It's written in Java using the Dropwizard framework. I've skimmed through the code and found references to Protobuf, GCM and Redis It's open source so you can

It's written in Java using the Dropwizard framework. I've skimmed through the code and found references to Protobuf, GCM and Redis It's open source so you can check the code here: https://github.com/signalapp/Signal-Server https://github.com/signalapp/Signal-Server