Docker is an open-source project that automates the deployment of applications inside software containers, by providing an additional layer of abstraction and automation of operating-system-level virtualization on Linux. Docker uses resource isolation features of the Linux kernel such as cgroups and kernel namespaces to allow independent "containers" to run within a single Linux instance, avoiding the overhead of starting and maintaining virtual machines.
The Linux kernel's support for namespaces mostly isolates an application's view of the operating environment, including process trees, network, user IDs and mounted file systems, while the kernel's cgroups provide resource isolation, including the CPU, memory, block I/O and network. Since version 0.9, Docker includes the libcontainer library as its own way to directly use virtualization facilities provided by the Linux kernel, in addition to using abstracted virtualization interfaces via libvirt, LXC (Linux Containers) and systemd-nspawn.
According to industry analyst firm 451 Research, "Docker is a tool that can package an application and its dependencies in a virtual container that can run on any Linux server. This helps enable flexibility and portability on where the application can run, whether on premise [sic], public cloud, private cloud, bare metal, etc."
Docker implements a high-level API to provide lightweight containers that run processes in isolation. Building on top of facilities provided by the Linux kernel (primarily cgroups and namespaces), a Docker container, as opposed to a traditional virtual machine, does not require or include a separate operating system. Instead, it relies on the kernel's functionality and uses resource isolation (CPU, memory, block I/O, network, etc.) and separate namespaces to isolate the application's view of the operating system. Docker accesses the Linux kernel's virtualization features either directly through the provided libcontainer library (which is available since Docker 0.9), or indirectly via libvirt, LXC (Linux Containers) or systemd-nspawn.
By using containers, resources can be isolated, services restricted, and processes provisioned to have an almost completely private view of the operating system with their own process ID space, file system structure, and network interfaces. Multiple containers share the same kernel, but each container can be constrained to only use a defined amount of resources such as CPU, memory and I/O.
Using Docker to create and manage containers may simplify the creation of highly distributed systems by allowing multiple applications, worker tasks, and other processes to run autonomously on a single physical machine or across a spectrum of virtual machines. This enables the deployment of nodes as resources are available or needed, thus providing platform as a service (PaaS)–style deployment and scaling for systems like Apache Cassandra, MongoDB or Riak, and simplification of the creation and operation of task or workload queues and other distributed systems.
Docker can be integrated into various infrastructure tools, including Amazon Web Services, Ansible, CFEngine, Chef, Google Cloud Platform, IBM Bluemix, Jelastic, Jenkins, Microsoft Azure, OpenStack Nova, OpenSVC, Puppet, Salt, and Vagrant.
The Cloud Foundry Diego project integrates Docker into the Cloud Foundry PaaS.
The GearD project aims to integrate Docker into the Red Hat's OpenShift Origin PaaS.
Solomon Hykes started Docker as an internal project within dotCloud, a platform as a service company, with initial contributions by other dotCloud engineers including Andrea Luzzardi and Francois-Xavier Bourlet. Jeff Lindsay was also involved as an independent collaborator. Docker represents an evolution of dotCloud's proprietary technology, which itself built on earlier open-source projects such as Cloudlets.
Docker was released as open source in March 2013. On March 13, 2014, with the release of version 0.9, Docker dropped LXC as the default execution environment and replaced it with its own libcontainer library written in Go language. As of April 13, 2015, the project had over 20,700 GitHub stars (making it the 20th most starred GitHub project), over 4,700 forks, and nearly 900 contributors.