Docker

Docker (Figure 5) stands among the ranks of established solutions because it has already been working the market for several years, and it has built up its own microcosmos of third-party software vendors. Thanks to Google's Kubernetes, for example, Docker can now also run in a cluster. The single Docker container on a single host is no longer important; instead, the focus has shifted to whole swarms of containers that can be rolled out to any number of hosts in an automated process.

Figure 5: Winner takes all: Docker is the modern solution when it comes to container virtualization for Linux.

Seen from a technical point of view, Docker's success is impressive, because it is very similar to LXC: Like LXC, Docker relies on cgroups and namespaces in the Linux kernel, and LXC containers can just as easily use a plain vanilla kernel as Docker. Although Docker bolsters each kernel function with many features, such as version management for containers, these features are not crucial when deciding for Docker or LXC. The developers behind Docker, however, did not just get the technology right, they consistently pushed forward with targeted marketing from the outset.

Docker seems to offer administrators a plethora of new options. For example, PaaS (Platform as a Service) is easy with Docker: A new container that houses the desired application can be created in the shortest possible time, and this is precisely the capability that Docker consistently and successfully promotes.

Dark Clouds

In the meantime, the hype surrounding Docker seems to be ebbing slightly, given the critical opinions being voiced about Docker of late. CoreOS recently departed from Docker and is now developing its own container format named Rocket.

The way in which Docker handles containers is not universally lauded. For example, admins criticize the substantial security risk they take in their daily routines with Docker containers. The principle behind the development of many Docker containers is that "it works for me," but that is not exactly a quality metric. For example, something that works for a developer in a container on their laptop will probably also work on a server, but any administrator hosting that container could be taking an incalculable risk – a fact that is often ignored. The CoreOS developers cited a similar view as their motivation for migrating from Docker to their own format.

Conclusions

When you compare the established container solutions for Linux, the future belongs to cgroups and namespaces. Both tools are an integral part of the Linux kernel. Popular solutions such as LXC and Docker already make intensive use of them, and other solutions that previously followed their own approaches (e.g., Virtuozzo) are preparing to migrate to the kernel functions or have completed a migration in part.

Linux-VServer is a special case among the candidates introduced in this article; the project made a conscious decision to continue with its own special approach. For the developers, this is a burden for the future and a factor of uncertainty for administrators; the solution can only hope to survive if someone can be found to keep providing the required kernel patches in the future.

In comparison, it is unlikely that cgroups and namespaces, and along with them LXC and Docker, will simply disappear. Too many corporations have invested time and money in developing the required interfaces, with no sign of an alternative. LXC and Docker are the Linux containers of the present day and are not alone in their dependence on cgroups and namespaces.

Linux has thus officially and finally acquired the functionality that containers (zones) in Solaris or jails in FreeBSD have had for years – a standardized interface right at the heart of the operating system that third-party vendors can use as needed.