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page_title: FAQ page_description: Most frequently asked questions. page_keywords: faq, questions, documentation, docker

FAQ

Most frequently asked questions

How much does Docker cost?

Docker is 100% free, it is open source, so you can use it without paying.

What open source license are you using?

We are using the Apache License Version 2.0, see it here: https://github.com/dotcloud/docker/blob/master/LICENSE

Does Docker run on Mac OS X or Windows?

Not at this time, Docker currently only runs on Linux, but you can use VirtualBox to run Docker in a virtual machine on your box, and get the best of both worlds. Check out the Mac OS X and Microsoft Windows installation guides. The small Linux distribution boot2docker can be run inside virtual machines on these two operating systems.

How do containers compare to virtual machines?

They are complementary. VMs are best used to allocate chunks of hardware resources. Containers operate at the process level, which makes them very lightweight and perfect as a unit of software delivery.

What does Docker add to just plain LXC?

Docker is not a replacement for LXC. "LXC" refers to capabilities of the Linux kernel (specifically namespaces and control groups) which allow sandboxing processes from one another, and controlling their resource allocations. On top of this low-level foundation of kernel features, Docker offers a high-level tool with several powerful functionalities:

  • Portable deployment across machines. Docker defines a format for bundling an application and all its dependencies into a single object which can be transferred to any Docker-enabled machine, and executed there with the guarantee that the execution environment exposed to the application will be the same. LXC implements process sandboxing, which is an important pre-requisite for portable deployment, but that alone is not enough for portable deployment. If you sent me a copy of your application installed in a custom LXC configuration, it would almost certainly not run on my machine the way it does on yours, because it is tied to your machine's specific configuration: networking, storage, logging, distro, etc. Docker defines an abstraction for these machine-specific settings, so that the exact same Docker container can run - unchanged - on many different machines, with many different configurations.

  • Application-centric. Docker is optimized for the deployment of applications, as opposed to machines. This is reflected in its API, user interface, design philosophy and documentation. By contrast, the lxc helper scripts focus on containers as lightweight machines - basically servers that boot faster and need less RAM. We think there's more to containers than just that.

  • Automatic build. Docker includes a tool for developers to automatically assemble a container from their source code, with full control over application dependencies, build tools, packaging etc. They are free to use make, maven, chef, puppet, salt, Debian packages, RPMs, source tarballs, or any combination of the above, regardless of the configuration of the machines.

  • Versioning. Docker includes git-like capabilities for tracking successive versions of a container, inspecting the diff between versions, committing new versions, rolling back etc. The history also includes how a container was assembled and by whom, so you get full traceability from the production server all the way back to the upstream developer. Docker also implements incremental uploads and downloads, similar to git pull, so new versions of a container can be transferred by only sending diffs.

  • Component re-use. Any container can be used as a "base image" to create more specialized components. This can be done manually or as part of an automated build. For example you can prepare the ideal Python environment, and use it as a base for 10 different applications. Your ideal Postgresql setup can be re-used for all your future projects. And so on.

  • Sharing. Docker has access to a public registry where thousands of people have uploaded useful containers: anything from Redis, CouchDB, Postgres to IRC bouncers to Rails app servers to Hadoop to base images for various Linux distros. The registry also includes an official "standard library" of useful containers maintained by the Docker team. The registry itself is open-source, so anyone can deploy their own registry to store and transfer private containers, for internal server deployments for example.

  • Tool ecosystem. Docker defines an API for automating and customizing the creation and deployment of containers. There are a huge number of tools integrating with Docker to extend its capabilities. PaaS-like deployment (Dokku, Deis, Flynn), multi-node orchestration (Maestro, Salt, Mesos, Openstack Nova), management dashboards (docker-ui, Openstack Horizon, Shipyard), configuration management (Chef, Puppet), continuous integration (Jenkins, Strider, Travis), etc. Docker is rapidly establishing itself as the standard for container-based tooling.

What is different between a Docker container and a VM?

There's a great StackOverflow answer [showing the differences]( http://stackoverflow.com/questions/16047306/ how-is-docker-io-different-from-a-normal-virtual-machine).

Do I lose my data when the container exits?

Not at all! Any data that your application writes to disk gets preserved in its container until you explicitly delete the container. The file system for the container persists even after the container halts.

How far do Docker containers scale?

Some of the largest server farms in the world today are based on containers. Large web deployments like Google and Twitter, and platform providers such as Heroku and dotCloud all run on container technology, at a scale of hundreds of thousands or even millions of containers running in parallel.

How do I connect Docker containers?

Currently the recommended way to link containers is via the link primitive. You can see details of how to work with links here.

Also of useful when enabling more flexible service portability is the Ambassador linking pattern.

How do I run more than one process in a Docker container?

Any capable process supervisor such as http://supervisord.org/, runit, s6, or daemontools can do the trick. Docker will start up the process management daemon which will then fork to run additional processes. As long as the processor manager daemon continues to run, the container will continue to as well. You can see a more substantial example that uses supervisord here.

What platforms does Docker run on?

Linux:

  • Ubuntu 12.04, 13.04 et al
  • Fedora 19/20+
  • RHEL 6.5+
  • Centos 6+
  • Gentoo
  • ArchLinux
  • openSUSE 12.3+
  • CRUX 3.0+

Cloud:

  • Amazon EC2
  • Google Compute Engine
  • Rackspace

How do I report a security issue with Docker?

You can learn about the project's security policy here and report security issues to this mailbox.

Why do I need to sign my commits to Docker with the DCO?

Please read [our blog post]( http://blog.docker.io/2014/01/ docker-code-contributions-require-developer-certificate-of-origin/) on the introduction of the DCO.

Can I help by adding some questions and answers?

Definitely! You can fork the repo and edit the documentation sources.

Where can I find more answers?

You can find more answers on:

Looking for something else to read? Checkout the Hello World example.