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202 lines
8.3 KiB
Markdown
202 lines
8.3 KiB
Markdown
Docker: the Linux container engine
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==================================
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Docker is an open source project to pack, ship and run any application
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as a lightweight container
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Docker containers are both *hardware-agnostic* and
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*platform-agnostic*. This means that they can run anywhere, from your
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laptop to the largest EC2 compute instance and everything in between -
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and they don't require that you use a particular language, framework
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or packaging system. That makes them great building blocks for
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deploying and scaling web apps, databases and backend services without
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depending on a particular stack or provider.
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Docker is an open-source implementation of the deployment engine which
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powers [dotCloud](http://dotcloud.com), a popular
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Platform-as-a-Service. It benefits directly from the experience
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accumulated over several years of large-scale operation and support of
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hundreds of thousands of applications and databases.
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## Better than VMs
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A common method for distributing applications and sandbox their
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execution is to use virtual machines, or VMs. Typical VM formats are
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VMWare's vmdk, Oracle Virtualbox's vdi, and Amazon EC2's ami. In
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theory these formats should allow every developer to automatically
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package their application into a "machine" for easy distribution and
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deployment. In practice, that almost never happens, for a few reasons:
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* *Size*: VMs are very large which makes them impractical to store
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and transfer.
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* *Performance*: running VMs consumes significant CPU and memory,
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which makes them impractical in many scenarios, for example local
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development of multi-tier applications, and large-scale deployment
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of cpu and memory-intensive applications on large numbers of
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machines.
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* *Portability*: competing VM environments don't play well with each
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other. Although conversion tools do exist, they are limited and
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add even more overhead.
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* *Hardware-centric*: VMs were designed with machine operators in
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mind, not software developers. As a result, they offer very
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limited tooling for what developers need most: building, testing
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and running their software. For example, VMs offer no facilities
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for application versioning, monitoring, configuration, logging or
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service discovery.
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By contrast, Docker relies on a different sandboxing method known as
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*containerization*. Unlike traditional virtualization,
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containerization takes place at the kernel level. Most modern
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operating system kernels now support the primitives necessary for
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containerization, including Linux with [openvz](http://openvz.org),
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[vserver](http://linux-vserver.org) and more recently
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[lxc](http://lxc.sourceforge.net), Solaris with
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[zones](http://docs.oracle.com/cd/E26502_01/html/E29024/preface-1.html#scrolltoc)
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and FreeBSD with
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[Jails](http://www.freebsd.org/doc/handbook/jails.html).
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Docker builds on top of these low-level primitives to offer developers
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a portable format and runtime environment that solves all 4
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problems. Docker containers are small (and their transfer can be
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optimized with layers), they have basically zero memory and cpu
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overhead, they are completely portable and are designed from the
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ground up with an application-centric design.
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The best part: because ``docker`` operates at the OS level, it can
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still be run inside a VM!
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## Plays well with others
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Docker does not require that you buy into a particular programming
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language, framework, packaging system or configuration language.
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Is your application a Unix process? Does it use files, tcp
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connections, environment variables, standard Unix streams and
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command-line arguments as inputs and outputs? Then ``docker`` can run
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it.
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Can your application's build be expressed as a sequence of such
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commands? Then ``docker`` can build it.
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## Escape dependency hell
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A common problem for developers is the difficulty of managing all
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their application's dependencies in a simple and automated way.
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This is usually difficult for several reasons:
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* *Cross-platform dependencies*. Modern applications often depend on
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a combination of system libraries and binaries, language-specific
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packages, framework-specific modules, internal components
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developed for another project, etc. These dependencies live in
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different "worlds" and require different tools - these tools
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typically don't work well with each other, requiring awkward
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custom integrations.
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* Conflicting dependencies. Different applications may depend on
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different versions of the same dependency. Packaging tools handle
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these situations with various degrees of ease - but they all
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handle them in different and incompatible ways, which again forces
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the developer to do extra work.
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* Custom dependencies. A developer may need to prepare a custom
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version of their application's dependency. Some packaging systems
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can handle custom versions of a dependency, others can't - and all
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of them handle it differently.
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Docker solves dependency hell by giving the developer a simple way to
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express *all* their application's dependencies in one place, and
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streamline the process of assembling them. If this makes you think of
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[XKCD 927](http://xkcd.com/927/), don't worry. Docker doesn't
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*replace* your favorite packaging systems. It simply orchestrates
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their use in a simple and repeatable way. How does it do that? With
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layers.
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Docker defines a build as running a sequence of Unix commands, one
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after the other, in the same container. Build commands modify the
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contents of the container (usually by installing new files on the
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filesystem), the next command modifies it some more, etc. Since each
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build command inherits the result of the previous commands, the
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*order* in which the commands are executed expresses *dependencies*.
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Here's a typical Docker build process:
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```bash
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from ubuntu:12.10
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run apt-get update
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run DEBIAN_FRONTEND=noninteractive apt-get install -q -y python
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run DEBIAN_FRONTEND=noninteractive apt-get install -q -y python-pip
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run pip install django
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run DEBIAN_FRONTEND=noninteractive apt-get install -q -y curl
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run curl -L https://github.com/shykes/helloflask/archive/master.tar.gz | tar -xzv
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run cd helloflask-master && pip install -r requirements.txt
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```
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Note that Docker doesn't care *how* dependencies are built - as long
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as they can be built by running a Unix command in a container.
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Getting started
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===============
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Docker can be installed on your local machine as well as servers - both bare metal and virtualized.
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It is available as a binary on most modern Linux systems, or as a VM on Windows, Mac and other systems.
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We also offer an interactive tutorial for quickly learning the basics of using Docker.
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For up-to-date install instructions and online tutorials, see the [Getting Started page](http://www.docker.io/gettingstarted/).
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Usage examples
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==============
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Docker can be used to run short-lived commands, long-running daemons (app servers, databases etc.),
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interactive shell sessions, etc.
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You can find a [list of real-world examples](http://docs.docker.io/en/latest/examples/) in the documentation.
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Under the hood
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--------------
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Under the hood, Docker is built on the following components:
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* The
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[cgroup](http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-24-c)
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and
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[namespacing](http://blog.dotcloud.com/under-the-hood-linux-kernels-on-dotcloud-part)
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capabilities of the Linux kernel;
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* [AUFS](http://aufs.sourceforge.net/aufs.html), a powerful union
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filesystem with copy-on-write capabilities;
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* The [Go](http://golang.org) programming language;
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* [lxc](http://lxc.sourceforge.net/), a set of convenience scripts to
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simplify the creation of Linux containers.
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Contributing to Docker
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======================
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Want to hack on Docker? Awesome! There are instructions to get you
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started [here](CONTRIBUTING.md).
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They are probably not perfect, please let us know if anything feels
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wrong or incomplete.
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### Legal
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*Brought to you courtesy of our legal counsel. For more context,
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please see the Notice document.*
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Transfers of Docker shall be in accordance with applicable export controls
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of any country and all other applicable legal requirements. Without limiting the
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foregoing, Docker shall not be distributed or downloaded to any individual or
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location if such distribution or download would violate the applicable US
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government export regulations.
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For more information, please see http://www.bis.doc.gov
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