What Is Docker And How It Works?

What is a VM (Virtual Machine)?

A virtual machine (VM) is a software implementation of a computer system that behaves like a physical machine. It allows multiple operating systems to run on a single physical machine, known as the host machine, and provides the image of separate physical machines for each virtual machine, known as guest machines. Each guest machine runs its own operating system and applications, and has its own virtualized hardware resources, such as CPU, memory, and storage.

What is Docker?

Docker is a platform that enables the development, deployment, and execution of applications within containers. Docker provides a way to decouple applications from the underlying infrastructure, allowing for faster delivery of software. Containers are a lightweight and portable way to package software with all its dependencies and run it consistently across different environments. Docker provides a way to build and share container images, which are the templates that define how containers should be configured and what software should be installed inside them.

Docker is based on the idea of containerization, which is a way of isolating applications and their dependencies from the underlying infrastructure. Containers are similar to virtual machines like Hyper-V or VmWare, in which virtual machines install the full image of the OS. That’s why it is not lightweight but they are much more lightweight and do not require a separate operating system. Instead, containers share the kernel of the host operating system and only contain the software and libraries necessary to run the application.

Docker provides a simple and consistent way to manage container images and run them on any platform. With Docker, you can create, share, and run container images on your local machine, in the cloud, or on any other infrastructure that supports Docker.

Docker Architecture

Docker follows a client-server architecture where the Docker client communicates with the Docker daemon. The Docker daemon is responsible for tasks like building, running, and distributing Docker containers. It is possible to install both the Docker client and daemon on a single system or to connect the client to a Docker daemon that is running remotely. Communication between the client and daemon is facilitated through a REST API, which can be established over a network interface or UNIX sockets. Additionally, Docker Compose provides another client that allows you to work with applications that consist of multiple containers.

Docker Daemon

The Docker daemon is responsible for receiving and processing Docker API requests and is in charge of managing different Docker objects like images, containers, networks, and volumes. It can also interact with other daemons to manage various Docker services.

Docker Client

Docker users can interact with Docker through the Docker client. When a user inputs commands, such as “docker run,” the Docker client relays these commands to dockerd, which is responsible for executing them.

Docker Desktop

Docker Desktop is a user-friendly application that can be installed on Mac, Windows, or Linux systems. This application allows you to create and distribute containerized applications and microservices with ease.

Docker Registries

A Docker registry stores Docker images. Docker Hub is a publicly available registry that can be utilized by anyone, and Docker is set up to search for images on Docker Hub by default. By executing the docker pull or docker run commands, the relevant images are retrieved from the registry that you have configured. Conversely, if you execute the docker push command, the image is uploaded to the registry that you have set up.

Docker Objects

Docker is a platform that enables the creation and usage of several objects, including images, containers, networks, volumes, plugins, and more. This section provides a concise introduction to these various components.

Images

An image is a self-contained package that contains all the necessary components required to execute an application, including the code, dependencies, libraries, and configuration files. It is a lightweight and executable package that enables easy deployment and portability of the application. It is a read-only template that serves as the basis for creating Docker containers. Docker images are used to create Docker containers, which are instances of the image that can be run on any system that supports Docker. It is possible to generate custom images, but there is also the option to utilize images produced by others and made available in a repository

Containers

A container is an executable version of an image. It can be created, started, stopped, moved or deleted through the Docker API or CLI. Additionally, a container can be linked to one or more networks, have storage attached to it, or even serve as the basis for a new image, all based on its current state.

When to use Docker ?

1. Development and Testing : Docker makes it easy to set up and manage development and testing environments.
2. Microservices : Docker is well-suited for building microservices, which are small, independent services that work together to form a larger application.
3. Continuous Integration and Deployment : Docker is often used in conjunction with continuous integration and deployment (CI/CD) pipelines. Container images can be built and tested automatically, and then deployed to production environments using tools like Kubernetes or Docker Swarm.
4. Cloud Migration : Docker can be used to migrate applications to the cloud, by packaging the application and its dependencies in a container image and running it on a cloud platform that supports Docker.

When not to use Docker ?

1. Legacy Applications: If you are working with legacy applications that are tightly coupled to the underlying operating system, it may be difficult to containerize them using Docker.
2. Resource-Intensive Applications: Docker containers are lightweight, but they still require resources to run. If you have an application that is very resource-intensive, it may not be practical to run it in a container.
3. Single-Node Environments: Docker is designed for distributed environments, where applications are run across multiple nodes. If you only need to run your application on a single node, there may be simpler solutions available.

The Key Benefits of Docker

1. Consistency : Docker ensures that all dependencies and configuration required to run an application are packaged together in a single container. This ensures that the application runs consistently across different environments, which can reduce errors and improve reliability.
2. Portability : Docker offers the ability to move containers between different environments seamlessly. For instance, containers can be moved from a developer’s computer to a testing environment or production server without any hassle. This makes it easier to deploy and scale applications, without worrying about differences in the underlying infrastructure.
3. Efficiency : Docker containers are lightweight and share resources with the host operating system. This means that they can be run more efficiently than virtual machines, which require their own operating system and resources.