Clustering Akka Applications with Docker — Version 3

Nov 27 2014

The SBT Native Packager plugin now offers first-class Docker support for building Scala based applications. My last post involved combining SBT Native Packager, SBT Docker, and a custom start script to launch our application. We can simplify the process in two ways:

1. Although the SBT Docker plugin allows for better customization of Dockerfiles it’s unnecessary for our use case. SBT Native Packager is enough.
2. A separate start script was required for IP address inspection so TCP traffic can be routed to the actor system. I recently contributed an update for better ENTRYPOINT support within SBT Native Packager which gives us options for launching our app in a container.

With this PR we can now add our IP address inspection snippet to our build removing the need for extraneous files. We could have added this snippet to bashScriptExtraDefines but that is a global change, requiring /sbin/ifconfig eth0 to be available wherever the application is run. This is definitely infrastructure bleed-out and must be avoided.

The new code, on GitHub, uses a shell with ENTRYPOINT exec mode to set our environment variable before launching the application:

dockerExposedPorts in Docker := Seq(1600)

dockerEntrypoint in Docker := Seq("sh", "-c", "CLUSTER_IP=`/sbin/ifconfig eth0 | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1 }'` bin/clustering $*")

The $* allows for command-line parameters to be honored when launching the container. Because the app leverages the Typesafe Config library we can also set via Java system properties:

docker run --rm -i -t --name seed mhamrah/clustering:0.3 -Dclustering.cluster.name=example-cluster

Launching the cluster is exactly as before:

docker run --rm -d --name seed mhamrah/clustering:0.3
docker run --rm -d --name member1 --link seed:seed mhamrah/clustering:0.3

For complex scripts it may be too messy to overload the ENTRYPOINT sequence. For those cases simply bake your own docker container as a base and use the ENTRYPOINT approach to call out to your script. SBT Native Packager will still upload all your dependencies and its bash script to /opt/docker/bin/<your app>. The Docker WORKDIR is set to /opt/docker so you can drop the /opt/docker as above.