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page_title: Understanding Docker
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page_description: Docker explained in depth
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-page_keywords: docker, introduction, documentation, about, technology, understanding, Dockerfile
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+page_keywords: docker, introduction, documentation, about, technology, understanding
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# Understanding Docker
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-*What is Docker? What makes it great?*
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+**What is Docker?**
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-Building development lifecycles, pipelines and deployment tooling is
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-hard. It's not easy to create portable applications and services.
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-There's often high friction getting code from your development
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-environment to production. It's also hard to ensure those applications
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-and services are consistent, up-to-date and managed.
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+Docker is a platform for developing, shipping, and running applications.
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+Docker is designed to deliver your applications faster. With Docker you
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+can separate your applications from your infrastructure AND treat your
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+infrastructure like a managed application. We want to help you ship code
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+faster, test faster, deploy faster and shorten the cycle between writing
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+code and running code.
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-Docker is designed to solve these problem for both developers and
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-sysadmins. It is a lightweight framework (with a powerful API) that
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-provides a lifecycle for building and deploying applications into
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-containers.
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+Docker does this by combining a lightweight container virtualization
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+platform with workflow and tooling that helps you manage and deploy your
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+applications.
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-Docker provides a way to run almost any application securely isolated
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-into a container. The isolation and security allows you to run many
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-containers simultaneously on your host. The lightweight nature of
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+At its core Docker provides a way to run almost any application securely
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+isolated into a container. The isolation and security allows you to run
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+many containers simultaneously on your host. The lightweight nature of
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containers, which run without the extra overload of a hypervisor, means
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you can get more out of your hardware.
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-**Note:** Docker itself is *shipped* with the Apache 2.0 license and it
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-is completely open-source — *the pun? very much intended*.
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+Surrounding the container virtualization, we provide tooling and a
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+platform to help you get your applications (and its supporting
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+components) into Docker containers, to distribute and ship those
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+containers to your teams to develop and test on them and then to deploy
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+those applications to your production environment whether it be in a
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+local data center or the Cloud.
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-### What are the Docker basics I need to know?
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+## What can I use Docker for?
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-Docker has three major components:
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+* Faster delivery of your applications
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-* Docker containers.
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-* Docker images.
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-* Docker registries.
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-
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-#### Docker containers
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-
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-Docker containers are like a directory. A Docker container holds
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-everything that is needed for an application to run. Each container is
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-created from a Docker image. Docker containers can be run, started,
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-stopped, moved and deleted. Each container is an isolated and secure
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-application platform. You can consider Docker containers the *run*
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-portion of the Docker framework.
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-
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-#### Docker images
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-
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-The Docker image is a template, for example an Ubuntu
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-operating system with Apache and your web application installed. Docker
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-containers are launched from images. Docker provides a simple way to
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-build new images or update existing images. You can consider Docker
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-images to be the *build* portion of the Docker framework.
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-
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-#### Docker Registries
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-
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-Docker registries hold images. These are public (or private!) stores
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-that you can upload or download images to and from. These images can be
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-images you create yourself or you can make use of images that others
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-have previously created. Docker registries allow you to build simple and
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-powerful development and deployment work flows. You can consider Docker
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-registries the *share* portion of the Docker framework.
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-
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-### How does Docker work?
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-
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-Docker is a client-server framework. The Docker *client* commands the Docker
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-*daemon*, which in turn creates, builds and manages containers.
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-
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-The Docker daemon takes advantage of some neat Linux kernel and
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-operating system features, like `namespaces` and `cgroups`, to build
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-isolated container. Docker provides a simple abstraction layer to these
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-technologies.
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-
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-> **Note:** If you would like to learn more about the underlying technology,
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-> why not jump to [Understanding the Technology](technology.md) where we talk about them? You can
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-> always come back here to continue learning about features of Docker and what
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-> makes it different.
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-
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-## Features of Docker
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-
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-In order to get a good grasp of the capabilities of Docker you should
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-read the [User's Manual](http://docs.docker.io). Let's look at a summary
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-of Docker's features to give you an idea of how Docker might be useful
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-to you.
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-
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-### User centric and simple to use
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-
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-*Docker is made for humans.*
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-
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-It's easy to get started and easy to build and deploy applications with
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-Docker: or as we say "*dockerize*" them! As much of Docker as possible
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-uses plain English for commands and tries to be as lightweight and
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-transparent as possible. We want to get out of the way so you can build
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-and deploy your applications.
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-
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-### Docker is Portable
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-
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-*Dockerize And Go!*
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-
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-Docker containers are highly portable. Docker provides a standard
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-container format to hold your applications:
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-
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-* You take care of your applications inside the container, and;
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-* Docker takes care of managing the container.
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-
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-Any machine, be it bare-metal or virtualized, can run any Docker
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-container. The sole requirement is to have Docker installed.
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-
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-**This translates to:**
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+Docker is perfect for helping you with the development lifecycle. Docker
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+can allow your developers to develop on local containers that contain
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+your applications and services. It can integrate into a continuous
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+integration and deployment workflow.
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- - Reliability;
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- - Freeing your applications out of the dependency-hell;
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- - A natural guarantee that things will work, anywhere.
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+Your developers write code locally and share their development stack via
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+Docker with their colleagues. When they are ready they can push their
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+code and the stack they are developing on to a test environment and
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+execute any required tests. From the testing environment you can then
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+push your Docker images into production and deploy your code.
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-### Lightweight
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+* Deploy and scale more easily
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-*No more resources waste.*
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+Docker's container platform allows you to have highly portable
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+workloads. Docker containers can run on a developer's local host, on
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+physical or virtual machines in a data center or in the Cloud.
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-Containers are lightweight, in fact, they are extremely lightweight.
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-Unlike traditional virtual machines, which have the overhead of a
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-hypervisor, Docker relies on operating system level features to provide
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-isolation and security. A Docker container does not need anything more
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-than what your application needs to run.
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+Docker's portability and lightweight nature also makes managing
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+workloads dynamically easy. You can use Docker to build and scale out
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+applications and services. Docker's speed means that scaling can be near
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+real time.
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-This translates to:
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+* Get higher density and run more workloads
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- - Ability to deploy a large number of applications on a single system;
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- - Lightning fast start up times and reduced overhead.
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+Docker is lightweight and fast. It provides a viable (and
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+cost-effective!) alternative to hypervisor-based virtual machines. This
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+is especially useful in high density environments, for example building
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+your own Cloud or Platform-as-a-Service. But it is also useful
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+for small and medium deployments where you want to get more out of the
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+resources you have.
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-### Docker can run anything
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+## What are the major Docker components?
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-*An amazing host! (again, pun intended.)*
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+Docker has two major components:
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-Docker isn't prescriptive about what applications or services you can run
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-inside containers. We provide use cases and examples for running web
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-services, databases, applications - just about anything you can imagine
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-can run in a Docker container.
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+* Docker: the open source container virtualization platform.
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+* [Docker.io](https://index.docker.io): our Software-as-a-Service
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+ platform for sharing and managing Docker containers.
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-**This translates to:**
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+**Note:** Docker is licensed with the open source Apache 2.0 license.
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- - Ability to run a wide range of applications;
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- - Ability to deploy reliably without repeating yourself.
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+## What is the architecture of Docker?
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-### Plays well with others
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+Docker has a client-server architecture. The Docker *client* talks to
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+the Docker *daemon* which does the heavy lifting of building, running
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+and distributing your Docker containers. Both the Docker client and the
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+daemon *can* run on the same system, or you can connect a Docker client
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+with a remote Docker daemon. The Docker client and service can
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+communicate via sockets or through a RESTful API.
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-*A wonderful guest.*
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+
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-Today, it is possible to install and use Docker almost anywhere. Even on
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-non-Linux systems such as Windows or Mac OS X thanks to a project called
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-[Boot2Docker](http://boot2docker.io).
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+### The Docker daemon
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-**This translates to running Docker (and Docker containers!) _anywhere_:**
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+As shown on the diagram above, the Docker daemon runs on a host machine.
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+The user does not directly interact with the daemon, but instead through
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+the Docker client.
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- - **Linux:**
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- Ubuntu, CentOS / RHEL, Fedora, Gentoo, openSUSE and more.
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- - **Infrastructure-as-a-Service:**
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- Amazon AWS, Google GCE, Rackspace Cloud and probably, your favorite IaaS.
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- - **Microsoft Windows**
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- - **OS X**
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+### The Docker client
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-### Docker is Responsible
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+The Docker client, in the form of the `docker` binary, is the primary user
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+interface to Docker. It is tasked with accepting commands from the user
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+and communicating back and forth with a Docker daemon.
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-*A tool that you can trust.*
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+### Inside Docker
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-Docker does not just bring you a set of tools to isolate and run
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-applications. It also allows you to specify constraints and controls on
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-those resources.
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+Inside Docker there are three concepts we’ll need to understand:
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-**This translates to:**
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-
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- - Fine tuning available resources for each application;
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- - Allocating memory or CPU intelligently to make most of your environment;
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-
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-Without dealing with complicated commands or third party applications.
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-
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-### Docker is Social
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-
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-*Docker knows that No One Is an Island.*
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-
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-Docker allows you to share the images you've built with the world. And
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-lots of people have already shared their own images.
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-
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-To facilitate this sharing Docker comes with a public registry called
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-[Docker.io](http://index.docker.io). If you don't want your images to be
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-public you can also use private images on [Docker.io](https://index.docker.io)
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-or even run your own registry behind your firewall.
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-
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-**This translates to:**
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-
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- - No more wasting time building everything from scratch;
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- - Easily and quickly save your application stack;
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- - Share and benefit from the depth of the Docker community.
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-
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-## Docker versus Virtual Machines
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-
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-> I suppose it is tempting, if the *only* tool you have is a hammer, to
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-> treat *everything* as if it were a nail.
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-> — **_Abraham Maslow_**
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-
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-**Docker containers are:**
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-
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- - Easy on the resources;
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- - Extremely light to deal with;
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- - Do not come with substantial overhead;
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- - Very easy to work with;
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- - Agnostic;
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- - Can work *on* virtual machines;
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- - Secure and isolated;
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- - *Artful*, *social*, *fun*, and;
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- - Powerful sand-boxes.
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-
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-**Docker containers are not:**
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-
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- - Hardware or OS emulators;
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- - Resource heavy;
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- - Platform, software or language dependent.
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-
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-## Docker Use Cases
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-
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-Docker is a framework. As a result it's flexible and powerful enough to
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-be used in a lot of different use cases.
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-
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-### For developers
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-
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- - **Developed with developers in mind:**
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- Build, test and ship applications with nothing but Docker and lean
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- containers.
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- - **Re-usable building blocks to create more:**
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- Docker images are easily updated building blocks.
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- - **Automatically build-able:**
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- It has never been this easy to build - *anything*.
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- - **Easy to integrate:**
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- A powerful, fully featured API allows you to integrate Docker into your tooling.
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-
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-### For sysadmins
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-
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- - **Efficient (and DevOps friendly!) lifecycle:**
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- Operations and developments are consistent, repeatable and reliable.
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- - **Balanced environments:**
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- Processes between development, testing and production are leveled.
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- - **Improvements on speed and integration:**
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- Containers are almost nothing more than isolated, secure processes.
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- - **Lowered costs of infrastructure:**
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- Containers are lightweight and heavy on resources compared to virtual machines.
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- - **Portable configurations:**
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- Issues and overheads with dealing with configurations and systems are eliminated.
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+* Docker images.
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+* Docker registries.
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+* Docker containers.
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-### For everyone
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+#### Docker images
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- - **Increased security without performance loss:**
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- Replacing VMs with containers provide security without additional
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- hardware (or software).
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- - **Portable:**
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- You can easily move applications and workloads from different operating
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- systems and platforms.
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+The Docker image is a read-only template, for example an Ubuntu operating system
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+with Apache and your web application installed. Docker containers are
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+created from images. You can download Docker images that other people
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+have created or Docker provides a simple way to build new images or
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+update existing images. You can consider Docker images to be the **build**
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+portion of Docker.
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-## Where to go from here
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+#### Docker Registries
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-### Learn about Parts of Docker and the underlying technology
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+Docker registries hold images. These are public (or private!) stores
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+that you can upload or download images to and from. The public Docker
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+registry is called [Docker.io](http://index.docker.io). It provides a
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+huge collection of existing images that you can use. These images can be
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+images you create yourself or you can make use of images that others
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+have previously created. You can consider Docker registries the
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+**distribution** portion of Docker.
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-Visit [Understanding the Technology](technology.md) in our Getting Started manual.
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+#### Docker containers
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-### Get practical and learn how to use Docker straight away
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+Docker containers are like a directory. A Docker container holds
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+everything that is needed for an application to run. Each container is
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+created from a Docker image. Docker containers can be run, started,
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+stopped, moved and deleted. Each container is an isolated and secure
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+application platform. You can consider Docker containers the **run**
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+portion of Docker.
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-Visit [Working with Docker](working-with-docker.md) in our Getting Started manual.
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+## So how does Docker work?
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-### Get the product and go hands-on
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+We've learned so far that:
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-Visit [Get Docker](get-docker.md) in our Getting Started manual.
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+1. You can build Docker images that hold your applications.
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+2. You can create Docker containers from those Docker images to run your
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+ applications.
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+3. You can share those Docker images via
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+ [Docker.io](https://index.docker.io) or your own registry.
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+
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+Let's look at how these elements combine together to make Docker work.
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+
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+### How does a Docker Image work?
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+
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+We've already seen that Docker images are read-only templates that
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+Docker containers are launched from. Each image consists of a series of
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+layers. Docker makes use of [union file
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+systems](http://en.wikipedia.org/wiki/UnionFS) to combine these layers
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+into a single image. Union file systems allow files and directories of
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+separate file systems, known as branches, to be transparently overlaid,
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+forming a single coherent file system.
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+
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+One of the reasons Docker is so lightweight is because of these layers.
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+When you change a Docker image, for example update an application to a
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+new version, this builds a new layer. Hence, rather than replacing the whole
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+image or entirely rebuilding, as you may do with a virtual machine, only
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+that layer is added or updated. Now you don't need to distribute a whole new image,
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+just the update, making distributing Docker images fast and simple.
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+
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+Every image starts from a base image, for example `ubuntu`, a base Ubuntu
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+image, or `fedora`, a base Fedora image. You can also use images of your
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+own as the basis for a new image, for example if you have a base Apache
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+image you could use this as the base of all your web application images.
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+
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+> **Note:**
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+> Docker usually gets these base images from [Docker.io](https://index.docker.io).
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+
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+Docker images are then built from these base images using a simple
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+descriptive set of steps we call *instructions*. Each instruction
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+creates a new layer in our image. Instructions include steps like:
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+
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+* Run a command.
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+* Add a file or directory.
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+* Create an environment variable.
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+* What process to run when launching a container from this image.
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+
|
|
|
+These instructions are stored in a file called a `Dockerfile`. Docker
|
|
|
+reads this `Dockerfile` when you request an image be built, executes the
|
|
|
+instructions and returns a final image.
|
|
|
+
|
|
|
+### How does a Docker registry work?
|
|
|
+
|
|
|
+The Docker registry is the store for your Docker images. Once you build
|
|
|
+a Docker image you can *push* it to a public registry [Docker.io](
|
|
|
+https://index.docker.io) or to your own registry running behind your
|
|
|
+firewall.
|
|
|
+
|
|
|
+Using the Docker client, you can search for already published images and
|
|
|
+then pull them down to your Docker host to build containers from them.
|
|
|
+
|
|
|
+[Docker.io](https://index.docker.io) provides both public and
|
|
|
+private storage for images. Public storage is searchable and can be
|
|
|
+downloaded by anyone. Private storage is excluded from search
|
|
|
+results and only you and your users can pull them down and use them to
|
|
|
+build containers. You can [sign up for a plan
|
|
|
+here](https://index.docker.io/plans).
|
|
|
+
|
|
|
+### How does a container work?
|
|
|
+
|
|
|
+A container consists of an operating system, user added files and
|
|
|
+meta-data. As we've discovered each container is built from an image. That image tells
|
|
|
+Docker what the container holds, what process to run when the container
|
|
|
+is launched and a variety of other configuration data. The Docker image
|
|
|
+is read-only. When Docker runs a container from an image it adds a
|
|
|
+read-write layer on top of the image (using a union file system as we
|
|
|
+saw earlier) in which your application is then run.
|
|
|
+
|
|
|
+### What happens when you run a container?
|
|
|
+
|
|
|
+The Docker client using the `docker` binary, or via the API, tells the
|
|
|
+Docker daemon to run a container. Let's take a look at what happens
|
|
|
+next.
|
|
|
+
|
|
|
+ $ docker run -i -t ubuntu /bin/bash
|
|
|
+
|
|
|
+Let's break down this command. The Docker client is launched using the
|
|
|
+`docker` binary with the `run` option telling it to launch a new
|
|
|
+container. The bare minimum the Docker client needs to tell the
|
|
|
+Docker daemon to run the container is:
|
|
|
+
|
|
|
+* What Docker image to build the container from, here `ubuntu`, a base
|
|
|
+ Ubuntu image;
|
|
|
+* The command you want to run inside the container when it is launched,
|
|
|
+ here `bin/bash` to shell the Bash shell inside the new container.
|
|
|
+
|
|
|
+So what happens under the covers when we run this command?
|
|
|
+
|
|
|
+Docker begins with:
|
|
|
+
|
|
|
+- **Pulling the `ubuntu` image:**
|
|
|
+ Docker checks for the presence of the `ubuntu` image and if it doesn't
|
|
|
+ exist locally on the host, then Docker downloads it from
|
|
|
+ [Docker.io](https://index.docker.io). If the image already exists then
|
|
|
+ Docker uses it for the new container.
|
|
|
+- **Creates a new container:**
|
|
|
+ Once Docker has the image it creates a container from it:
|
|
|
+ * **Allocates a filesystem and mounts a read-write _layer_:**
|
|
|
+ The container is created in the file system and a read-write layer is
|
|
|
+ added to the image.
|
|
|
+ * **Allocates a network / bridge interface:**
|
|
|
+ Creates a network interface that allows the Docker container to talk to
|
|
|
+ the local host.
|
|
|
+ * **Sets up an IP address:**
|
|
|
+ Finds and attaches an available IP address from a pool.
|
|
|
+- **Executes a process that you specify:**
|
|
|
+ Runs your application, and;
|
|
|
+- **Captures and provides application output:**
|
|
|
+ Connects and logs standard input, outputs and errors for you to see how
|
|
|
+ your application is running.
|
|
|
+
|
|
|
+Now you have a running container! From here you can manage your running
|
|
|
+container, interact with your application and then when finished stop
|
|
|
+and remove your container.
|
|
|
+
|
|
|
+## The underlying technology
|
|
|
+
|
|
|
+Docker is written in Go and makes use of several Linux kernel features to
|
|
|
+deliver the features we've seen.
|
|
|
+
|
|
|
+### Namespaces
|
|
|
+
|
|
|
+Docker takes advantage of a technology called `namespaces` to provide an
|
|
|
+isolated workspace we call a *container*. When you run a container,
|
|
|
+Docker creates a set of *namespaces* for that container.
|
|
|
+
|
|
|
+This provides a layer of isolation: each aspect of a container runs in
|
|
|
+its own namespace and does not have access outside it.
|
|
|
+
|
|
|
+Some of the namespaces that Docker uses are:
|
|
|
+
|
|
|
+ - **The `pid` namespace:**
|
|
|
+ Used for process isolation (PID: Process ID).
|
|
|
+ - **The `net` namespace:**
|
|
|
+ Used for managing network interfaces (NET: Networking).
|
|
|
+ - **The `ipc` namespace:**
|
|
|
+ Used for managing access to IPC resources (IPC: InterProcess
|
|
|
+Communication).
|
|
|
+ - **The `mnt` namespace:**
|
|
|
+ Used for managing mount-points (MNT: Mount).
|
|
|
+ - **The `uts` namespace:**
|
|
|
+ Used for isolating kernel and version identifiers. (UTS: Unix Timesharing
|
|
|
+System).
|
|
|
+
|
|
|
+### Control groups
|
|
|
+
|
|
|
+Docker also makes use of another technology called `cgroups` or control
|
|
|
+groups. A key need to run applications in isolation is to have them only
|
|
|
+use the resources you want. This ensures containers are good
|
|
|
+multi-tenant citizens on a host. Control groups allow Docker to
|
|
|
+share available hardware resources to containers and if required, set up to
|
|
|
+limits and constraints, for example limiting the memory available to a
|
|
|
+specific container.
|
|
|
+
|
|
|
+### Union file systems
|
|
|
+
|
|
|
+Union file systems or UnionFS are file systems that operate by creating
|
|
|
+layers, making them very lightweight and fast. Docker uses union file
|
|
|
+systems to provide the building blocks for containers. We learned about
|
|
|
+union file systems earlier in this document. Docker can make use of
|
|
|
+several union file system variants including: AUFS, btrfs, vfs, and
|
|
|
+DeviceMapper.
|
|
|
+
|
|
|
+### Container format
|
|
|
+
|
|
|
+Docker combines these components into a wrapper we call a container
|
|
|
+format. The default container format is called `libcontainer`. Docker
|
|
|
+also supports traditional Linux containers using
|
|
|
+[LXC](https://linuxcontainers.org/). In future Docker may support other
|
|
|
+container formats, for example integration with BSD Jails or Solaris
|
|
|
+Zones.
|
|
|
+
|
|
|
+## Next steps
|
|
|
+
|
|
|
+### Learning how to use Docker
|
|
|
+
|
|
|
+Visit [Working with Docker](working-with-docker.md).
|
|
|
+
|
|
|
+### Installing Docker
|
|
|
+
|
|
|
+Visit the [installation](/installation/#installation) section.
|
|
|
|
|
|
### Get the whole story
|
|
|
|
|
|
[https://www.docker.io/the_whole_story/](https://www.docker.io/the_whole_story/)
|
|
|
+
|
Sven Dowideit