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Docker containers provide a consistent, compact and flexible means of packaging application builds. Delivering applications with Docker on Mesos promises a truly elastic, efficient and consistent platform for delivering a range of applications on premises or in the cloud.

Service Launch and Discovery

Let's launch a service in a Docker container and use Marathon's discovery features to find and connect to it from another Docker container. In this example, we'll launch Redis within a Docker container and then connect to it using the Redis CLI.

Launching Docker containers with Marathon is a matter of assigning the "executor" parameter to point to a Docker executor and making the container name the first word of the command.

:;  http POST http://localhost:8080/v1/apps/start \
              id=multidis instances=2 mem=512 cpus=1 \
              executor=/var/lib/mesos/executors/docker \
              cmd='johncosta/redis'

>>

HTTP/1.1 204 No Content
Content-Type: application/json
Server: Jetty(8.y.z-SNAPSHOT)

Once we have two Redis instances running, we can query Marathon to find them and connect to them.

:;  http GET http://localhost:8080/v1/endpoints

>>

HTTP/1.1 200 OK
Content-Type: text/plain
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked

multidis 10445 ip-10-184-7-73.ec2.internal:31001 ip-10-184-7-73.ec2.internal:31000

Using one of the ip:port pairs above, we can connect to one of our Redis instances, using the same Docker image, to get access to a working Redis CLI:

:;  sudo docker run -i -t johncosta/redis \
         redis-cli -h ip-10-184-7-73.ec2.internal -p 31001

redis ip-10-184-7-73.ec2.internal:31001> SET x 7
OK
redis ip-10-184-7-73.ec2.internal:31001> GET x
"7"

There's no need to install Redis or its supporting libraries on your Mesos hosts.

Reproducing This Example

Installation of the many components used here is straightforward. Mesos and the Python bindings for Mesos are available as packages:

Marathon, a Scala program, can be run from a standalone Jar. We've made an Upstart script for it, too.

With Mesos and Marathon, you have everything you need to start running reliable services across your cluster.

The Docker project provides Ubuntu 13.04 installation instructions.

To follow along with the examples above, install the Docker executor as /var/lib/mesos/executors/docker.

Docker Executor for Mesos

For a standalone installation, we've put together a script that installs all the needed components and configures them on Ubuntu 13.04. Just pipe it to a shell running under sudo and you're all set:

curl -fL https://raw.github.com/mesosphere/mesos-docker/master/bin/mesos-docker-setup | sudo bash