Before knowing about Docker, we need to have an idea about Virtual Machines and why those VM’s can be replaced by Docker. Virtual Machine can be defined as Virtualization or emulation of the physical computer system. VM’s are often referred to as guests while the physical system they run is referred as host.

Virtualization makes it possible to create multiple Virtual Machines (VM) each having their own Operating System and applications on a single physical machine. VM requires light weight software layer called Hypervisor to coordinate between it and underlying physical hardware. Though several VM’s running at a time speaks about efficiency, they lead to unstable performance. As the guest Operating System (OS) would have its own kernel, set of libraries, and dependencies, this would utilize a large part of system resources. Applications installed on VMs are tied to the underlying hardware, moving a VM to another machine requires new configurations, new applications bugs, and reinstallation of the entire environment. VM’s can also be slow to boot. The concept of Containerization helps to overcome above mentioned flaws. Docker is one of those containerization platforms.
 

Docker:
 

Docker is an open-source software that originated to streamline application development. It comes under the category of PAAS (platform-as-a-service) products that helps to create virtual environments for developing, deploying, and testing applications by using containers. Container refers to the lightweight, stand-alone, and executable package of a piece of software that contains all the libraries, configuration files, dependencies, and other necessary parameters to operate the application.
 

How Docker Works:
 

Docker Engine, Docker Image, and Docker Container play an important role in this phase.
 

Docker Engine is primarily a client-server technology that helps to build and run the container using Docker components. When we refer to Docker, it defines Docker engine which includes the Docker daemon, REST API, and Command Line Interface (CLI). CLI enables users in interacting with Docker daemon through Docker Engine API. Docker Engine consists of the tasks and workflows required to develop, deploy, and run applications using containers. The engine creates a daemon process which is server-side that hosts images, containers, networks, and storage volumes.
 

Docker Engine was initially developed for Linux systems but later updates are provided to operate natively on Windows referred to as Docker Windows Containers and MAC-OS.
 

Docker Image can be defined as an immutable (unchangeable) file that contains the source code, libraries, dependencies, tools, and other files required for an application to execute. Because of their read-only feature, the mentioned images are also referred to as snapshots. They constitute for a developed app and its virtualized environment at a particular point of time. This dependability is one of the available characteristics of Docker. This allows application developers to test and deploy software in secure and uniform conditions.
 

Docker Images can be started only in container. We can use that snapshot as a foundation to build a container. After the container is created, we can modify the immutable images because a layer is added to write. The container which is created on top of the images exists independently and can’t be changed. When the container is started, you essentially design a read-write copy of those docker images in the container.

Docker provides users with a feature to create many Docker images from the base image. Every time the initial state of an image is changed and the newly updated state is saved, a new template will be created in addition to a new layer. So it can be said that Docker images consist of a group of layers, every layer is different but also developed from the previous one. Image layers appear as read-only files to which a container layer is added once you use it to start a virtualized environment. Docker Images behave as containers at runtime.

The command - docker image build – builds an image from the docker file. There are many child tags that start with docker image to carry out the required functionality and these can be found in the technology reference site for Docker.
 

Docker Container is a run-time virtualized environment where applications are isolated by users from the original system. These created containers are compact and shippable units in which applications can be started by users rapidly in a simple manner. The computing environment is standardized present in the container. Not only this ensures your developed application works in a similar scenario, but it also makes it easier to share with other developers of the team.

The technology behind Docker Container is unique because it focuses primarily on the requirement specifications of developers and systems operators to separate application dependencies from infrastructure. Containers provide stronger isolation, confirming that they do not interrupt other containers which are running, even the server which backs those containers. Docker says that these containers “provide the strongest isolation capabilities in the industry”. This provides an assurance about protecting your machine while developing an application.

Docker packages and runs containers created by developers and this technology is available on top of the operating system. Docker allows us to embed multiple containers on the same OS because it uses resource isolation in the OS kernel. All these containers share services of the underlying OS. This is different from VM’s which group an entire OS. As Docker images contain all the configurations and dependencies needed to execute code inside a container, so containers that move between different Docker environments with the same OS work with no modifications.

The software that is Containerized using Docker Engine will always run the same, regardless of the infrastructure. Containers isolate software from its environment and ensure that it works uniformly despite differences for example between development, staging, and QA environments. The following are different types of Containers specified by Docker:
 

• Standard: Basic for Container creation which is portable
   
• Lightweight: Containers share the machine’s OS system kernel and therefore do not require a different OS for each application, enabling higher server efficiency and reducing operational costs
   
• Secure: Applications are safer in containers and Docker provides the strongest default isolation capabilities in the industry
   

Uses of Docker:
 

• Docker can be used as a version control system for your entire app development and it’s OS
   
• When you want to distribute/collaborate on your app's operating system with a team
   
• When developers want to run their code on a laptop in the same environment as they have on their server
   
• Whenever an application needs to go through multiple phases of development and testing (Dev/Test/QA/Prod)  

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Summary:
 

With this, we can summarize that Docker Container is an alternative to VM because Containers take up less space than VMs (container images are typically tens of MBs in size), can handle more applications, and incur less operational costs for an application. The final outcome after including Docker Container is Packaged Software for Development and Deployment.