Digital transformation is on the rise. To be successful, businesses must adopt new models, processes and technology infrastructure in response to changes that are brought about by new technologies such as mobile, social media and cloud computing. The speed at which these transformations happen has increased exponentially over recent years, and organizations must be agile to be competitive. Traditional delivery and deployment methods can’t keep pace with the rate of change.
Companies are turning to more flexible and efficient methods. Docker is a system that enables fast and reliable code deployment at scale. What is docker and how is it used? This beginner's guide will answer this question and more.
What Is Docker Used For?
Docker is a virtualization engine based on containerization techniques. It helps to create and run software environments called containers. Containers contain all the parts of an application including code, runtime, system tools and libraries, configuration files and dependencies. One of the main benefits of the platform is it allows you to package your software into a container image and use this image across different platforms, operating systems, cloud providers, etc. It is also extremely helpful for teams using DevOps and DevSecOps procedures for deployment.
What Is Docker, and How Does It Differ From Virtual Machines?
Containers are different from virtual machines in that they run on top of one operating system instead of a separate environment. This makes them more lightweight than virtual machines, as they share the host machine’s kernel and libraries (libraries provided by the hosting OS) with each other.
The Docker Architecture
Server: The server is the physical server that is used to host multiple virtual machines.
Host: The host OS is the base machine such as Linux or Windows.
Engine: This is what makes the tool work. Thus, there is no need for extra hardware for the guest operating system.
Docker Components
The platform is composed of multiple components that each work together to make it one of the most efficient containerization platforms available. Those components are:
- Docker Engine
- Docker Containers
- Docker images
- Docker Client
- Docker daemon
Let’s move into talking about each.
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Docker Engine
The engine creates and runs containers. It is a client-server based application that is made of:
- A continuously running service called the daemon process
- A REST API interacts with the daemon to instruct the engine on what to do
- A command-line interface
Containers
The Engine is a container management system that helps to manage and deploy applications inside containers. The engine uses Linux kernel namespaces as its foundation for isolation which allows each process in the host operating system to run in its own isolated environment with limited access to hardware resources or other processes running on the same computer.
It uses capabilities such as control groups (Cgroups) for allocating resources among processes and namespaces for restricting access or visibility. Containers are manipulated and modified using commands while they're in a "running" state.
What Is a Dockerfile?
A Dockerfile is like a recipe of instructions for building an image. It is a text document that contains all the commands needed to create an image. Each command in the file is called a build step. These steps are what compile and package an application into an image.
Images
An image is a package that contains everything needed to set up an instance of an application. The image can be used on different machines and even across different cloud providers without having to manually set them up. Images are made of the following components:
Image Layers
When developers make changes to an image, a new top layer is created. This top layer replaces the previous version. Previous versions can be saved for rollbacks.
Container Layer
Each time Docker launches a container from an image, it adds a thin writable layer to the container. Each layer stores all changes to the container throughout its runtime.
Parent Image
The first layer of the image is called the parent. It has various pre-made components, which are used to create an environment. There is a wide variety of ready-made images on public registry services (sometimes called package registries) such as Docker Hub or you could use one of your own images to create a new image.
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Base Image
A base image is an empty layer to build images from the ground up. However, these are generally intended for more advanced users and provide full control over the contents of the image.
Manifest
Images also include a manifest, which is essentially a description of the image in JSON format. This includes information on tags and digital signatures along with configuration details for different types of containers.
Docker Client
The client is a terminal-based application that provides users with an interface for running, creating and managing container images and services on the local or remote host machine(s). The client can be used as part of your build pipeline, as well as in standalone mode from the command line. You may also use it to interact directly with Docker API calls without using any other tools such as bash scripts or Jenkins jobs.
What Is Docker Desktop?
Docker Desktop is the official GUI for Windows, Mac OS X and Linux systems. This tool enables users to create and manage containers from within a user-friendly interface. It provides an easy way to use containers on a local machine or in the cloud. This mode is not recommended for production environments, but it's the easiest way for beginners to get started with the platform.
What Is Docker Daemon?
The docker daemon process is used to control and manage the containers. The daemon listens to only Docker API requests and handles images, containers, networks and volumes. It also communicates with other daemons to manage services.
What Is Docker Hub?
Docker Hub is a repository that serves as a registry. It's where developers upload their images to share or collaborate with the community. Developers can push and pull images, It is a cloud-based tool and contains both public and private registries.
The Benefits of Containers
As mentioned, containers are an efficient way of deploying and running applications. Besides that, there are a host of other benefits which include:
Portability: LXC, or Linux Containers, are usually used to create a machine-specific configuration on a single host. Containers run without modification across any desktop, data center and cloud environment.
Lightweight: With LXC, multiple processes can run simultaneously in a single container. With containers, only one process is allowed per container. With this approach, an application can continue running while parts of the application need to be taken down for updates or repairs.
Automated: The platform automatically builds a container based on the application’s source code.
Versioning: The platform has robust versioning and tracks versions and can roll back to a previous version when necessary. It can also track who built a version. Lastly, it can even upload deltas between versions.
Reuse: Existing images can be used as templates for new images
Shared Libraries: A shared library allows developers access to thousands of open-source user-contributed containers.
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