Introduction To Docker

What is Docker?

Docker is a command-line program, a background daemon, and a set of remote services that take a logistical approach to solving common software problems and simplifying your experience installing, running, publishing, and removing software. It accomplishes this using a UNIX technology called containers. 

OR

Docker is the world’s leading software container platform

What Docker means to Developers, Operations, and Enterprises?

For Developers: 

Developers use Docker to eliminate “works on my machine” problems when collaborating on code with co-workers.

For Operations: 

Operators use Docker to run and manage apps side-by-side in isolated containers to get better compute density.

For Enterprise: 

Enterprises use Docker to build agile software delivery pipelines to ship new features faster, more securely and with confidence for both Linux and Windows Server apps.

What problems does Docker solve?

Using software is complex. Before installation, you have to consider what operating system you’re using, the resources the software requires, what other software is already installed, and what other software it depends on. You need to decide where it should be installed. Then you need to know how to install it. It’s surprising how drastically installation processes vary today. The list of considerations is long and unforgiving. Installing software is at best inconsistent and over complicated.

Most computers have more than one application installed and running. And most applications have dependencies on other software. What happens when two or more applications you want to use don’t play well together? Disaster. Things are only made more complicated when two or more applications share dependencies:

• What happens if one application needs an upgraded dependency but the other does not?
• What happens when you remove an application? Is it really gone?
• Can you remove old dependencies?
• Can you remember all the changes you had to make to install the software you now want to remove?

Is Docker a Framework?

or

Is Docker a Programming Language?

or

Is Docker a Tool?

Docker As A Tool: Docker is a tool that helps solve common problems like installing, removing, upgrading, distributing, trusting, and managing software.

The Docker platform

Docker provides the ability to package and run an application in a loosely isolated environment called a container. The isolation and security allow you to run many containers simultaneously on a given host. Containers are lightweight because they don’t need the extra load of a hypervisor, but run directly within the host machine’s kernel. This means you can run more containers on a given hardware combination than if you were using virtual machines. You can even run Docker containers on host machines that are actually virtual machines!

Docker Engine

Docker Engine is a client-server application with these major components:

• A server which is a type of long-running program called a daemon process (the dockerd command).
• A REST API which specifies interfaces that programs can use to talk to the daemon and instruct it what to do.
• A command line interface (CLI) client (the docker command).

Note: Docker is licensed under the open source Apache 2.0 license.

Docker architecture

Docker uses a client-server architecture. The Docker client talks to the Docker daemon, which does the heavy lifting of building, running, and distributing your Docker containers. The Docker client and daemon can run on the same system, or you can connect a Docker client to a remote Docker daemon. The Docker client and daemon communicate using a REST API, over UNIX sockets or a network interface.

The Docker daemon

The Docker daemon (dockerd) listens for Docker API requests and manages Docker objects such as images, containers, networks, and volumes. A daemon can also communicate with other daemons to manage Docker services.

The Docker client

The Docker client (docker) is the primary way that many Docker users interact with Docker. When you use commands such as docker run, the client sends these commands to dockerd, which carries them out. The docker command uses the Docker API. The Docker client can communicate with more than one daemon.

Docker registries

A Docker registry stores Docker images. Docker Hub and Docker Cloud are public registries that anyone can use, and Docker is configured to look for images on Docker Hub by default. You can even run your own private registry. If you use Docker Datacenter (DDC), it includes Docker Trusted Registry (DTR).

When you use the docker pull or docker run commands, the required images are pulled from your configured registry. When you use the docker push command, your image is pushed to your configured registry.

Docker store allows you to buy and sell Docker images or distribute them for free. For instance, you can buy a Docker image containing an application or service from a software vendor and use the image to deploy the application into your testing, staging, and production environments. You can upgrade the application by pulling the new version of the image and redeploying the containers.

Docker objects

When you use Docker, you are creating and using images, containers, networks, volumes, plugins, and other objects. This section is a brief overview of some of those objects.

IMAGES

An image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.

An image is a read-only template with instructions for creating a Docker container. Often, an image is based on another image, with some additional customization. For example, you may build an image which is based on the ubuntu image but installs the Apache web server and your application, as well as the configuration details needed to make your application run.

You might create your own images or you might only use those created by others and published in a registry. To build your own image, you create a Dockerfile with a simple syntax for defining the steps needed to create the image and run it. Each instruction in a Dockerfile creates a layer in the image. When you change the Dockerfile and rebuild the image, only those layers which have changed are rebuilt. This is part of what makes images so lightweight, small, and fast when compared to other virtualization technologies.

CONTAINERS

A container is a runtime instance of an image – what the image becomes in memory when actually executed. It runs completely isolated from the host environment by default, only accessing host files and ports if configured to do so.

A container is a runnable instance of an image. You can create, run, stop, move, or delete a container using the Docker API or CLI. You can connect a container to one or more networks, attach storage to it, or even create a new image based on its current state.

By default, a container is relatively well isolated from other containers and its host machine. You can control how isolated a container’s network, storage, or other underlying subsystems are from other containers or from the host machine.

A container is defined by its image as well as any configuration options you provide to it when you create or run it. When a container stops, any changes to its state that are not stored in persistent storage disappears.

Example docker run command

The following command runs an ubuntu container, attaches interactively to your local command-line session, and runs /bin/bash.

$ docker run -i -t ubuntu /bin/bash

When you run this command, the following happens (assuming you are using the default registry configuration):

1. If you do not have the ubuntu image locally, Docker pulls it from your configured registry, as though you had run docker pull ubuntu manually.
2. Docker creates a new container, as though you had run a docker create command manually.
3. Docker allocates a read-write filesystem to the container, as its final layer. This allows a running container to create or modify files and directories in its local filesystem.
4. Docker creates a network interface to connect the container to the default network since you did not specify any networking options. This includes assigning an IP address to the container. By default, containers can connect to external networks using the host machine’s network connection.
5. Docker starts the container and executes /bin/bash. Because the container is run interactively and attached to your terminal (due to the -i and -t) flags, you can provide input using your keyboard and output is logged to your terminal.
6. When you type exit to terminate the /bin/bash command, the container stops but is not removed. You can start it again or remove it.

SERVICES

Services allow you to scale containers across multiple Docker daemons, which all work together as a swarm with multiple managers and workers. Each member of a swarm is a Docker daemon, and the daemons all communicate using the Docker API. A service allows you to define the desired state, such as the number of replicas of the service that must be available at any given time. By default, the service is load-balanced across all worker nodes. To the consumer, the Docker service appears to be a single application. Docker Engine supports swarm mode in Docker 1.12 and higher.