The primary responsibility of Kubernetes is container orchestration. This means making sure that all the containers that execute various workloads are scheduled to run on physical or virtual machines. The containers must be packed efficiently and follow the constraints of the deployment environment and the cluster configuration. In addition, Kubernetes must keep an eye on all running containers and replace dead, unresponsive, or otherwise unhealthy containers. Kubernetes provides many more capabilities that you will learn about in the following chapters. In this section, the focus is on containers and their orchestration.
Understanding container orchestration
Physical machines, virtual machines, and containers
It all starts and ends with hardware. In order to run your workloads, you need some real hardware provisioned. That includes actual physical machines, with certain compute capabilities (CPUs or cores), memory, and some local persistent storage (spinning disks or SSDs). In addition, you will need some shared persistent storage and the networking to hook up all these machines so they can find and talk to each other. At this point, you can run multiple virtual machines on the physical machines or stay at the bare-metal level (no virtual machines). Kubernetes can be deployed on a bare-metal cluster (real hardware) or on a cluster of virtual machines. Kubernetes in turn can orchestrate the containers it manages directly on bare-metal or virtual machines. In theory, a Kubernetes cluster can be composed of a mix of bare-metal and virtual machines, but this is not very common.
The benefits of containers
Containers represent a true paradigm shift in the development and operation of large, complicated software systems. Here are some of the benefits compared to more traditional models:
- Agile application creation and deployment
- Continuous development, integration, and deployment
- Dev and ops separation of concerns
- Environmental consistency across development, testing, and production
- Cloud- and OS-distribution portability
- Application-centric management
- Loosely coupled, distributed, elastic, liberated microservices
- Resource isolation
- Resource utilization
Containers in the cloud
Containers are ideal to package microservices because, while providing isolation to the microservice, they are very lightweight, and you don't incur a lot of overhead when deploying many microservices as you do with virtual machines. That makes containers ideal for cloud deployment, where allocating a whole virtual machine for each microservice would be cost prohibitive.
All major cloud providers, such as Amazon AWS, Google's GCE, Microsoft's Azure and even Alibaba Cloud, provide container-hosting services these days. Google's GKE has always been based on Kubernetes. AWS ECS is based on their own orchestration solution. Microsoft Azure's container service was based on Apache Mesos. Kubernetes can be deployed on all cloud platforms, but it wasn't deeply intergated with other services until today. But at the end of 2017, all cloud providers announced direct support for Kubernetes. Microsofts launched AKS, AWS released EKS, and Alibaba Cloud started working on a Kubernetes controller manager to integrate Kubernetes seamlessly.
Cattle versus pets
In the olden days, when systems were small, each server had a name. Developers and users knew exactly what software was running on each machine. I remember that, in many of the companies I worked for, we had multi-day discussions to decide on a naming theme for our servers. For example, composers and Greek mythology characters were popular choices. Everything was very cozy. You treated your servers like beloved pets. When a server died, it was a major crisis. Everybody scrambled to figure out where to get another server, what was even running on the dead server, and how to get it working on the new server. If the server stored some important data, then hopefully you had an up-to-date backup and maybe you'd even be able to recover it.
Obviously, that approach doesn't scale. When you have a few tens or hundreds of servers, you must start treating them like cattle. You think about the collective and not individuals. You may still have some pets, but your web servers are just cattle.
Kubernetes takes the cattle approach to the extreme and takes full responsibility for allocating containers to specific machines. You don't need to interact with individual machines (nodes) most of the time. This works best for stateless workloads. For stateful applications, the situation is a little different, but Kubernetes provides a solution called StatefulSet, which we'll discuss soon.
In this section, we covered the idea of container orchestration and discussed the relationships between hosts (physical or virtual) and containers, as well as the benefits of running containers in the cloud, and finished with a discussion about cattle versus pets. In the following section, we will get to know the world of Kubernetes and learn its concepts and terminology.
