What is Docker? what is Docker Hub.

What is Docker

Docker is a computer program that performs operating-system-level virtualization, also known as "containerization".It was first released in 2013 and is developed by Docker, Inc.

                    Docker is used to run software packages called "containers". Containers are isolated from each other and bundle their own application tools, libraries, and configuration files. they can communicate with each other through well-defined channels. All containers are run by a single operating system kernel and are thus more lightweight than virtual machines. Containers are created from "images" that specify their precise contents. Images are often created by combining and modifying standard images downloaded from public repositories.

Below Definition also clear you what is Docker

                    Docker is a software platform designed to make it easier to create, deploy, and run applications by using containers. It allows developers to package up an application with all the parts it needs in a container, and then ship it out as one package.


Docker History
Docker came along in March 2013, when the code, invented by Solomon Hykes, was released as open source. It’s also the name of a company founded by Hykes that supports and develops Docker code.

Both the Docker open source container and company’s approach have appeal, especially for cloud applications and agile development. Because many different applications can run on top of a single OS instance, this can be a more efficient way to run applications.

The company’s approach also speeds up applications development and testing, because software developers don’t have to worry about shipping special versions of the code for different operating systems. Because of the lightweight nature of its containers, the approach can also improve the portability of applications. Containers are an efficient and fast way to move pieces of software around in the cloud.

The company received $40 million in venture capital funding from Sequoia Inc. in September of 2014, and several reports at the time said the valuation was close to $400M. The platform consists of the Docker Engine, a runtime, and software packaging tool, and Docker Hub, a service for sharing applications in the cloud.

Docker Adopted by different OS Distribution 

• On September 19, 2013, Red Hat and Docker announced a collaboration around Fedora, Red Hat Enterprise Linux, and OpenShift.
• In November 2014 Docker container services were announced for the Amazon Elastic Compute Cloud (EC2).
• On November 10, 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.
• 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.
• In April 2016, Windocks, an independent ISV released a port of Docker's open source project to Windows, supporting Windows Server 2012 R2 and Server 2016, with all editions of SQL Server 2008 onward. 
• A May 2016 analysis showed the following organizations as main contributors to Docker: The Docker team, Cisco, Google, Huawei, IBM, Microsoft, and Red Hat.
• On October 4, 2016, Solomon Hykes announced InfraKit as a new self-healing container infrastructure effort for Docker container environments.
• A January 2017 analysis of LinkedIn profile mentions showed Docker presence grew by 160% in 2016.The software has been downloaded more than 13 billion times as of 2017.

Components:

The Docker software is a service consisting 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. The Docker client program, called docker, provides a command-line interface that allows users to interact with Docker daemons.

• Objects: Docker objects are various entities used to assemble an application in Docker. The main classes of Docker objects are images, containers, and services.
• A Docker container is a standardized, encapsulated environment that runs applications.                  A container is managed using the Docker API or CLI.
• A Docker image is a read-only template used to build containers. Images are used to store and ship applications.
• 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.

• 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. Two main public registries are Docker Hub and Docker Cloud. Docker Hub is the default registry where Docker looks for images.

Portability and Scalability

Some software gurus argue that the real benefit of container technology allows for a much larger scale of applications in virtualized environments, because of the efficiencies of virtualizing the OS. Others argue that the real benefit is in DevOps and testing because the applications can be built and tested much more quickly.

The downside of open source container technology is that it is limited to use in Linux environments. Also, as an application technology, it requires specific expertise and security safeguards geared toward a container architecture.

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