How to understand the roles of Kubernetes scheduler, controller manager, and etcd

Introduction

This tutorial provides an overview of the Kubernetes architecture, focusing on the key components that manage the state and operations of a Kubernetes cluster. You will learn about the roles of the Kubernetes Scheduler, Controller Manager, and etcd, and how they work together to ensure the reliable and scalable deployment of your containerized applications.

Skills Graph

Overview of Kubernetes Architecture

Kubernetes is an open-source container orchestration platform that automates the deployment, scaling, and management of containerized applications. It provides a comprehensive set of features and components that work together to ensure the reliable and scalable operation of your applications.

At the heart of Kubernetes is the concept of a cluster, which consists of a set of worker nodes and a set of control plane components that manage the cluster. The worker nodes are the machines (physical or virtual) that run your containerized applications, while the control plane components are responsible for managing the overall state of the cluster.

The main components of a Kubernetes cluster include:

• API Server: The central control point that receives and processes API requests, validates and configures data for the API objects, and updates the state of the cluster accordingly.
• Scheduler: Responsible for placing Pods (the smallest deployable units in Kubernetes) on the appropriate nodes based on resource availability and other constraints.
• Controller Manager: Responsible for maintaining the desired state of the cluster, such as ensuring that the correct number of Pods are running, managing replication, and handling failover.
• etcd: A distributed key-value store that serves as the backbone of the Kubernetes cluster, storing all the critical data that the Kubernetes control plane needs to manage the state of the cluster.

Here's an example of how you can deploy a simple Nginx web server using Kubernetes:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.14.2
          ports:
            - containerPort: 80

This YAML file defines a Deployment resource that will create three replicas of an Nginx web server. The Kubernetes control plane will ensure that these Pods are scheduled and managed according to the desired state specified in the Deployment.

Kubernetes Scheduler: Placing Pods on Nodes

The Kubernetes Scheduler is a crucial component responsible for placing Pods on the appropriate nodes within the cluster. It plays a vital role in ensuring that the available resources are utilized efficiently and that the application's requirements are met.

The Scheduler's primary function is to match Pods to Nodes based on various factors, such as resource availability, constraints, and user-defined policies. It evaluates the current state of the cluster, including the resource capacity of each node and the resource requirements of each Pod, and then makes scheduling decisions to optimize the overall cluster utilization.

The Scheduler's scheduling process can be divided into the following key steps:

1. Filtering: The Scheduler first filters the available nodes based on the Pod's resource requirements and constraints, such as node affinity, taints, and tolerations.
2. Scoring: After filtering, the Scheduler scores the remaining nodes based on various factors, such as resource availability, node utilization, and user-defined priorities.
3. Binding: Finally, the Scheduler binds the Pod to the highest-scoring node, updating the cluster state accordingly.

Here's an example of how you can configure the Scheduler's behavior using a custom scheduling policy:

apiVersion: v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000000
globalDefault: false
description: "This priority class should be used for high-priority workloads."
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: high-priority-app
spec:
  replicas: 3
  priorityClassName: high-priority
  ## Other Deployment configuration

In this example, we define a custom PriorityClass called "high-priority" with a value of 1000000. We then apply this priority class to a Deployment, ensuring that the Pods belonging to this Deployment will be scheduled with higher priority than the default Pods.

By understanding the Kubernetes Scheduler's functionality and leveraging its scheduling policies, you can optimize the placement of your Pods and ensure that your applications are running efficiently on the available resources.

Kubernetes Controller Manager and etcd: Maintaining Cluster State

The Kubernetes Controller Manager and etcd are two critical components responsible for maintaining the desired state of the Kubernetes cluster.

The Kubernetes Controller Manager is a control plane component that runs a collection of controllers, each of which is responsible for monitoring the state of the cluster and taking appropriate actions to ensure that the actual state matches the desired state. These controllers include the Replication Controller, Endpoint Controller, and Node Controller, among others.

The Controller Manager continuously monitors the cluster state and performs various tasks, such as:

• Ensuring that the correct number of Pods are running for each Deployment or ReplicaSet.
• Managing the lifecycle of Nodes, including the detection and handling of failed nodes.
• Updating Endpoint objects to reflect the current set of Pods that are members of a Service.

On the other hand, etcd is a distributed key-value store that serves as the backbone of the Kubernetes cluster. It stores all the critical data that the Kubernetes control plane needs to manage the state of the cluster, including:

• Cluster configuration
• Resource definitions (Pods, Services, Deployments, etc.)
• Cluster state information

The Controller Manager relies on etcd to store and retrieve the necessary information to perform its tasks. When changes are made to the cluster, the Controller Manager updates the corresponding data in etcd, ensuring that the cluster state is accurately maintained.

Here's an example of how you can interact with etcd to retrieve the current state of a Kubernetes resource:

## Install the etcdctl command-line tool
sudo apt-get install etcd-client

## Retrieve the current state of a Deployment
etcdctl get /registry/deployments/default/nginx-deployment

By understanding the roles of the Kubernetes Controller Manager and etcd, you can gain a deeper understanding of how Kubernetes maintains the desired state of the cluster and ensures the reliable operation of your applications.

Summary

In this tutorial, you have learned about the core components of the Kubernetes architecture, including the Scheduler, Controller Manager, and etcd. The Scheduler is responsible for placing Pods on the appropriate nodes, the Controller Manager maintains the desired state of the cluster, and etcd serves as the distributed key-value store that stores critical cluster data. Understanding the roles and interactions of these components is essential for effectively managing and scaling your Kubernetes-based applications.