What is Basho Didgeridoo?

Basho Didgeridoo is a fork of Basho Banjo, and is an experiment in using Riak Pipe (instead of Riak Core directly) to create a distributed orchestra powered by midi files. It serves as a simple example of one way to create a distributed application with Riak Pipe.

How Does it Work?

To understand 'Didgeridoo, you first need to understand a little about Riak Pipe. Riak Pipe is an open-source Erlang library that allows you to create masterless distributed applications by building atop Riak Core to use the principles described in Amazon's Dynamo Paper. For more information about Riak Core, you can view this presentation. For more about Riak Pipe, the README is a pretty good starting point.

For our purposes, there are two main things to consider when creating an application on Riak Pipe:

1. Inputs are the items that you send to the system for processing. Didgeridoo inputs consist of a command to play a note of a certain frequency/volume/duration.

2. The Fitting is a pluggable module that conforms to an interface provided by Riak Pipe that accepts incoming inputs and acts on them. The Didgeridoo Fitting accepts the incoming command to play a note, generates a wave file (if it doesn't already exist) and then plays the file using aplay or afplay, depending on the platform. (Mac and Linux are supported... Windows is not.)

With these two things implemented, Riak Pipe does the rest of the work. It uses a consistent hash to route notes to specific VNodes (where the fittings are evaluated), and Riak Core takes care of assigning VNodes (a.k.a. Virtual Nodes) to physical nodes, and re-assigning the VNodes when you add/remove physical nodes to/from the cluster.

The last part of the equation is some code that uses an Erlang midi library developed by Jim Menard to parse midi files and extract notes. The code than iterates through the notes with the correct timing, generating play commands to send to Riak Core.

How Do I Run Basho Didgeridoo?

Get the Code and Compile

git clone https://github.com/beerriot/BashoDidgeridoo.git
cd Didgeridoo
make rel

Start Basho Didgeridoo

cd rel/basho_didgeridoo
bin/didgeridoo console

Play a Note (from the Erlang console)

play(78).

Play a Midi Tune

play("../../midi/mario.mid").
play("../../midi/zelda.mid").

Connect to Another Didgeridoo Node

If you want to run multiple nodes on a single computer, you can:

1. Copy the files in rel/basho_didgeridoo to another directory.
2. Edit etc/vm.args and change the node name to something like didgeridoo1@127.0.0.
3. Edit etc/app.config and change the handoff port to avoid a port conflict.
4. Start the new node.
5. Join the new node to the existing node from the Erlang console: join('didgeridoo1@hostname').
6. Then, play a midi tune, which should play across both nodes: play("../../midi/mario.mid").