Chapter 1: Kubernetes Architecture
Traditional applications, such as web applications, are known to follow a modular architecture, splitting code into an application layer, business logic, a storage layer, and a communication layer. Despite the modular architecture, the components are packaged and deployed as a monolith. A monolith application, despite being easy to develop, test, and deploy, is hard to maintain and scale. This led to the growth of microservices architecture. Development of container runtimes like Docker and Linux Containers (LXC) has eased deployment and maintenance of applications as microservices.
Microservices architecture splits application deployment into small and interconnected entities. The increasing popularity of microservices architecture has led to the growth of orchestration platforms such as Apache Swarm, Mesos, and Kubernetes. Container orchestration platforms help manage containers in large and dynamic environments.
Kubernetes is an open source...
The rise of Docker and the trend of microservices
Before we start looking into Kubernetes, it's important to understand the growth of microservices and containerization. With the evolution of a monolithic application, developers face inevitable problems as the applications evolve:
- Scaling: A monolith application is difficult to scale. It's been proven that the proper way to solve a scalability problem is via a distributed method.
- Operational cost: The operation cost increases with the complexity of a monolith application. Updates and maintenance require careful analysis and enough testing before deployment. This is the opposite of scalability; you can't scale down a monolithic application easily as the minimum resource requirement is high.
- Longer release cycle: The maintenance and development barrier is significantly high for monolith applications. For developers, when there is a bug, it takes a lot of time to identify the root cause in a complex and...
Kubernetes follows a client-server architecture. In Kubernetes, multiple master nodes control multiple worker nodes. Each master and worker has a set of components that are required for the cluster to work correctly. A master node generally has
kube-scheduler. The worker nodes have
kube-proxy, a Container Runtime Interface (CRI) component, a Container Storage Interface (CRI) component, and so on. We will go through each of them in detail now:
kube-apiserver: The Kubernetes API server (
kube-apiserver) is a control-plane component that validates and configures data for objects such as pods, services, and controllers. It interacts with objects using REST requests.
etcdis a high-availability key-value store used to store data such as configuration, state, and metadata. The watch functionality of
etcdprovides Kubernetes with the ability to listen for updates...
The storage and compute resources of the system are classified into different objects that reflect the current state of the cluster. Objects are defined using a
.yaml spec and the Kubernetes API is used to create and manage the objects. We are going to cover some common Kubernetes objects in detail.
A pod is a basic building block of a Kubernetes cluster. It's a group of one or more containers that are expected to co-exist on a single host. Containers within a pod can reference each other using localhost or inter-process communications (IPCs).
Kubernetes deployments help scale pods up or down based on labels and selectors. The YAML spec for a deployment consists of
replicas, which is the number of instances of pods that are required, and
template, which is identical to a pod specification.
A Kubernetes service is an abstraction of an application. A service enables network access for pods. Services and deployments work...
In the Kubernetes ecosystem, Kubernetes is the flagship among all variations. However, there are some other ships that play very important roles. Next, we will introduce some Kubernetes-like platforms, which serve different purposes in the ecosystem.
Minikube is the single-node cluster version of Kubernetes that can be run on Linux, macOS, and Windows platforms. Minikube supports standard Kubernetes features, such as
Ingress, container runtimes, and developer-friendly features such as add-ons and GPU support.
Minikube is a great starting place to get hands-on experience with Kubernetes. It's also a good place to run tests locally, especially cluster dependency or working on proof of concepts.
K3s is a lightweight Kubernetes platform. Its total size is less than 40 MB. It is great for Edge, Internet of Things (IoT), and ARM, previously Advanced RISC Machine, originally Acorn RISC Machine...
Kubernetes and cloud providers
A lot of people believe that Kubernetes is the future of infrastructure, and there are some people who believe that everything will end up on the cloud. However, this doesn't mean you have to run Kubernetes on the cloud, but it does work really well with the cloud.
Kubernetes as a service
Containerization makes applications more portable so that locking down with a specific cloud provider becomes unlikely. Although there are some great open source tools, such as
kops, that can help DevOps create Kubernetes clusters, Kubernetes as a service offered by a cloud provider still sounds attractive. As the original creator of Kubernetes, Google has offered Kubernetes as a service since 2014. It is called Google Kubernetes Engine (GKE). In 2017, Microsoft offered its own Kubernetes service, called Azure Kubernetes Service (AKS). AWS offered Elastic Kubernetes Service (EKS) in 2018.
The trend of microservices and the rise of Docker has enabled Kubernetes to become the de facto platform for DevOps to deploy, scale, and manage containerized applications. Kubernetes abstracts storage and computing resources as Kubernetes objects, which are managed by components such as
etcd, and so on.
Kubernetes can be created in a private data center or on the cloud or hybrid. This allows DevOps to work with multiple cloud providers and not get locked down to any one of them. Although Kubernetes is in GA as of 2018, it is still young and evolving very fast. As Kubernetes gets more and more attention, the attacks targeted at Kubernetes also become more notable.
In the next chapter, we are going to cover the Kubernetes network model and understand how microservices communicate with each other in Kubernetes.
The following links contain more detailed information about Kubernetes,
kops, and the OpenShift platform. You will find them useful when starting to build a Kubernetes cluster: