Docker is a set of platform as a service (PaaS) products that use OS-level virtualization to deliver software in packages called containers.
Docker can package an application and its dependencies in a virtual container that can run on a Linux server. This enables the application to run in a variety of locations, such as on-premises, in a public cloud, and/or in a private cloud.
Docker uses the resource isolation features of the Linux kernel (such as cgroups and kernel namespaces) and a union-capable file system (such as OverlayFS) to allow containers to run within a single Linux instance, avoiding the overhead of starting and maintaining virtual machines.
This is free and open source software.
- Portable deployment across machines. Docker defines a format for bundling an application and all its dependencies into a single object called a container. This container can be transferred to any Docker-enabled machine. The container can be executed there with the guarantee that the execution environment exposed to the application is the same in development, testing, and production.
- Application-centric. Docker is optimized for the deployment of applications, as opposed to machines. This is reflected in its API, user interface, design philosophy and documentation.
- Automatic build. Docker includes a tool for developers to automatically assemble a container from their source code, with full control over application dependencies, build tools, packaging etc. They are free to use make, maven, chef, puppet, salt, Debian packages, RPMs, source tarballs, or any combination of the above, regardless of the configuration of the machines.
- Versioning. Docker includes git-like capabilities for tracking successive versions of a container, inspecting the diff between versions, committing new versions, rolling back etc. The history also includes how a container was assembled and by whom, so you get full traceability from the production server all the way back to the upstream developer. Docker also implements incremental uploads and downloads, similar to git pull, so new versions of a container can be transferred by only sending diffs.
- Component re-use. Any container can be used as a parent image to create more specialized components. This can be done manually or as part of an automated build. For example you can prepare the ideal Python environment, and use it as a base for 10 different applications. Your ideal PostgreSQL setup can be re-used for all your future projects.
- Sharing. Docker has access to a public registry on Docker Hub where thousands of people have uploaded useful images: anything from Redis, CouchDB, PostgreSQL to IRC bouncers to Rails app servers to Hadoop to base images for various Linux distros. The registry also includes an official “standard library” of useful containers maintained by the Docker team. The registry itself is open-source, so anyone can deploy their own registry to store and transfer private containers, for internal server deployments for example.
- Tool ecosystem. Docker defines an API for automating and customizing the creation and deployment of containers. There are a huge number of tools integrating with Docker to extend its capabilities. PaaS-like deployment (Dokku, Deis, Flynn), multi-node orchestration (Maestro, Salt, Mesos, Openstack Nova), management dashboards (docker-ui, Openstack Horizon, Shipyard), configuration management (Chef, Puppet), continuous integration (Jenkins, Strider, Travis), etc.
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