Docker is an open source containerization platform. It enables developers to package applications into containers—standardized executable components combining application source code with the operating system (OS) libraries and dependencies required to run that code in any environment. Containers simplify delivery of distributed applications, and have become increasingly popular as organizations shift to cloud-native development and hybrid multicloud environments.
Developers can create containers without Docker, but the platform makes it easier, simpler, and safer to build, deploy and manage containers. Docker is essentially a toolkit that enables developers to build, deploy, run, update, and stop containers using simple commands and work-saving automation through a single API.
Docker also refers to Docker, Inc. (link resides outside IBM), the company that sells the commercial version of Docker, and to the Docker open source project (link resides outside IBM), to which Docker, Inc. and many other organizations and individuals contribute.
Docker is so popular today that “Docker” and “containers” are used interchangeably. But the first container-related technologies were available for years — even decades (link resides outside IBM) — before Docker was released to the public in 2013.
Most notably, in 2008, LinuXContainers (LXC) was implemented in the Linux kernel, fully enabling virtualization for a single instance of Linux. While LXC is still used today, newer technologies using the Linux kernel are available. Ubuntu, a modern, open-source Linux operating system, also provides this capability.
Docker enhanced the native Linux containerization capabilities with technologies that enable:
Improved—and seamless—portability: While LXC containers often reference machine-specific configurations, Docker containers run without modification across any desktop, data center and cloud environment.
Even lighter weight and more granular updates: With LXC, multiple processes can be combined within a single container. With Docker containers, only one process can run in each container. This makes it possible to build an application that can continue running while one of its parts is taken down for an update or repair.
Automated container creation: Docker can automatically build a container based on application source code.
Container versioning: Docker can track versions of a container image, roll back to previous versions, and trace who built a version and how. It can even upload only the deltas between an existing version and a new one.
Container reuse: Existing containers can be used as base images—essentially like templates for building new containers.
Shared container libraries: Developers can access an open-source registry containing thousands of user-contributed containers.
Today Docker containerization also works with Microsoft Windows server. And most cloud providers offer specific services to help developers build, ship and run applications containerized with Docker.
For these reasons, Docker adoption quickly exploded and continues to surge. At this writing, Docker Inc. reports 11 million developers and 13 billion container image downloads every month.
DockerFile
Every Docker container starts with a simple text file containing instructions for how to build the Docker container image. DockerFile automates the process of Docker image creation. It’s essentially a list of command-line interface (CLI) instructions that Docker Engine will run in order to assemble the image.
Docker images
Docker images contain executable application source code as well as all the tools, libraries, and dependencies that the application code needs to run as a container. When you run the Docker image, it becomes one instance (or multiple instances) of the container.
It’s possible to build a Docker image from scratch, but most developers pull them down from common repositories. Multiple Docker images can be created from a single base image, and they’ll share the commonalities of their stack.
Docker images are made up of layers, and each layer corresponds to a version of the image. Whenever a developer makes changes to the image, a new top layer is created, and this top layer replaces the previous top layer as the current version of the image. Previous layers are saved for rollbacks or to be re-used in other projects.
Each time a container is created from a Docker image, yet another new layer called the container layer is created. Changes made to the container—such as the addition or deletion of files—are saved to the container layer only and exist only while the container is running. This iterative image-creation process enables increased overall efficiency since multiple live container instances can run from just a single base image, and when they do so, they leverage a common stack.
Docker containers
Docker containers are the live, running instances of Docker images. While Docker images are read-only files, containers are live, ephemeral, executable content. Users can interact with them, and administrators can adjust their settings and conditions using docker commands.
Docker Hub
Docker Hub (link resides outside IBM) is the public repository of Docker images that calls itself the “world’s largest library and community for container images.” It holds over 100,000 container images sourced from commercial software vendors, open-source projects, and individual developers. It includes images that have been produced by Docker, Inc., certified images belonging to the Docker Trusted Registry, and many thousands of other images.
All Docker Hub users can share their images at will. They can also download predefined base images from the Docker filesystem to use as a starting point for any containerization project.
Docker daemon
Docker daemon is a service running on your operating system, such as Microsoft Windows or Apple MacOS or iOS. This service creates and manages your Docker images for you using the commands from the client, acting as the control center of your Docker implementation.
Docker registry
A Docker registry is a scalable open-source storage and distribution system for docker images. The registry enables you to track image versions in repositories, using tagging for identification. This is accomplished using git, a version control tool.
