WHAT IS DOCKER

Docker?is a set of?platform as a service?(PaaS) products that use?OS-level virtualization?to deliver software in packages called?containers.[5]?The service has both free and premium tiers. The software that hosts the containers is called?Docker Engine.[6]?It was first started in 2013 and is developed by?Docker, Inc.[7]

Docker is a tool that is used to automate the deployment of applications in lightweight containers so that applications can work efficiently in different environments in isolation.

Background[edit]

Containers are isolated from one another and bundle their own software,?libraries?and configuration files; they can communicate with each other through well-defined channels.[8]?Because all of the containers share the services of a single?operating system kernel, they use fewer resources than?virtual machines.[6]

Operation[edit]

Docker can package an application and its dependencies in a virtual container that can run on any Linux, Windows, or macOS computer. This enables the application to run in a variety of locations, such as?on-premises, in?public?(see?decentralized computing,?distributed computing, and?cloud computing) or?private cloud.[10]?When running on 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)[11]?to allow containers to run within a single Linux instance, avoiding the overhead of starting and maintaining?virtual machines.[12]?Docker on?macOS?uses a Linux?virtual machine?to run the containers.[13]

Because Docker containers are lightweight, a single server or virtual machine can run several containers simultaneously.[14]?A 2018 analysis found that a typical Docker use case involves running eight containers per host, and that a quarter of analyzed organizations run 18 or more per host.[15]?It can also be installed on a single board computer like the?Raspberry Pi.[16]

The Linux kernel's support for namespaces mostly[17]?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 for memory and CPU.[18]?Since version 0.9, Docker includes its own component (called "libcontainer") to use virtualization facilities provided directly by the Linux kernel, in addition to using abstracted virtualization interfaces via?libvirt,?LXC?and?systemd-nspawn.[19][9][10][20]

Docker implements a high-level?API?to provide lightweight containers that run processes in isolation.[21]

Licensing model[edit]

• The Docker Engine is licensed under the?Apache License 2.0. Docker Desktop distributes some components that are licensed under the GNU General Public License.
• The Dockerfile files can be licensed under an open-source license themselves. It is vital to realize that the scope of this license statement is only the Dockerfile and not the container image.

Components[edit]

The Docker software as a service offering consists of three components:

• Software:?The Docker?daemon, called?dockerd, is a persistent process that manages Docker containers and handles container objects. The daemon listens for requests sent via the Docker Engine API.[22][23]?The Docker client program, called?docker, provides a?command-line interface?(CLI), that allows users to interact with Docker daemons.[22][24]
• Objects:?Docker objects are various entities used to assemble an application in Docker. The main classes of Docker objects are images, containers, and services.[22]
• A Docker container is a standardized, encapsulated environment that runs applications.[25]?A container is managed using the Docker API or?CLI.[22]
• A Docker image is a read-only template used to build containers. Images are used to store and ship applications.[22]
• A Docker service allows containers to be scaled across multiple Docker daemons. The result is known as a?swarm, a set of cooperating daemons that communicate through the Docker API.[22]
• Registries:?A Docker registry is a repository for Docker images. Docker clients connect to registries to download ("pull") images for use or upload ("push") images that they have built. Registries can be public or private. The main public registry is Docker Hub. Docker Hub is the default registry where Docker looks for images.[22][26]?Docker registries also allow the creation of notifications based on events.[27]

Tools[edit]

• Docker Compose?is a tool for defining and running multi-container Docker applications.[28]?It uses?YAML?files to configure the application's services and performs the creation and start-up process of all the containers with a single command. The?docker-compose?CLI utility allows users to run commands on multiple containers at once, for example, building images,?scaling?containers, running containers that were stopped, and more.[29]?Commands related to image manipulation, or user-interactive options, are not relevant in Docker Compose because they address one container.[30]?The?docker-compose.yml?file is used to define an application's services and includes various configuration options. For example, the?build?option defines configuration options such as the Dockerfile path, the?command?option allows one to override default Docker commands, and more.[31]?The first public beta version of Docker Compose (version 0.0.1) was released on December 21, 2013.[32]?The first production-ready version (1.0) was made available on October 16, 2014.[33]
• Docker Swarm?provides native?clustering?functionality for Docker containers, which turns a group of Docker engines into a single virtual Docker engine.[34]?In Docker 1.12 and higher, Swarm mode is integrated with Docker Engine.[35]?The?docker swarm?CLI[36]?utility allows users to run Swarm containers, create discovery tokens, list nodes in the cluster, and more.[37]?The?docker node?CLI utility allows users to run various commands to manage nodes in a swarm, for example, listing the nodes in a swarm, updating nodes, and removing nodes from the swarm.[38]?Docker manages swarms using the?Raft?consensus?algorithm. According to Raft, for an update to be performed, the majority of Swarm nodes need to agree on the update.[39][40]
• Docker Volume?facilitates the independent persistence of data, allowing data to remain even after the container is deleted or re-created.[41]

History[edit]

Docker Inc. was founded by Kamel Founadi, Solomon Hykes, and Sebastien Pahl[42]?during the?Y Combinator?Summer 2010 startup incubator group and launched in 2011.[43]?The startup was also one of the 12 startups in?Founder's Den?first cohort.[44]?Hykes started the Docker project in France as an internal project within?dotCloud, a?platform-as-a-service?company.[45]

Docker debuted to the public in Santa Clara at?PyCon?in 2013.[46]?It was released as?open-source?in March 2013.[21]?At the time, it used?LXC?as its default execution environment. One year later, with the release of version 0.9, Docker replaced LXC with its own component,?libcontainer, which was written in the?Go?programming language.[19][47]

In 2017, Docker created the Moby project for open research and development.[48]