What is Docker, And Why Use It?

Understanding Docker

Docker is an open-source containerisation software that allows developers to package applications and their dependencies into a single “container”. This container can then be deployed on any system that supports Docker, making it an ideal tool for developers who need to deploy applications in different environments.

Docker’s most obvious use case is deploying micro-services: small, independent services that work together to form a larger application. Docker's containerisation technology allows developers to deploy micro-services quickly and easily, without worrying about compatibility issues or dependencies. One of the key benefits of using Docker is that it allows developers to separate their applications from the underlying infrastructure. This means developers can focus on building and testing their applications without worrying about the underlying infrastructure. Docker also makes it easy to manage applications, as developers can use the same tools to manage both the application and the infrastructure. Another benefit of using Docker is that it is highly scalable. Docker's containerisation technology allows developers to deploy applications quickly and easily and to scale them up or down as needed. This makes it ideal for applications need to handle a large amount of traffic. Overall, Docker is an essential tool for developers who need to deploy applications quickly and easily while ensuring that they are scalable and easy to manage. With its open-source nature, Docker is constantly evolving and improving, making it an ideal choice for developers who want to stay up-to-date with the latest trends in application development.

Key Components of Docker

Docker has several key components that work together to provide a complete solution for containerisation.

Docker Daemon

The Docker Daemon is the core component of the Docker platform. It is responsible for managing the lifecycle of Docker containers, including starting and stopping containers, creating and deleting images, and managing the Docker network. The Docker Daemon runs on the host machine and communicates with the Docker Client to execute commands.

Docker Client

The Docker Client is a command-line tool that allows developers to interact with the Docker Daemon. It provides a simple and intuitive interface for managing Docker containers, images, and networks. The Docker Client can run on any machine with access to the Docker Daemon, including remote machines.

Docker Images

Docker Images are the building blocks of Docker containers. They are lightweight, portable, and self-contained packages with all the necessary dependencies and configuration files to run an application. Docker Images are stored in a registry, such as Docker Hub, and can be shared and reused across different environments.

Docker Containers

Docker Containers are instances of Docker Images running in an isolated environment. Each container has its own file system, network interface, and process space, which makes it possible to run multiple containers on the same host machine without conflicts. Docker Containers are ephemeral and can be started, stopped, and deleted at any time.

Dockerfile

A “Dockerfile” is a text file containing instructions for building a Docker Image. It specifies the base image, the application code, and any dependencies required to run the application. Dockerfiles are used to automate building Docker Images and ensure consistency across different environments. In summary, Docker has several key components that work together to provide a complete solution for containerisation. The Docker Daemon and Docker Client are the core components. Docker Images, Docker Containers, and Dockerfiles are used to build, package, and deploy applications in a portable and efficient way.

Working with Docker

Working with Docker involves using key components, including the Docker CLI, Docker Compose, Docker Hub, and Docker Desktop.

Docker CLI

The Docker CLI, or Command Line Interface, is a tool that allows developers to interact with Docker from the command line. It provides a set of commands for managing Docker containers, images, networks, and volumes. With the Docker CLI, developers can create, start, stop, and remove containers and build and push images to Docker Hub.

Docker Compose

Docker Compose is a tool that allows developers to define and run multi-container Docker applications. It uses a YAML file to define an application’s services, networks, and volumes, making it easy to manage complex applications with multiple containers. With Docker Compose, developers can start and stop an entire application with a single command.

Docker Hub

Docker Hub is a cloud-based registry that allows developers to store and share Docker images. It provides a central location for developers to find and download images and a platform for sharing images with the community. Docker Hub also provides tools for managing images, including versioning, tagging, and automated builds.

Docker Desktop

Docker Desktop is a tool that allows developers to run Docker on their local machine. It provides a user-friendly interface for managing containers, images, networks, volumes and tools for building and pushing images to Docker Hub. Docker Desktop is available for Windows, Mac, and Linux, making it easy for developers to work with Docker on their preferred platform. In summary, working with Docker involves using the Docker CLI, Docker Compose, Docker Hub, and Docker Desktop tools. They allow developers to create, deploy, and manage Docker containers and applications with ease.

Docker Architecture and Workflow

Docker is built on a client-server architecture. The Docker client talks to the Docker daemon, which does the heavy lifting of building, running, and distributing Docker containers. Docker uses a layered filesystem, container lifecycle, and Docker API to provide a flexible and efficient platform for developing, shipping, and running applications.

Docker Layered Filesystem

Docker uses a layered filesystem to build and store Docker images. Each layer represents a change to the filesystem, such as adding a file or modifying a configuration. Layers are stacked; the final layer is the container's read-write layer. This layered approach allows Docker to reuse layers across different images, reducing the amount of disk space needed to store images.

Container Lifecycle

Containers are isolated environments that run a single application or process. Docker provides a simple and efficient way to manage container lifecycles, from creating and starting containers to stopping and removing them. The container lifecycle includes the following steps:

• Create: Docker creates a container from an image.
• Start: Docker starts the container and runs the specified command.
• Pause: Docker pauses the container and saves its state to disk.
• Unpause: Docker resumes a paused container.
• Stop: Docker stops the container and saves its state to disk.
• Restart: Docker restarts a stopped container.
• Remove: Docker removes the container and its associated filesystem.

Docker API

The Docker API is a programmatic interface that communicates with the Docker daemon. With the Docker API, developers can perform tasks like starting, stopping, and deleting containers or downloading and uploading Docker images. The Docker API also makes networks and volumes possible and manages user permissions and access. In summary, Docker's architecture and workflow provide a powerful platform for developing, shipping and running applications. The layered filesystem, container lifecycle, and Docker API make it easy to manage Docker containers and images while also providing flexibility and efficiency.

Docker and Virtualisation

Docker is often compared to virtualisation technologies such as Virtual Machines (VMs). While both technologies aim to provide isolation, they differ in how they achieve it.

Docker Vs Virtual Machines

Virtual Machines (VMs) virtualise the hardware layer, allowing multiple guest operating systems to run on a single host machine. Each guest operating system runs on a virtual hardware layer, which is created by the hypervisor. The hypervisor is responsible for managing the virtual hardware resources and isolating each guest operating system from the others. On the other hand, Docker uses kernel namespaces and cgroups to provide isolation. Kernel namespaces allow Docker to create multiple isolated environments on a single host machine, each with its view of the system resources. Cgroups allow Docker to limit the amount of resources containers can use, ensuring that one container does not monopolise the resources of the host machine.

Container Isolation

Docker containers are isolated from each other and the host machine but share the same kernel. This means that each container does not need to run the guest operating system, which reduces overhead and makes it possible to run more containers on a single host machine. Docker containers are also portable, so they can be easily moved between different host machines without requiring any changes. This makes it easy to deploy applications across different environments, from development to production. In summary, Docker provides a lightweight and efficient way to isolate applications and their dependencies. While it shares some similarities with virtualisation technologies such as VMs, it uses a different approach to achieve isolation. Docker containers are portable and can be easily moved between different host machines, making deploying applications across different environments easy.

Docker in Software Development

Docker has become an essential tool for software developers, providing an efficient and reliable way to build, test, and deploy applications. Docker containers enable developers to package an application and its dependencies into a single unit, making it easier to move between different environments.

Docker in Development Environment

In the development environment, Docker provides a consistent and reproducible environment for developers to work on their code. With Docker, developers can easily set up a development environment that closely mimics the production environment, reducing the risk of bugs and errors that might arise from different environments. Docker also makes it easier to manage dependencies and libraries, ensuring that all developers are working with the same versions of software. This consistency helps to reduce the time spent on debugging and troubleshooting.

Docker in Production

In the production environment, Docker provides a reliable and scalable way to deploy applications. Docker containers can be easily deployed to different environments, making it easier to scale up or down as needed. Docker also provides a way to isolate applications, reducing the risk of conflicts between different applications running on the same server. This isolation also helps to improve security, as any vulnerabilities in one container will not affect other containers running on the same server. Overall, Docker has become an essential tool in software development, providing a consistent and reliable way to build, test, and deploy applications. With Docker, developers can focus on writing code, knowing that the application will run consistently across different environments.

Docker and Cloud Services

Docker has become a popular technology for deploying applications in the cloud. Docker containers provide a lightweight and portable way to package and run applications, making moving applications between different environments easier. This section will explore how Docker integrates with some of the most popular cloud services, including AWS and Kubernetes.

Docker and AWS

Amazon Web Services (AWS) is one of the most popular cloud service providers, and Docker can be used with AWS to deploy applications in the cloud. AWS provides various services that can be used with Docker, including Elastic Container Service (ECS) and Elastic Kubernetes Service (EKS). ECS is a managed container orchestration service that allows you to run Docker containers in the cloud. ECS provides a range of features, including automatic scaling, load balancing, and service discovery. You can use ECS to deploy Docker containers on EC2 instances or Fargate, a serverless compute engine for containers. EKS is a managed Kubernetes service that allows you to run Kubernetes clusters in the cloud. Kubernetes is an open-source container orchestration system that provides a range of features for deploying and managing containerised applications. EKS provides a managed Kubernetes control plane, making deploying and managing Kubernetes clusters in the cloud easier.

Docker and Kubernetes

Kubernetes is an open-source container orchestration system that can be used with Docker to deploy and manage containerised applications. Kubernetes provides a range of features, including automatic scaling, load balancing, and service discovery. Docker can be used with Kubernetes to package and deploy applications in containers. Kubernetes provides a range of features for managing containers, including rolling updates, health checks, and self-healing. Kubernetes also provides a range of tools for managing containerised applications, including kubectl, a command-line tool for managing Kubernetes clusters. In summary, Docker integrates well with cloud services like AWS and Kubernetes, providing a lightweight and portable way to package and deploy applications in the cloud. Whether deploying applications on ECS or EKS, or using Kubernetes to manage containers, Docker can help streamline the deployment process and provide a consistent environment for running applications in the cloud.

Advanced Docker Concepts

Docker Volumes

Docker volumes are a way to store and share data between containers. They serve to persist data, even if a container is deleted or recreated. Volumes can be used to share data between containers or to make data available to other services on the host. Volumes can be created using the docker volume create command. Once a volume is created, it can be mounted to a container using the docker run command. Volumes can also be mounted to multiple containers, allowing for easy sharing of data.

Docker Networking

Docker networking gives containers the means to communicate with each other and with services outside of the container environment. Docker provides several networking options, including bridge networks, overlay networks, and host networks. Bridge networks are the default networking option for Docker. They allow containers to communicate with each other on the same host, but not with services outside of the container environment. Overlay networks allow containers to communicate with each other across multiple hosts. Host networks allow containers to use the networking stack of the host machine.

Docker Security

Docker provides several security features to help protect containers and the host system. Docker containers are isolated from the host system, and each other, using Linux namespaces and cgroups. Docker also provides several options for controlling container access, including user namespaces, SELinux, and AppArmor. Docker also provides several options for securing the Docker daemon, including TLS encryption and client-server authentication. Docker images can also be scanned for vulnerabilities using tools like Docker Security Scanning. Overall, Docker volumes, networking, and security are advanced concepts that can help make Docker more reliable, portable, and scalable. Using these features, users can ensure that their Docker containers are secure and that data is persistently stored and easily shared between containers.

Getting Started with Docker

In this section, we will cover the basics of getting started with Docker.

Installation

Before you can start using Docker, you need to install it on your computer. Docker provides installation packages for Windows, macOS, and Linux. Once you have installed Docker, you can use it from the command line.

Running a Container

The first thing you will want to do with Docker is to run a container. A container is an instance of an image running as a separate process on your computer. To run a container, specify the image you want to use and any options you want to pass to the container. For example, to run a container based on the official nginx image, you can use the following command:

docker run --name my-nginx-container -p 8080:80 nginx

This command will:

• start a new container based on the nginx image,
• give it a name of my-nginx-container, and
• map port 8080 on your computer to port 80 in the container.

Building an Image

If you want to create your own Docker image, you can do so using a Dockerfile. A Dockerfile is a text file that contains instructions for building an image. You can use any text editor to create a Dockerfile. For example, the following Dockerfile will create a new image that installs nginx and copies a custom configuration file:

FROM nginx
COPY nginx.conf /etc/nginx/nginx.conf

To build the image, you can use the following command:

docker build -t my-nginx-image .

This command will build a new image with the tag my-nginx-image based on the Dockerfile in the current directory.

Deploying an Application

Once you have built your Docker image, you can deploy it to a production environment. Docker provides several tools for deploying applications, including Docker Swarm and Kubernetes. Docker Swarm is a native clustering and orchestration solution for Docker. It allows you to create and manage a cluster of Docker nodes, and deploy your applications across the cluster. Kubernetes is a popular open-source platform for managing containerised workloads and services. It provides powerful features for deploying, scaling, and managing containerised applications. In conclusion, Docker is a powerful tool for developing, shipping and running applications. Following the steps outlined in this section, you can start with Docker and build and deploy your own containerised applications.