Thread by @iamShantanu_D

What is Docker?

Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all the parts it needs, such as libraries and other dependencies, and ship it

all out as one package.

What Docker is?

“Docker is an open-source project that automates the deployment of software applications inside containers by providing an additional layer of abstraction and automation of OS-level virtualization on Linux.”

To understand that let’s take a look at how traditional virtualization platforms work. So, there are two kinds of Virtualization Platforms:
1. Type-1 (VMware ESX, ESX-I)
2. Type-2 (VMware Workstation, Windows Hyper-V, Linux KVM)
And here is the basic diagram of how they work:

So, traditional virtualization platforms need us to do:
1. Get Hardware
2. Deploy a Host OS (Optional)
3. Deploy Hypervisor (Type 1 or Type2)
4. Deploy Guest OS
5. Deploy dependencies
6. Deploy Application

Now let’s compare them with how Docker works by looking at the following diagram:

Here is how Containers work:
1. Get Hardware
2. Deploy Host OS
4. Deploy your Containers
So, Docker is not a traditional virtualization platform but it creates an Abstraction layer on top of your Host OS so that all your containers can use same Kernel to complete their work

without introducing unnecessary overhead.
Docker Components
In Docker terminology there are 3 main things:
1. Docker Image
2. Container
3. Docker Engine

Docker Image
To define Docker Image here is what official docker site says about it:
“An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. An image typically contains a union of

layered filesystems stacked on top of each other. An image does not have state and it never changes”
An image is an inert, immutable, file that’s essentially a snapshot of a container.
Image will contain all your application code and its dependencies.

You can mention all your dependencies in a dockerfile. What makes it different than traditional build is Docker Images store their changes as a stacked layer of filesystem changes. So, if your latest image is introducing any bugs you can execute docker history to

see previous versions of the same image and can load more stable image from there.
2. Docker Containers
According to official site of Docker:
“A Docker container is a lightweight, standalone, executable package of software that includes everything needed to run an application:

code, runtime, system tools, system libraries and settings.”
So, Container is a running instance of your Image. Consider this in VM terminology, if you want to test your build then you will deploy your OVF file to create a VM and start the VM.

In docker you just execute docker start <<imagename>> and it will start one instance of your image which is called a container.
3. Docker Engine
Docker engine is the heart of whole Docker platform. Here is what it does:
- Give an Abstract Virtualized layer on top of Host OS

- Segregate all deployed container dependencies
- Manage deployed containers
- Create custom Docker Images

One other thing to note here is that if you can see the diagram of difference between VM and Docker, Docker uses same libraries for containers of same images. In case of VMs you will just have to create another VM which will introduce overhead of an extra Storage and Memory

overhead of a Guest OS and dependencies. In conclusion, your docker images will be running on your docker engine.