Understanding Kubernetes Architecture

In one sentence, Kubernetes is a platform to orchestrate the deployment, scaling, and management of container-based applications. You have probably read about Kubernetes, and maybe even dipped your toes in and used it in a side project or maybe even at work. But to understand what Kubernetes is all about, how to use it effectively, and what the best practices are requires much more.

Kubernetes is a big open source project and ecosystem with a lot of code and a lot of functionality. Kubernetes came out of Google, but joined the Cloud Native Computing Foundation (CNCF) and became the clear leader in the space of container-based applications.

In this chapter, we will build the foundation necessary to utilize Kubernetes to its full potential. We will start by understanding what Kubernetes is, what Kubernetes isn't, and what container orchestration means exactly. Then we will cover important Kubernetes concepts that will...

What is Kubernetes?

Kubernetes is a platform that encompasses a huge number of services and capabilities that keeps growing. The core functionality is scheduling workloads in containers across your infrastructure, but it doesn't stop there. Here are some of the other capabilities Kubernetes brings to the table:

• Mounting storage systems
• Distributing secrets
• Checking application health and readiness
• Replicating application instances
• Using the Horizontal Pod Autoscaler
• Using Cluster Autoscaling
• Naming and service discovery
• Balancing loads
• Rolling updates
• Monitoring resources
• Accessing and ingesting logs
• Debugging applications
• Providing authentication and authorization

We will cover all these capabilities in great detail throughout the book. At this point, just absorb and appreciate how much value Kubernetes can add to your system.

Kubernetes has impressive scope, but it is also important...

Kubernetes concepts

In this section, I'll briefly introduce many important Kubernetes concepts and give you some context as to why they are needed and how they interact with other concepts. The goal is to get familiar with these terms and concepts. Later, we'll see how these concepts are woven together and organized into API groups and resource categories to achieve awesomeness. You can consider many of these concepts as building blocks. Some of the concepts, such as nodes and masters, are implemented as a set of Kubernetes components. These components are at a different abstraction level, and I discuss them in detail in a dedicated section later in this chapter – Kubernetes components.

Here is the famous Kubernetes architecture diagram:

Figure 1.1: Kubernetes architecture diagram

Clusters

A cluster is a collection of hosts (nodes) that provide compute, memory, storage, and networking resources. Kubernetes uses these resources to run the various...

Diving into Kubernetes architecture in depth

Kubernetes has very ambitious goals. It aims to manage and simplify the orchestration, deployment, and management of distributed systems across a wide range of environments and cloud providers. It provides many capabilities and services that should work across all that diversity while evolving and remaining simple enough for mere mortals to use. This is a tall order. Kubernetes achieves this by following a crystal-clear, high-level design and well-thought-out architecture that promotes extensibility and pluggability. Many parts of Kubernetes are still hardcoded or environment-aware, but the trend is to refactor them into plugins and keep the core small, generic, and abstract. In this section, we will peel Kubernetes like an onion, starting with various distributed system design patterns and how Kubernetes supports them, then go over the surface of Kubernetes, which is its set of APIs, and then take a look at the actual components that comprise...

Kubernetes runtimes

Kubernetes originally only supported Docker as a container runtime engine. But that is no longer the case. Kubernetes now supports several different runtimes:

• Docker (via a CRI shim)
• rkt (direct integration to be replaced with Rktlet)
• CRI-O
• Frakti (Kubernetes on the Hypervisor, previously Hypernetes)
• rktlet (CRI implementation for rkt)
• CRI-containerd

The major design policy is that Kubernetes itself should be completely decoupled from specific runtimes. The Container Runtime Interface (CRI) enables it.

In this section, you'll get a closer look at the CRI and get to know the individual runtime engines. At the end of this section, you'll be able to make a well-informed decision about which runtime engine is appropriate for your use case and under what circumstances you may switch or even combine multiple runtimes in the same system.

The container runtime interface (CRI)

The CRI is a collection...

Continuous integration and deployment

Kubernetes is a great platform for running your microservice-based applications. But, at the end of the day, it is an implementation detail. Users, and often most developers, may not be aware that the system is deployed on Kubernetes. But Kubernetes can change the game and make things that were too difficult before possible.

In this section, we'll explore the CI/CD pipeline and what Kubernetes brings to the table. At the end of this section, you'll be able to design CI/CD pipelines that take advantage of Kubernetes properties such as easy scaling and development-production parity to improve the productivity and robustness of day-to-day development and deployment.

What is a CI/CD pipeline?

A CI/CD pipeline is a set of tools and steps that takes a set of changes by developers or operators that modify the code, data, or configuration of a system, tests them, and deploys them to production (and possibly other environments). Some...

Summary

In this chapter, we covered a lot of ground, and you got to understand the design and architecture of Kubernetes. Kubernetes is an orchestration platform for microservice-based applications running as containers. Kubernetes clusters have master and worker nodes. Containers run within pods. Each pod runs on a single physical or virtual machine. Kubernetes directly supports many concepts, such as services, labels, and persistent storage. You can implement various distributed system design patterns on Kubernetes. Container runtimes just need to implement the CRI. Docker, rkt, hyper containers, and more are supported.

In Chapter 2, Creating Kubernetes Clusters, we will explore the various ways to create Kubernetes clusters, discuss when to use different options, and build a multi-node cluster.