Deimos is a Docker plugin for Mesos, providing external containerization as described in MESOS-816.

NOTE: Deimos is currently under heavy development and relies on very recent changes in Mesos master that have not been released yet. Be prepared for some bumps and changing APIs. If you run into trouble, please file an issue on Github. Official support for external containerizers like Deimos will be available in the next Mesos release (0.19), at which point there will be a stable release of Deimos as well.

For a complete installation walkthrough, see this Gist.

Deimos can be installed from the Cheeseshop.

pip install deimos

Although Deimos does not have a library dependency on a particular Mesos version, one does need a development version of the Mesos package (for which there are also Python bindings), available only for very recent Ubuntu, to try out external containerizers. If you want to use Deimos together with Marathon, please use this patched Marathon. Official support for external containerizers will be available in the next Mesos release (0.19).

In Mesos, every successful resource offer is ultimately followed up with a TaskInfo that describes the work to be done. Within the TaskInfo is a CommandInfo and within the CommandInfo there is a ContainerInfo (following MESOS-816). The ContainerInfo structure allows specification of an image URL and container options. For example:

{
  container = ContainerInfo {
    image = "docker:///ubuntu"
    options = ["-c", "10240"]
  }
}

Deimos handles image URLs beginning with docker:/// by stripping the prefix and using the remainder as the image name. The container options are passed to docker run when the task is launched.

If no ContainerInfo is present in a task, Deimos will still containerize it, by using the --default_container_image passed to the slave, or taking a reasonable guess based on the host's distribution and release.

Some options for Docker, like -H, do not apply only to docker run. These options should be set in the Deimos configuration file.

Deimos recognizes Mesos resources that specify ports, CPUs and memory and translates them to appropriate Docker options.

Deimos logs to the console when run interactively and to syslog when run in the background. You can configure logging explicitly in the Deimos configuration file.

There is an example configuration file in example.cfg which documents all the configuration options. The two config sections that are likely to be most important in production are:

• [docker]: global Docker options (--host)
• [log]: logging settings

Configuration files are searched in this order:

./deimos.cfg
~/.deimos
/etc/deimos.cfg
/usr/etc/deimos.cfg
/usr/local/etc/deimos.cfg

Only one configuration file -- the first one found -- is loaded. To see what Deimos thinks its configuration is, run deimos config.

Deimos creates a state directory for each container, by default under /tmp/deimos, where it tracks the container's status, start time and PID. File locks are maintained for each container to coordinate invocations of Deimos that start, stop and probe the container.

To clean up state directories belonging to exited containers, invoke Deimos as follows:

deimos state --rm

This task can be run safely from Cron at a regular interval. In the future, Deimos will not require separate invocation of the state subcommand for regular operation.

Only the slave needs to be configured. Set these options:

--containerizer_path=/usr/local/bin/deimos --isolation=external

The packaged version of Mesos can also load these options from files:

echo /usr/local/bin/deimos    >    /etc/mesos-slave/containerizer_path
echo external                 >    /etc/mesos-slave/isolation