SkillRary

Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and deploy it as one package. By doing so, thanks to the container, the developer can rest assured that the application will run on any other Linux machine regardless of any customized settings that the machine might have that could differ from the machine used for writing and testing the code.

In a way, Docker is a bit like a virtual machine. But unlike a virtual machine, rather than creating a whole virtual operating system, Docker allows applications to use the same Linux kernel as the system that they're running on and only requires applications to be shipped with things not already running on the host computer. This gives a significant performance boost and reduces the size of the application.

And importantly, Docker is open source. This means that anyone can contribute to Docker and extend it to meet their own needs if they need additional features that aren't available out of the box.

Docker is a tool that is designed to benefit both developers and system administrators, making it a part of many DevOps (developers + operations) toolchains. For developers, it means that they can focus on writing code without worrying about the system that it will ultimately be running on. It also allows them to get a head start by using one of the thousands of programs already designed to run in a Docker container as a part of their application. For operations staff, Docker gives flexibility and potentially reduces the number of systems needed because of its small footprint and lower overhead.

Real-world data backs up the conclusion that Docker is being widely adopted. DataDog, a cloud-monitoring system found that by March 2016, "13.6 percent of Datadog's customers had adopted Docker. One year later that number has grown to 18.8 percent. That's almost 40 percent market-share growth in 12 months." RightScale observed in its RightScale 2018 State of the Cloud report that Docker's adoption by the industry has increased to 49 percent from 35 percent in 2017.

Docker packages an application and all its dependencies in a virtual container that can run on any Linux server. This is why we call them containers. Because they have all the necessary dependencies contained in a single piece of software.

Docker is composed of the following elements:

• a Daemon, which is used to build, run, and manage the containers
• a high-level API that allows the user to communicate with the Daemon,
• and a CLI, the interface we use to make this all available.

Containers are abstractions of the app layer. They package all the code, libraries, and dependencies together. This makes it possible for multiple containers to run in the same host, so you can use that host's resources more efficiently.

Each container runs as an isolated process in the user space and takes up less space than regular VMs due to their layered architecture. These layers are called intermediate images, and these images are created every time you run a new command in the Dockerfile.

Today, Docker, and its open-source father now named Moby, is bigger than ever. According to Docker, over 3.5 million applications have been placed in containers using Docker technology and over 37 billion containerized applications have been downloaded.