Difference between Docker Image and Container?

Docker is open-source, virtualization software designed to ease specific application development. It is based on PaaS (platform-as-a-service) products model aimed at isolating virtualized environments to deploy, build, and test applications that are usually incompatible or not meant to work with the current OS.

The software in itself is fairly easy to master but the terminology related to cocker can be quite baffling at times. As docker creates novel craze in virtualization, terms like images, images, containers, Dockerfiles, and volumes will need to be mastered.

It is a good idea to try to understand the basic roles of these entities. It will speed up learning on how you can work with them. One of the things many users stumble upon is the difference between a Docker image and a container.

This brief wiki will explain how the two differ, and also how they relate. Read more to find out.

What is a Docker Image?

A Docker image is non-changeable file containing libraries, source code, tools and other files needed to run applications.

Due to their read-only characteristics, docker images are also referred to as snapshots. The snapshots represent application and it's VE (virtual environment) at a given point of time. It is this consistency that makes Docker one of the most touted virtualization applications ever. The ability to stamp time allows developers to experiment and test apps in various conditions.

'Images' may mean pictures in traditional sense but when docker is under the radar, 'images' become copies of an entire system in one, movable file. Because images but templates, you cannot run or execute them. But what you can do is use those templates and build a container on top of it. Container will ultimately be running images. Every time you create a container, it creates a writable layer right on top of an unchangeable image. This is it, you can now modify it the way you want.

Docker Container

Docker container is nothing but an environment virtualized during run-time to allow users to isolate applications from the system underpinning it. These containers are compact, portable units in which you can start up an application quickly and easily.

A notable feature is the addition of standardized computing environment that runs inside the container at all times. Having this computing region is essential because it ensures that applications continue running the way they should ideally be running. Apart from facilitating computing, it also simplifies sharing among teammates.

As docker containers are entirely autonomous, they render essential service components by providing strong isolation, thereby ensuring containers don't interfere with one another, or with the server wherein they have been configured.  As per Docker claims, the units have the strongest isolation capability in the industry existing today.

Containers are different than Virtual Machine that they deploy a different level of virtualization. Unlike VMs where virtualization takes place in the hardware, containers are virtualized in the app layer. It is often called the OS-level virtualization and its only downside is, both host and guest OS needs to be alike. Yet, however, they can make use of one machine, partition its kernel, virtualize the operating system and run isolated processes as if the fellow OS never existed.

Difference between Docker Image and Container?

Docker is a platform that runs each and every application segregated and securely by the use of kernel containerization feature. It is a highly efficient and lightweight platform in terms of resources it uses. It uses the host underlying kernel containerization feature rather than creating its own hypervisor.

Docker Image is a set of files which has no state, whereas Docker Container is the instantiation of Docker Image. In other words, Docker Container is the run time instance of images.

Docker Image vs Container

Docker is a tool for creating and deploying applications easily. When using Docker, there are 2 main concepts you need to have clarity on: images and containers.

Docker Image

A docker image is an immutable file that contains all the dependencies required to run an application correctly. As images cannot be changed, they are also called snapshots. Since images are mere templates, they cannot be run on their own. You can use the image as a foundation to build a container. Once the container has been created, an editable layer is added on the top of the image, meaning you can modify the newly created copy of the image.

Docker images can have a series of layers, each of which differs, but also originates from, the previous one. These layers are immutable files to which a container layer can be added once you use them to build a virtual environment.

Docker Container

A docker container is a virtualized environment that includes everything required to run an application-code, runtime, tools and libraries. These portable units can be used to start an application quickly. A container allows for standardization of the computing environment so that your application not only works in identical circumstances but can also be shared within the team.

Being an autonomous entity, containers are completely isolated from other neighboring containers as well as from the server that supports them. Unlike a virtual machine where virtualization happens at the level of the hardware, containers virtualize at the application layer. This also makes them extremely light weight in comparison to a virtual machine.

Now that you have a fair understanding of both the concepts, you may be able to figure out how they relate to each other

An image can exist without a container, but vice versa cannot happen. A container needs an image to run. Containers depend on images to create a run-time environment for running and deploying applications. All in all, images and containers are two phases in running a Docker container, a running container being the final phase of the entire process.

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