We learned docker, docker-compose. We know how to containerize our web app. Why do we need to learn yet another tool?
Note that docker and docker-compose runs on single host.
With help of Kubernetes we can run our app in multi-host environment.
With Kubernetes, we can easily deploy and run your software components without having to know about the actual servers underneath.
It doesn't matter if it has single host, or it has multiple-host or clusters.
When deploying a multi-component application through Kubernetes, it selects a server for each component, deploys it.
It also makes it easy to find and communicate with all the other components of your application.
Kubernetes enables you to run your applications on thousands of computer nodes as if all those nodes were a single, enormous computer.
It abstracts away the underlying infrastructure and, by doing so, simplifies development, deployment.
Kubernetes can be thought of as an operating system for the cluster
Kubernetes' architecture in its simplest form has two most important component.
Kubernetes Master Kubernetes Worker
Cloud
+----------------------+
| +----------------+ |
User+-----------------> | Kubernetes | |
| | Master | |
| +-------+--------+ |
| | |
| | |
| +-------v--------+ |
| | Kubernetes | |
| | Worker | |
| +----------------+ |
+----------------------+
As a user we always communicate to Kubernetes Master. We will use kubectl tool to interact with Kubernetes Master.
Kubernetes Master (Control Plane) has a subcomponents
Kubernetes API Server, which you and the other Control Plane components communicate withScheduler, which schedules your apps (assigns a worker node to each deployable component of your application)Controller Manager, which performs cluster-level functions, such as replicating components, keeping track of worker nodes, handling node failures, and so onetcd, a reliable distributed data store that persistently stores the cluster configuration.
+-------------------------------------+
| +--------------+ |
| | Scheduler | |
| +--------------+ |
| |
| +-----------+ +--------------+ |
| | API | | Controller | |
| | Server | | Manager | |
| +-----------+ +--------------+ |
| |
| +--------------+ |
| | etcd | |
| +--------------+ |
+-------------------------------------+
Kubernetes Worker the run your containerized applications.The task of running, monitoring, and providing services to your applications is done Workers.
Kubernetes Worker have different components.
Container Runtime: Docker, rkt, or another container runtime, which runs your containers.Kubelet, which talks to the API server and manages containers on its node.Kubernetes Service Proxy (kube-proxy), which load-balances network traffic between application components.
+------------------------------------------------+
| +------------+ +-----------------+ |
| | kublet | | kube-proxy | |
| +------------+ +-----------------+ |
| |
| +-----------------------+ |
| | Container Runtime | |
| +-----------------------+ |
+------------------------------------------------+
Within a cloud we can have lots of nodes, which may or may not be on same machine.
It is a part of Kubernetes to abstract out these details.
We always communicate with Master which may be on same node as Worker or may be on different.
Cloud
+------------------------------------+
| +----------+ |
| | Worker | |
| | Node | |
| +----------+ |
| |
| +-----------+ +----------+ |
| | Master | | Worker | |
| | Node | | Node | |
| +-----------+ +----------+ |
| |
| +----------+ |
| | Worker | |
| | Node | |
| +----------+ |
+------------------------------------+