Docker & Kubernetes

What is Docker?

• An open source platform that allows developers to build, ship, and run applications (with all required dependencies) within isolated, lightweight environments called containers.

• It simplifies the process of developing, testing, and deploying software by ensuring consistent environments across different stages and machines.

Key Docker Concepts

• Containers: Docker packages an application and all its dependencies (code, libraries, system tools, and runtime) into a self-contained unit called a container. This ensures the application runs consistently regardless of the underlying infrastructure.

• Images: Docker images are read-only templates used to create containers. They act as blueprints that define everything needed to run an application. Images can be built from Dockerfiles or pulled from registries like Docker Hub.

• Docker Engine: This is the core of Docker, a client-server application that manages containers. It includes a daemon (dockerd) that runs on the host machine, APIs for interaction, and a command-line interface (CLI) client.

• Portability and Consistency: Docker addresses the "it works on my machine" problem by providing a consistent environment. A container built on one machine will run identically on any other machine with Docker installed.

• Isolation and Efficiency: Containers provide process and resource isolation, allowing multiple containers to run securely on a single host without interfering with each other. They are also more lightweight and efficient than traditional virtual machines.

• Ecosystem: Docker includes tools like Docker Compose for managing multi-container applications and Docker Hub for sharing and finding pre-built images.

• Dockerfile: A text file containing a set of instructions used to build a Docker image. It specifies the base image, necessary installations, and configurations for the application.

• Docker Compose: A tool for defining and managing multi-container applications. Using a YAML file, you can define an application's services and start them all with a single command

How Does Docker Work?

Docker uses a client-server architecture composed of three main components: 

• Docker client: The primary interface for users to interact with Docker. When you run a docker run command, the client sends this command to the Docker daemon.

• Docker daemon: The persistent background service (dockerd) that manages Docker objects, such as containers, images, networks, and volumes. It listens for API requests from the Docker client and carries out the heavy lifting of building and running containers.

• Docker registry: A repository that stores Docker images. The most popular public registry is Docker Hub. When you run or pull an image, the daemon fetches it from the registry. You can also push your own images to a registry. 

Benefits of Using Docker?

• Consistency: Docker packages an application and its environment together, ensuring it behaves the same way whether it's on a developer's laptop, a test server, or a production environment.

• Portability: Docker containers can be deployed seamlessly across different platforms, including local machines, on-premises data centers, or any major cloud provider.

• Resource efficiency: By sharing the host's operating system kernel, containers use far fewer resources than traditional virtual machines, allowing for more applications to be run on the same hardware.

• Speed and scalability: The lightweight nature of containers allows them to start up in seconds. With orchestration tools like Kubernetes or Docker Swarm, you can easily scale applications up or down by adding or removing containers.

• Isolation and security: Each container is isolated from the others and from the host machine, providing a layer of security by preventing conflicts and unauthorised access between applications.

• Simplified deployment: Docker streamlines the application deployment process by bundling all dependencies, which reduces manual installation and configuration steps.

What is Kubernetes?

• It is an open-source platform designed to automate the deployment, scaling, and management of containerized applications.

Kubernetes Features

• Container Orchestration: It manages the lifecycle of containers, ensuring they are deployed, started, stopped, and scaled as needed across a cluster of machines (nodes).

• Automated Deployment and Updates: Kubernetes automates the process of deploying new versions of applications and managing updates without downtime, using strategies like rolling updates.

• Scaling and Load Balancing: It can automatically scale applications up or down based on demand and distribute network traffic across multiple instances of an application for optimal performance and availability.

• Self-Healing Capabilities: Kubernetes monitors the health of applications and automatically restarts failed containers or replaces unhealthy ones, contributing to high availability.

• Resource Management: It efficiently allocates resources like CPU, memory, and storage to containers, optimizing hardware utilization.

• Declarative Configuration: Users define the desired state of their applications and infrastructure using configuration files (e.g., YAML), and Kubernetes works to maintain that state.

How Kubernetes Works

Kubernetes works by managing a cluster, which is a group of physical or virtual machines called nodes. It is responsible for making sure the "desired state" of your application is continuously maintained. 

The basic architecture includes:

• Control Plane: The "brain" of the cluster that manages all the operational tasks. It includes the API server, scheduler, and controller manager.

• Nodes: The worker machines in the cluster where your applications actually run.

• Pods: The smallest, most fundamental deployable unit in Kubernetes. A Pod contains one or more containers that are guaranteed to be co-located on the same node and share resources.

• Services: An abstraction that defines a logical set of Pods and a policy for accessing them, which enables load balancing and service discovery.

Kubernetes Functions and Benefits

• Automated rollouts and rollbacks: Kubernetes progressively rolls out changes to your application and will automatically roll back the change if something goes wrong.

• Self-healing capabilities: If a container or node fails, Kubernetes can automatically restart, replace, or reschedule the failed component to ensure application availability.

• Automatic scaling: Applications can be scaled up or down automatically based on CPU usage or other metrics to handle changes in workload and optimize costs.

• Load balancing: Kubernetes automatically distributes network traffic across multiple application instances to ensure stability and consistent performance.

• Portability: As an open-source, cloud-agnostic platform, Kubernetes can run consistently across various environments, including on-premises data centers, public clouds (like Google Cloud, AWS, and Azure), and hybrid clouds.