2. An Overview of Kubernetes

Overview

In this chapter, we will have our first hands-on introduction to Kubernetes. This chapter will give you a brief overview of the different components of Kubernetes and how they work together. We will also try our hand at working with some fundamental Kubernetes components.

By the end of this chapter, you will have a single-node Minikube environment set up where you can run many of the exercises and activities in this book. You will be able to understand the high-level architecture of Kubernetes and identify the roles of the different components. You will also learn the basics required to migrate containerized applications to a Kubernetes environment.

Introduction

We ended the previous chapter by providing a brief and abstract introduction to Kubernetes, as well as some of its advantages. In this chapter, we will provide you with a much more concrete high-level understanding of how Kubernetes works. First, we will walk you through how to install Minikube, which is a handy tool that creates a single-node cluster and provides a convenient learning environment for Kubernetes. Then, we will take a 10,000-foot overview of all the components, including their responsibilities and how they interact with each other. After that, we will migrate the Docker application that we built in the previous chapter to Kubernetes and illustrate how it can enjoy the benefits afforded by Kubernetes, such as creating multiple replicas, and version updates. Finally, we will explain how the application responds to external and internal traffic.

Having an overview of Kubernetes is important before we dive deeper into the different aspects of it so that...

Setting up Kubernetes

Had you asked the question, "How do you easily install Kubernetes?" three years ago, it would have been hard to give a compelling answer. Embarrassing, but true. Kubernetes is a sophisticated system, and getting it installed and managing it well isn't an easy task.

However, as the Kubernetes community has expanded and matured, more and more user-friendly tools have emerged. As of today, based on your requirements, there are a lot of options to choose from:

• If you are using physical (bare-metal) servers or virtual machines (VMs), Kubeadm is a good fit.
• If you're running on cloud environments, Kops and Kubespray can ease Kubernetes installation, as well as integration with the cloud providers. In fact, we will teach you how to deploy Kubernetes on AWS using Kops in Chapter 11, Build Your Own HA Cluster, and we will take another look at the various options we can use to set up Kubernetes.
• If you want to drop the burden of managing...

Kubernetes Components Overview

By completing the previous exercise, you have a single-node Kubernetes cluster up and running. Before playing your first concert, let's hold on a second and pull the curtains aside to take a look backstage to see how Kubernetes is architected behind the scenes, and then check how Minikube glues its various components together inside its VM.

Kubernetes has several core components that make the wheels of the machine turn. They are as follows:

• API server
• etcd
• Controller manager
• Scheduler
• Kubelet

These components are critical for the functioning of a Kubernetes cluster.

Besides these core components, you would deploy your applications in containers, which are bundled together as pods. We will learn more about pods in Chapter 5, Pods. These pods, and several other resources, are defined by something called API objects.

An API object describes how a certain resource should be honored in Kubernetes. We usually define...

Kubernetes Architecture

In the previous section, we gained a first impression of the core Kubernetes components: etcd, the API server, the scheduler, the controller manager, and the kubelet. These components, plus other add-ons, comprise the Kubernetes architecture, which can be seen in the following diagram:

Figure 2.8: Kubernetes architecture

At this point, we won't look at each component in too much detail. However, at a high-level view, it's critical to understand how the components communicate with each other and why they're designed in that way.

The first thing to understand is which components the API server can interact with. From the preceding diagram, we can easily tell that the API server can talk to almost every other component (except the container runtime, which is handled by the kubelet) and that it also serves to interact with end-users directly. This design makes the API server act as the "heart" of Kubernetes. Additionally...

Migrating Containerized Application to Kubernetes

In the previous chapter, we built a simple HTTP server called k8s-for-beginners, and it runs as a Docker container. It works perfectly for a sample application. However, what if you have to manage thousands of containers, and coordinate and schedule them properly? How can you upgrade a service without downtime? How do you keep a service healthy upon unexpected failure? These problems exceed the abilities of a system that simply uses containers alone. What we need is a platform that can orchestrate, as well as manage, our containers.

We have told you that Kubernetes is the solution that we need. Next, we will walk you through a series of exercises regarding how to orchestrate and run containers in Kubernetes using a Kubernetes native approach.

Pod Specification

A straightforward thought is that we wish to see what the equivalent API call or command to run a container in Kubernetes is. As explained in Chapter 1, Introduction...

Delivering Kubernetes-Native Applications

In the previous sections, we migrated a Docker-based application to Kubernetes and successfully accessed it from inside the Minikube VM, as well as externally. Now, let's see what other benefits Kubernetes can provide if we design our application from the ground up so that it can be deployed using Kubernetes.

Along with the increasing usage of your application, it may be common to run several replicas of certain pods to serve a business functionality. In this case, grouping different containers in a pod alone is not sufficient. We need to go ahead and create groups of pods that are working together. Kubernetes provides several abstractions for groups of pods, such as Deployments, DaemonSets, Jobs, CronJobs, and so on. Just like the Service object, these objects can also be created by using a spec that's been defined in a YAML file.

To start understanding the benefits of Kubernetes, let's use a Deployment to demonstrate...

Pod Life Cycle and Kubernetes Components

The previous sections in this chapter briefly described the Kubernetes components and how they work internally with each other. On the other hand, we also demonstrated how to use some Kubernetes API objects (Pods, Services, and Deployments) to compose your applications.

But how is a Kubernetes API object managed by different Kubernetes components? Let's consider a pod as an example. Its life cycle can be illustrated as follows:

Figure 2.20: The process behind the creation of a pod

This entire process can be broken down as follows:

1. A user starts to deploy an application by sending a Deployment YAML manifest to the Kubernetes API server. The API server verifies the request and checks whether it's valid. If it is, it persists the Deployment API object to its backend datastore (etcd).

   Note

   For any step that evolves by modifying API objects, interactions have to happen between etcd and the API server, so...

Summary

In this chapter, we used Minikube to provision a single-node Kubernetes cluster and gave a high-level overview of Kubernetes' core components, as well as its key design rationale. After that, we migrated an existing Docker container to Kubernetes and explored some basic Kubernetes API objects, such as pods, Services, and Deployments. Lastly, we intentionally broke a Kubernetes cluster and restored it one component at a time, which allowed us to understand how the different Kubernetes components work together to get a pod up and running on a node.

Throughout this chapter, we have used kubectl to manage our cluster. We provided a quick introduction to this tool, but in the following chapter, we will take a closer look at this powerful tool and explore the various ways in which we can use it.