WHAT IS DOCKER
Tags: docker, devops, containers
Published: 2021-11-21 06:29:13

Docker is a software platform for developing applications based on containers, which are tiny and efficient execution environments that share the operating system kernel but execute in isolation. While containers as a concept have been around for a while, Docker, an open-source project founded in 2013, helped popularize the technology and helped push the cloud-native development trend of containerization and microservices in software development.

## What are containers?
One of the aims of contemporary software development is to keep apps running on the same host or cluster isolated from one another so that they don’t interfere with each other’s operation or maintenance. Due to the packages, libraries, and other software components necessary for them to execute, this might be tricky. Virtual machines are one answer to this problem, since they keep programs on the same hardware completely distinct, reducing conflicts between software components and rivalry for hardware resources to a minimum. Virtual machines, on the other hand, are large (each has its own operating system, therefore they’re generally terabytes in size) and difficult to manage and upgrade.

Containers, on the other hand, separate the execution contexts of programs while sharing the underlying OS kernel. They’re usually measured in megabytes, utilize a fraction of the resources of virtual machines, and boot up virtually instantly. They can be packed much closer together on the same hardware and whirled up and down in mass with significantly less effort and overhead. Containers offer a very efficient and granular method for merging software components into the kind of application and service stacks required in today’s company, as well as for keeping those software components updated and maintained.

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ghg97mlfm4v7of6gvs8i.png)

## What is Docker?
Docker is an open source project that enables creating containers and container-based applications simple. Docker was originally designed for Linux, but it now works on Windows and MacOS as well. Let’s look at some of the components you’d utilize to construct Docker-containerized apps to have a better understanding of how Docker works.

## Dockerfile
A Dockerfile is the foundation of every Docker container. A Dockerfile is a text file with instructions on how to construct a Docker image in an easy-to-understand syntax (more on that in a moment). A Dockerfile describes the container’s operating system, as well as the languages, environmental variables, file locations, network ports, and other components it requires — and, of course, what the container will perform once we execute it.

## Docker image
After you’ve generated your Dockerfile, you’ll use the Docker build tool to generate an image based on it. A Docker image is a portable file providing the specifications for which software components the container will execute and how, whereas a Dockerfile is a collection of instructions that informs build how to generate the image. Because a Dockerfile will almost always include instructions for downloading software packages from online repositories, you should make sure to mention the correct versions explicitly, otherwise your Dockerfile may generate inconsistent images depending on when it is executed. However, once a picture is made, it cannot be changed.

## Docker run
The run command in Docker is what really runs a container. Each container is an image instance. Containers are intended to be ephemeral and ephemeral, however they may be stopped and resumed, putting them back in the same condition as when they were first stopped. Additionally, many container instances of the same image can run at the same time (as long as each container has a unique name).

## Docker Hub
While creating containers is simple, don’t get the impression that you’ll have to create each of your photos from scratch. Official Docker images from open-source projects and software companies, as well as unauthorized images from the general public, are available on Docker Hub, a SaaS repository for sharing and managing containers. You may either download or upload container images containing valuable code, distribute them publicly or make them private. If you like, you may also construct a local Docker registry.

## Docker Engine
Docker Engine is the underlying client-server technology that builds and operates containers, and it is at the heart of Docker. When someone mentions Docker in a general sense and isn’t referring to the firm or the project as a whole, they’re usually referring to Docker Engine. Docker Engine is available in two versions: Docker Engine Enterprise and Docker Engine Community.

### The story of Docker’s conquest of the container world
For decades, Unix operating systems such as BSD and Solaris have included the concept that a particular process can be run with some degree of isolation from the rest of its operating environment. LXC, the original Linux container technology, is a way for running many isolated Linux systems on a single host that uses OS-level virtualization. LXC was made feasible by two Linux features: namespaces, which surround a collection of system resources and present them to a process as though they were devoted to that process; and cgroups, which manage the isolation and use of system resources like CPU and memory for a group of processes.Containers separate programs from operating systems, allowing users to keep their Linux operating system clean and simple while running everything else in one or more isolated containers.
You may transfer a container across any Linux server that supports the container runtime environment because the operating system is separated away from containers.
LXC has undergone numerous substantial improvements as a result of Docker’s modifications, making containers more portable and versatile to utilize. You can deploy, replicate, transfer, and back up a workload even faster and easier using Docker containers than you can with virtual machines. Docker gives any infrastructure that can run containers cloud-like flexibility. Docker’s container image tools were also an improvement over LXC, allowing developers to create image libraries, compose apps from multiple images, and deploy those containers and apps on local or remote infrastructure.

## Docker Compose, Docker Swarm, and Kubernetes
![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/9bm5c1c7gqldg5doqtwk.png)

Docker also makes it easy to coordinate container actions and, as a result, construct application stacks by connecting containers. Docker Compose was developed by Docker to make the development and testing of multi-container applications easier. It’s a command-line program that uses a specially designed descriptor file to construct programs from several containers and run them in parallel on a single host, similar to the Docker client.

More advanced versions of these behaviors — what’s called container orchestration — are offered by other products, such as Docker Swarm and Kubernetes. But Docker provides the basics. Even though Swarm grew out of the Docker project, Kubernetes has become the de facto Docker orchestration platform of choice.

Source : https://www.infoworld.com/article/3204171/what-is-docker-the-spark-for-the-container-revolution.html