Learn Docker Beginner to Expert

What is Docker?

Docker is a set of platform as a service products that use OS-level virtualisation to deliver software in packages called containers. Containers are isolated from one another and bundle their own software, 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.[8]

What is Container?

Docker Container is a standardised unit which can be created on the fly to deploy a particular application or environment. It could be an Ubuntu container, CentOs container, etc. to full-fill the requirement from an operating system point of view. Also, it could be an application oriented container like CakePHP container or a Tomcat-Ubuntu container etc.

Docker concepts

Docker is a platform for developers and sysadmins to build, share, and run applications with containers. The use of containers to deploy applications is called containerisation. Containers are not new, but their use for easily deploying applications is.

Containerisation is increasingly popular because containers are:

• Flexible: Even the most complex applications can be containerised.
• Lightweight: Containers leverage and share the host kernel, making them much more efficient in terms of system resources than virtual machines.
• Portable: You can build locally, deploy to the cloud, and run anywhere.
• Loosely coupled: Containers are highly self sufficient and encapsulated, allowing you to replace or upgrade one without disrupting others.
• Scalable: You can increase and automatically distribute container replicas across a datacenter.
• Secure: Containers apply aggressive constraints and isolations to processes without any configuration required on the part of the user.

Welcome! We are excited that you want to learn Docker. The Docker Get Started Tutorial teaches you how to:

1. Set up your Docker environment (on this page)
2. Build an image and run it as one container
3. Set up and use a Kubernetes environment on your development machine
4. Set up and use a Swarm environment on your development machine
5. Share your containerized applications on Docker Hub

Images and containers

Fundamentally, a container is nothing but a running process, with some added encapsulation features applied to it in order to keep it isolated from the host and from other containers. One of the most important aspects of container isolation is that each container interacts with its own, private filesystem; this filesystem is provided by a Docker image. An image includes everything needed to run an application — the code or binary, runtimes, dependencies, and any other filesystem objects required.

Containers and virtual machines

A container runs natively on Linux and shares the kernel of the host machine with other containers. It runs a discrete process, taking no more memory than any other executable, making it lightweight.

By contrast, a virtual machine (VM) runs a full-blown “guest” operating system with virtual access to host resources through a hypervisor. In general, VMs incur a lot of overhead beyond what is being consumed by your application logic.

lets me summarise the learning till now:

• Virtual Machines are slow and take a lot of time to boot.
• Containers are fast and boots quickly as it uses host operating system and shares the relevant libraries.
• Containers do not waste or block host resources unlike virtual machines.
• Containers have isolated libraries and binaries specific to the application they are running.
• Containers are handled by Containerisation engine.
• Docker is one of the containerisation platforms which can be used to create and run containers.

Why do we use docker?

So we have discussed what Docker is. However, what is the need for the Docker? Well, Docker containers are lightweight and they are super easy to create and deploy.

Docker provides us with containers. And containerization consists of an entire runtime environment, an application, all its dependencies, libraries, binaries and configuration files needed to run it, bundled into one package. Each application runs separately from the other. Docker solves the dependency problem by keeping the dependency contained inside the containers. It unites developers against dependency of their project.

Benefits of using Containers over Virtual Machines

Now let’s discuss what is the benefit of Docker over VMs.

• Unlike VMs( Virtual Machines ) that run on a Guest OS, using a hypervisor, Docker containers run directly on a host server (for Linux), using a Docker engine, making it faster and lightweight.

• Docker containers can be easily integrated compared to VMs.
• With a fully virtualized system, you get more isolation. However, it requires more resources. With Docker, you get less isolation. However, as it requires fewer resources, you can run thousands of container on a host.
• A VM can take a minimum of one minute to start, while a Docker container usually starts in a fraction of seconds.
• Containers are easier to break out of than a Virtual Machine.
• Unlike VMs there is no need to preallocate the RAM. Hence docker containers utilize less RAM compared to VMs. So only the amount of RAM that is required is used.

How does Docker work?

Since we now understand the benefits of using Docker. Let’s talk above the functioning of Docker. Docker has a docker engine, which is the heart of Docker system. It is a client-server application. It has three main components:

• A server which is a type of long-running process called a daemon process.
• A client which is Docker CLI( Command Line Interface), and
• A REST API which is used to communicate between the client( Docker CLI ) and the server ( Docker Daemon )

The Docker daemon receives the command from the client and manages Docker objects, such as images, containers, networks, and volumes. The Docker client and daemon can either run on the same system, or you can connect a Docker client to a remote Docker daemon. They can communicate using a REST API, over UNIX sockets or a network interface.

In Linux, Docker host runs docker daemon and docker client can be accessed from the terminal.

In Windows/OS X, there is an additional tool called Docker toolbox. This toolbox installs the docker environment on Win/OS system. This toolbox installs the following: Docker Client, Compose, Kitematic, Machine, and Virtual Box

Technology Used in Docker

The programming language used in Docker is GO. Docker takes advantage of various features of Linux kernel like namespaces and cgroups.

namespaces: Docker uses namespaces to provide isolated workspace called containers. When a container is run, docker creates a set of namespaces for it, providing a layer of isolation. Each aspect of a container runs in a separate namespace and its access is limited to that namespace.

cgroups( control groups ): croups are used to limit and isolate the resource usage( CPU, memory, Disk I/O, network etc ) of a collection of processes. cgroups allow Docker engine to share the available hardware resources to containers and optionally enforce limit and constraints.

UnionFS( Union file systems ): are file systems that operate by creating layers, making them very lightweight and fast.It is used by Docker engine to provide the building blocks for containers.

Docker Engine combines the namespaces, cgroups, and UnionFS into a wrapper called a container format. The default container format is libcontainer.

Install Docker Desktop

The best way to get started developing containerized applications is with Docker Desktop, for OSX or Windows. Docker Desktop will allow you to easily set up Kubernetes or Swarm on your local development machine, so you can use all the features of the orchestrator you’re developing applications for right away, no cluster required. Follow the installation instructions appropriate for your operating system:

• OSX
• Windows

Docker Daily use commands

1. docker –version

This command is used to get the currently installed version of docker

2. docker pull

Usage: docker pull <image name>

This command is used to pull images from the docker repository(hub.docker.com)

3. docker run

Usage: docker run -it -d <image name>

This command is used to create a container from an image

4. docker ps

This command is used to list the running containers

5. docker ps -a

This command is used to show all the running and exited containers

6. docker exec

Usage: docker exec -it <container id> bash

This command is used to access the running container

7. docker stop

Usage: docker stop <container id>

This command stops a running container

8. docker kill

Usage: docker kill <container id>

This command kills the container by stopping its execution immediately. The difference between ‘docker kill’ and ‘docker stop’ is that ‘docker stop’ gives the container time to shutdown gracefully, in situations when it is taking too much time for getting the container to stop, one can opt to kill it

9. docker commit

Usage: docker commit <conatainer id> <username/imagename>

This command creates a new image of an edited container on the local system

10. docker login

This command is used to login to the docker hub repository

11. docker push

Usage: docker push <username/image name>

This command is used to push an image to the docker hub repository

12. docker images

This command lists all the locally stored docker images

13. docker rm

Usage: docker rm <container id>

This command is used to delete a stopped container

14. docker rmi

Usage: docker rmi <image-id>

This command is used to delete an image from local storage

15. docker build

Usage: docker build <path to docker file>

This command is used to build an image from a specified docker file

Creating Our First Docker Application

Let say we have a PHP application and want to deploy it to our staging or production server. First, we make sure we have docker configuration script included in the root directory of the application.

1. Create a Dockerfile in your application

Create a file with name Dockerfile at the root of your application and include the code below to tell docker what to do when running in the production or staging environment

FROM PHP:7.2-Apache
COPY src/ /var/www/html/
EXPOSE 80

Above is a sample docker script which configures PHP version 7.2 on a staging or production server, copy the PHP files from /src directory to /var/www/html/ and expose the port 80 to be reached on.

2. Installing Docker on Staging Or Production Server

For Mac get docker here.

For Windows go here.

3. Running Docker

After docker is installed on the staging or production server, click on the whale icon to run docker

4. Deploying Your Application

Copy the application to the staging or production server and do the following

• Navigate to the project directory on the terminal and create a docker image.

Run the following command in the terminal and it will create a docker image of the application and download all the necessary dependencies needed for the application to run successfully

docker build -t <name to give to your image>

• Convert Docker image of the Application into a Running container.

Run the following command in terminal and it will use create a running container with all the needed dependencies and start the application.

docker run -p 9090:80 <name to give to your container>

The 9090 is the port we want to access our application on. 80 is the port the container is exposing for the host to access.

Below are some useful Docker commands

Stopping a running image

docker stop <id-of-image>

Starting an image which is not running

docker start <id-of-image>

Removing an image from docker

docker rmi <id-of-image>

Removing a container from docker

docker rm <id-of-container>