for registering the node with the API Server, and managing the containers within the node. It must be noted that the Kubelet only manages the containers that are created through Kubernetes, so any other containers on your nodes that are not managed by Kubernetes are not managed by the Kubelet.
2. Kube Proxy
It handles the network related operations. Remember we talked about services in the previous chapter? The Kube Proxy handles just that.
3. Container Runtime
It is responsible for running containers on the nodes.
Now that we've gone through the definitions, let's see how it looks in a diagram.
The following diagram is taken from the official Kubernetes documentation, I've added a distinction to show the control plane and data plane.
To show the interaction between the components, we'll go through behind the scene working of a Kubernetes deployment in the next chapter. This will help you truly understand and appreciate how the control plane and data plane work together.