Docker

 1. What is Docker?

·    Docker is an open platform that provides the capability to develop, test, ship and run your application in an isolated environment. This isolated environment is called container. Containers are light weighted as they directly run within the host machine's kernel. 

·    Docker enables you to keep your application separate from your Infrastructure and it reduces the delay between developing your application and deploying it in production.

·    Docker runs the applications in isolation with security, it allows you to run many containers on a host machine.

2. What is the Docker Engine?

Docker Engine is an application which has client-server type architecture with these major components.

• Daemon Process ('dockerd' command) - It's a server which is a type of long running program.
• A Rest API - It provides an interface that programs or CLI use to talk to daemon processes and to instruct it.
• A CLI (Command Line Interface) client ('docker' command).

Source: Docker Overview

3. What can you use Docker for?

You can use the docker for:

• Fast and consistent delivery of applications- means dockers allows developers to streamline their work by allowing them to work on local containers which provide their applications and services. Containers are best suited for CI (Continuous Integration) and CD (Continuous Delivery)
• Scaling and Responsive deployment - means docker's container based platform allows for highly portable workloads and docker containers can run on developers laptop, on physical or virtual machines, on cloud provider or in a hybrid environment.
• Running more workloads on the same hardware - means docker provides a cost effective alternative of hypervisor based virtual machines so that you can consume more compute capacity without changing the existing hardware. Docker is very lightweight and fast.

4. Give some Docker Example scenarios.

As Docker provides the consistent and fast delivery of applications, these are the below examples for that.

• Developers write the code and can share that with their team members using dockers containers.
• Docker is used to push the code and execute automated or manual tests in a Test environment.
• Developers can fix the bugs in the development environment and can push them to the test environment for testing.
• Giving fixes or updated applications to customers is easy as you can push the updated image to the Production.

5. Explain the Docker architecture.

Docker is based on client-server architecture. Docker client talks to docker daemon (A long running program) via REST API over a network interface or UNIX sockets. Docker client and daemon can run on the same system or remote. Docker daemon is responsible for building, running and distributing the containers.
  

Source: Docker Architecture

• Docker Daemon - responsible to listen to API calls and manage docker objects. To manage Docker services it can communicate with other docker daemon as well.
• Docker Client - is used to communicate with docker daemon via REST APIs and can communicate with more than one daemon.
• Docker Registries - Docker images are stored in Docker registry. Docker Hub is a public registry and a default location for docker images configured in docker.
• Docker Objects - When you are using docker means you are going to use many things like images, containers, volume, networks and many others, these are called docker objects.

6. What is DTR (Docker Trusted Registry)?

If you are using Docker Data Center (DDC), then Docker provides an enterprise grade image storage solution called Docker Trusted Registry (DTR). DTR can be installed on virtual private networks or on-premises so that you can store your images in a secure way and behind your firewall. DTR also provides a User Interface that can be accessed by authorized users to view and manage the repositories.

7. What are the common Docker objects?

With Docker you use many things like Images, Containers, Registries, Services, Volumes etc. These all are Docker objects.

8. Explain the Docker Images.

A Docker Image is a read-only template with instructions that forms the basis of a container. It's an order collection of file-system changes. Image is based on another image with some customizations. For example, you can create an image that is based on 'ubuntu' image but you can install other application dependencies.
Images contain a set of run parameters (starting executable file) that will run when the container starts.

9. Why Images are light weight, fast and small?

Docker allows you to create your own image using a docker file with simple instructions needed to create an image. Each instruction creates layers in docker image. So when you make any changes in a docker file then only changed layers are rebuilt, not all, this is the reason that docker images are fast, small and light weight in comparison to other virtualization systems.

10. Describe Docker Containers?

Containers are created from Images or you can say Container is a runnable instance of an image. When you build your image and deploy the application with all the dependencies then multiple containers can be instantiated. Each container is isolated from one another and from the host machine as well. So A Docker Container is defined by an image and other configurations provided when you start or create it.

11. Explain the underlying technology of Docker.

Docker is developed in Go Language and has capability to use many Linux kernel features to deliver the functionality. 

12. What is Dockerfile?

·      Dockerfile is a file that holds a set of instructions to create an image. 

·      Each instruction of Dockerfile is responsible for creating a layer in the image. 

·      When you rebuild the image, only changed layers are rebuilt.

 13.Explain some quick facts about Docker.

Developers are considering Docker as a good choice to deploy the application any time, anywhere - OnPrem or Cloud. Let's know some facts about it.

• Docker was launched by Solomon Hykes in 2013. Now Solomon Hykes is the CTO and chief Architect of Docker.
• Docker allows us to build, ship, run and orchestrate the applications in an isolated environment.
• Docker is much faster than starting a virtual machine, but virtual machines are not obsolete yet.
• Docker Hub offers free options to host public repositories by developers and paid options for private repositories.
• Docker Desktop and Docker Compose usage has reduced the local development environment setup time that helps the developers to be productive.

14. What is the future of Docker?

• Docker offers a quick way to build, develop, ship and orchestrate distributed applications in isolated environments.
• Docker is being used by many companies to make developer's processes faster. Docker also provides automatic deployment management. So most companies are adopting this containerized approach for their application development and deployment.
• Docker also provides integration with many hundreds of tools like Bitbucket, Jenkins, Kubernetes, Ansible, Amazon EC2 etc. 

 15. What is Docker Desktop?

        Docker Desktop is an easy-to-install application for your Mac, Windows or Linux environment that enables you to build and share containerized applications and microservices. Docker Desktop includes the Docker daemon (dockerd), the Docker client (docker), Docker Compose, Docker Content Trust, Kubernetes, and Credential Helper.

16. What is Docker Compose?

        Docker Compose is a tool that was developed to help define and share multi-container applications. With Compose, we can create a YAML file to define the services and with a single command, can spin everything up or tear it all down.

The big advantage of using Compose is you can define your application stack in a file, keep it at the root of your project repo (it’s now version controlled), and easily enable someone else to contribute to your project. Someone would only need to clone your repo and start the compose app.

Example:-

command:

docker run -d \

  --network todo-app --network-alias mysql \

  -v todo-mysql-data:/var/lib/mysql \

  -e MYSQL_ROOT_PASSWORD=secret \

  -e MYSQL_DATABASE=todos \

  mysql:8.0

Compose:

services:

  app:

    image: node:18-alpine

    command: sh -c "yarn install && yarn run dev"

    ports:

      - 3000:3000

    working_dir: /app

    volumes:

      - ./:/app

    environment:

      MYSQL_HOST: mysql

      MYSQL_USER: root

      MYSQL_PASSWORD: secret

      MYSQL_DB: todos

mysql:

    image: mysql:8.0

    volumes:

      - todo-mysql-data:/var/lib/mysql

    environment:

      MYSQL_ROOT_PASSWORD: secret

      MYSQL_DATABASE: todos

17. Create the Compose file?

•  At the root of the app project, create a file named docker-compose.yml
•  In the compose file, we’ll start off by defining the list of services (or containers) we want to run as part of our application.
• First, let’s define the service entry and the image for the container. We can pick any name for the service. The name will automatically become a network alias, which will be useful when defining our MySQL service. 
• Typically, you will see the command close to the image definition, although there is no requirement on ordering. So, let’s go ahead and move that into our file.
• Let’s migrate the -p 3000:3000 part of the command by defining the ports for the service. 
• Next, we’ll migrate both the working directory (-w /app) and the volume mapping (-v "$(pwd):/app") by using the working_dir and volumes definitions.
• Finally, we need to migrate the environment variable definitions using the environment key.