What it is
Underground is an attempt to build a HA messaging broker, inspired by LMAX' disruptor, Kafka, and other stuff. It is written in Scala.
The architecture is simple: messages flow in, are handed off to a disruptor, which is setup to replicate to the slave and persist to the disk first, then send it to the actuall message broker logic. The message broker spits out responses to a second disruptor, which is wired up to transmit them on the network on the master and do nothing on the slave. In this way, master and slave stay in sync.
Scaling is currently no concern. A single setup should be able to handle most messaging requirements, the target is to be able to saturate at least one 1GigE port.
How it is built
It's fully test-driven, using Scalatest with JMock. HA is built in from the start, not slapped on as an afterthought as many messaging products seem to have been done. HA also will be fully tested.
Not even alpha :)