Docker is an open-source project that automates the deployment of applications inside software containers. It is promoted by the company Docker, Inc.
Docker provides an additional layer of abstraction and automation of operating-system-level virtualization on Windows and Linux. 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 and others 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 limiting, 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.
Docker can use different interfaces to access virtualization features of the Linux kernel.
As actions are done to a Docker base image, union file system layers are created and documented, such that each layer fully describes how to recreate an action. This strategy enables Docker's lightweight images, as only layer updates need to be propagated (compared to full VMs, for example).
According to a Linux.com article,
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 premises, 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, unlike a 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 using the libcontainer library, which is available as of Docker 0.9, or indirectly via libvirt, LXC (Linux Containers) or systemd-nspawn.
Because Docker containers are so lightweight, a single server or virtual machine can run several containers simultaneously. A 2016 analysis found that a typical Docker use case involves running five containers per host, but that many organizations run 10 or more.
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 multiple virtual machines. This allows the deployment of nodes to be performed as the resources become available or when more nodes are needed, allowing a platform as a service (PaaS)-style of deployment and scaling for systems like Apache Cassandra, MongoDB or Riak. Docker also simplifies 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, HPE Helion Stackato, Jelastic,Jenkins,Kubernetes,Microsoft Azure,OpenStack Nova,OpenSVC, Oracle Container Cloud Service,Puppet,Salt,Vagrant, and VMware vSphere Integrated Containers.
The Cloud Foundry Diego project integrates Docker into the Cloud Foundry PaaS.
Red Hat's OpenShift PaaS integrates Docker and related project (Kubernetes, Geard, Project Atomic and others) since v3 (June 2015).
The Apprenda PaaS integrates Docker containers in version 6.0 of its product.
Solomon Hykes started Docker in France 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 also became involved as an independent collaborator. Docker represents an evolution of dotCloud's proprietary technology, which is 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 the Go programming language. As of October 24, 2015 , the project had over 25,600 GitHub stars (making it the 20th most-starred GitHub project), over 6,800 forks, and nearly 1,100 contributors.
A May 2016 analysis showed the following organizations as main contributors to Docker: The Docker team, Cisco, Google, Huawei, IBM, Microsoft, and Red Hat.
A January 2017 analysis of LinkedIn profile mentions showed Docker presence grew by 160% in 2016.
- On September 19, 2013, Red Hat and Docker announced a collaboration around Fedora, Red Hat Enterprise Linux, and OpenShift.
- On October 15, 2014, Microsoft announced integration of the Docker engine into the next (2016) Windows Server release, and native support for the Docker client role in Windows.
- On 10 November 2014, Docker announced a partnership with Stratoscale.
- On December 4, 2014, IBM announced a strategic partnership with Docker that enables Docker to integrate more closely with the IBM Cloud.
- On June 22, 2015, Docker and several other companies announced that they are working on a new vendor and operating-system-independent standard for software containers.
- On June 8, 2016, Microsoft announced that Docker now could be used natively on Windows 10 with Hyper-V Containers, to build, ship and run containers utilizing the Windows Server 2016 Technical Preview 5 Nano Server container OS image.
- On October 4, 2016, Solomon Hykes announced InfraKit as a new self-healing container infrastructure effort for Docker container environments.
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Red Hat has also started a second community project, called GearD, to integrate Docker into its PaaS (platform-as-a-service) hosting software, OpenShift Origin.
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The 6.0 release integrates Docker's flexibility and portability with the compliance, governance and security capabilities that enterprises need from PaaS.
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