Docker: Lightweight Linux Containers for Consistent Development and Deployment

Take on "dependency hell" with Docker containers, the lightweight and nimble cousin of VMs. Learn how Docker makes applications portable and isolated by packaging them in containers based on LXC technology.

Imagine being able to package an application along with all of its dependencies easily and then run it smoothly in disparate development, test and production environments. That is the goal of the open-source Docker project. Although it is still not officially production-ready, the latest release (0.7.x at the time of this writing) brought Docker another step closer to realizing this ambitious goal.

Docker tries to solve the problem of "dependency hell". Modern applications often are assembled from existing components and rely on other services and applications. For example, your Python application might use PostgreSQL as a data store, Redis for caching and Apache as a Web server. Each of these components comes with its own set of dependencies that may conflict with those of other components. By packaging each component and its dependencies, Docker solves the following problems:

  • Conflicting dependencies: need to run one Web site on PHP 4.3 and another on PHP 5.5? No problem if you run each version of PHP in a separate Docker container.

  • Missing dependencies: installing applications in a new environment is a snap with Docker, because all dependencies are packaged along with the application in a container.

  • Platform differences: moving from one distro to another is no longer a problem. If both systems run Docker, the same container will execute without issues.

Docker: a Little Background

Docker started life as an open-source project at dotCloud, a cloud-centric platform-as-a-service company, in early 2013. Initially, Docker was a natural extension of the technology the company had developed to run its cloud business on thousands of servers. It is written in Go, a statically typed programming language developed by Google with syntax loosely based on C. Fast-forward six to nine months, and the company has hired a new CEO, joined the Linux Foundation, changed its name to Docker Inc., and announced that it is shifting its focus to the development of Docker and the Docker ecosystem. As further indication of Docker's popularity, at the time of this writing, it has been starred on GitHub 8,985 times and has been forked 1,304 times. Figure 1 illustrates Docker's rising popularity in Google searches. I predict that the shape of the past 12 months will be dwarfed by the next 12 months as Docker Inc. delivers the first version blessed for production deployments of containers and the community at large becomes aware of Docker's usefulness.

Figure 1. Google Trends Graph for "Docker Software" for Past 12 Months

Under the Hood

Docker harnesses some powerful kernel-level technology and puts it at our fingertips. The concept of a container in virtualization has been around for several years, but by providing a simple tool set and a unified API for managing some kernel-level technologies, such as LXCs (LinuX Containers), cgroups and a copy-on-write filesystem, Docker has created a tool that is greater than the sum of its parts. The result is a potential game-changer for DevOps, system administrators and developers.

Docker provides tools to make creating and working with containers as easy as possible. Containers sandbox processes from each other. For now, you can think of a container as a lightweight equivalent of a virtual machine.

Linux Containers and LXC, a user-space control package for Linux Containers, constitute the core of Docker. LXC uses kernel-level namespaces to isolate the container from the host. The user namespace separates the container's and the host's user database, thus ensuring that the container's root user does not have root privileges on the host. The process namespace is responsible for displaying and managing only processes running in the container, not the host. And, the network namespace provides the container with its own network device and virtual IP address.

Another component of Docker provided by LXC are Control Groups (cgroups). While namespaces are responsible for isolation between host and container, control groups implement resource accounting and limiting. While allowing Docker to limit the resources being consumed by a container, such as memory, disk space and I/O, cgroups also output lots of metrics about these resources. These metrics allow Docker to monitor the resource consumption of the various processes within the containers and make sure that each gets only its fair share of the available resources.

In addition to the above components, Docker has been using AuFS (Advanced Multi-Layered Unification Filesystem) as a filesystem for containers. AuFS is a layered filesystem that can transparently overlay one or more existing filesystems. When a process needs to modify a file, AuFS creates a copy of that file. AuFS is capable of merging multiple layers into a single representation of a filesystem. This process is called copy-on-write.

The really cool thing is that AuFS allows Docker to use certain images as the basis for containers. For example, you might have a CentOS Linux image that can be used as the basis for many different containers. Thanks to AuFS, only one copy of the CentOS image is required, which results in savings of storage and memory, as well as faster deployments of containers.

An added benefit of using AuFS is Docker's ability to version container images. Each new version is simply a diff of changes from the previous version, effectively keeping image files to a minimum. But, it also means that you always have a complete audit trail of what has changed from one version of a container to another.

Traditionally, Docker has depended on AuFS to provide a copy-on-write storage mechanism. However, the recent addition of a storage driver API is likely to lessen that dependence. Initially, there are three storage drivers available: AuFS, VFS and Device-Mapper, which is the result of a collaboration with Red Hat.

As of version 0.7, Docker works with all Linux distributions. However, it does not work with most non-Linux operating systems, such as Windows and OS X. The recommended way of using Docker on those OSes is to provision a virtual machine on VirtualBox using Vagrant.