Docker: the Linux container engine
==================================

Docker is an open source project to pack, ship and run any application
as a lightweight container

Docker containers are both *hardware-agnostic* and
*platform-agnostic*. This means that they can run anywhere, from your
laptop to the largest EC2 compute instance and everything in between -
and they don't require that you use a particular language, framework
or packaging system. That makes them great building blocks for
deploying and scaling web apps, databases and backend services without
depending on a particular stack or provider.

Docker is an open-source implementation of the deployment engine which
powers [dotCloud](http://dotcloud.com), a popular
Platform-as-a-Service.  It benefits directly from the experience
accumulated over several years of large-scale operation and support of
hundreds of thousands of applications and databases.

![Docker L](docs/theme/docker/static/img/dockerlogo-h.png "Docker")

## Better than VMs

A common method for distributing applications and sandbox their
execution is to use virtual machines, or VMs. Typical VM formats are
VMWare's vmdk, Oracle Virtualbox's vdi, and Amazon EC2's ami. In
theory these formats should allow every developer to automatically
package their application into a "machine" for easy distribution and
deployment. In practice, that almost never happens, for a few reasons:

  * *Size*: VMs are very large which makes them impractical to store
     and transfer.
  * *Performance*: running VMs consumes significant CPU and memory,
    which makes them impractical in many scenarios, for example local
    development of multi-tier applications, and large-scale deployment
    of cpu and memory-intensive applications on large numbers of
    machines.
  * *Portability*: competing VM environments don't play well with each
     other. Although conversion tools do exist, they are limited and
     add even more overhead.
  * *Hardware-centric*: VMs were designed with machine operators in
    mind, not software developers. As a result, they offer very
    limited tooling for what developers need most: building, testing
    and running their software. For example, VMs offer no facilities
    for application versioning, monitoring, configuration, logging or
    service discovery.

By contrast, Docker relies on a different sandboxing method known as
*containerization*. Unlike traditional virtualization,
containerization takes place at the kernel level. Most modern
operating system kernels now support the primitives necessary for
containerization, including Linux with [openvz](http://openvz.org),
[vserver](http://linux-vserver.org) and more recently
[lxc](http://lxc.sourceforge.net), Solaris with
[zones](http://docs.oracle.com/cd/E26502_01/html/E29024/preface-1.html#scrolltoc)
and FreeBSD with
[Jails](http://www.freebsd.org/doc/handbook/jails.html).

Docker builds on top of these low-level primitives to offer developers
a portable format and runtime environment that solves all 4
problems. Docker containers are small (and their transfer can be
optimized with layers), they have basically zero memory and cpu
overhead, they are completely portable and are designed from the
ground up with an application-centric design.

The best part: because ``docker`` operates at the OS level, it can
still be run inside a VM!

## Plays well with others

Docker does not require that you buy into a particular programming
language, framework, packaging system or configuration language.

Is your application a Unix process? Does it use files, tcp
connections, environment variables, standard Unix streams and
command-line arguments as inputs and outputs? Then ``docker`` can run
it.

Can your application's build be expressed as a sequence of such
commands? Then ``docker`` can build it.


## Escape dependency hell

A common problem for developers is the difficulty of managing all
their application's dependencies in a simple and automated way.

This is usually difficult for several reasons:

  * *Cross-platform dependencies*. Modern applications often depend on
    a combination of system libraries and binaries, language-specific
    packages, framework-specific modules, internal components
    developed for another project, etc. These dependencies live in
    different "worlds" and require different tools - these tools
    typically don't work well with each other, requiring awkward
    custom integrations.

  * Conflicting dependencies. Different applications may depend on
    different versions of the same dependency. Packaging tools handle
    these situations with various degrees of ease - but they all
    handle them in different and incompatible ways, which again forces
    the developer to do extra work.
  
  * Custom dependencies. A developer may need to prepare a custom
    version of their application's dependency. Some packaging systems
    can handle custom versions of a dependency, others can't - and all
    of them handle it differently.


Docker solves dependency hell by giving the developer a simple way to
express *all* their application's dependencies in one place, and
streamline the process of assembling them. If this makes you think of
[XKCD 927](http://xkcd.com/927/), don't worry. Docker doesn't
*replace* your favorite packaging systems. It simply orchestrates
their use in a simple and repeatable way. How does it do that? With
layers.

Docker defines a build as running a sequence of Unix commands, one
after the other, in the same container. Build commands modify the
contents of the container (usually by installing new files on the
filesystem), the next command modifies it some more, etc. Since each
build command inherits the result of the previous commands, the
*order* in which the commands are executed expresses *dependencies*.

Here's a typical Docker build process:

```bash
from ubuntu:12.10
run apt-get update
run DEBIAN_FRONTEND=noninteractive apt-get install -q -y python
run DEBIAN_FRONTEND=noninteractive apt-get install -q -y python-pip
run pip install django
run DEBIAN_FRONTEND=noninteractive apt-get install -q -y curl
run curl -L https://github.com/shykes/helloflask/archive/master.tar.gz | tar -xzv
run cd helloflask-master && pip install -r requirements.txt
```

Note that Docker doesn't care *how* dependencies are built - as long
as they can be built by running a Unix command in a container.


Getting started
===============

Docker can be installed on your local machine as well as servers - both bare metal and virtualized.
It is available as a binary on most modern Linux systems, or as a VM on Windows, Mac and other systems.

We also offer an interactive tutorial for quickly learning the basics of using Docker.


For up-to-date install instructions and online tutorials, see the [Getting Started page](http://www.docker.io/gettingstarted/).


Usage examples
==============

Docker can be used to run short-lived commands, long-running daemons (app servers, databases etc.),
interactive shell sessions, etc.

You can find a [list of real-world examples](http://docs.docker.io/en/latest/examples/) in the documentation.

Under the hood
--------------

Under the hood, Docker is built on the following components:

* The
  [cgroup](http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-24-c)
  and
  [namespacing](http://blog.dotcloud.com/under-the-hood-linux-kernels-on-dotcloud-part)
  capabilities of the Linux kernel;
* [AUFS](http://aufs.sourceforge.net/aufs.html), a powerful union
  filesystem with copy-on-write capabilities;
* The [Go](http://golang.org) programming language;
* [lxc](http://lxc.sourceforge.net/), a set of convenience scripts to
  simplify the creation of Linux containers.



Contributing to Docker
======================

Want to hack on Docker? Awesome! There are instructions to get you
started [here](CONTRIBUTING.md).

They are probably not perfect, please let us know if anything feels
wrong or incomplete.


### Legal

*Brought to you courtesy of our legal counsel. For more context,
please see the Notice document.*

Transfers of Docker shall be in accordance with applicable export
controls of any country and all other applicable legal requirements.
Docker shall not be distributed or downloaded to or in Cuba, Iran,
North Korea, Sudan or Syria and shall not be distributed or downloaded
to any person on the Denied Persons List administered by the U.S.
Department of Commerce.

