Happening Plugins use Javascript (or Coffeescript compiled to Javascript) for backend logic with simple APIs for Push notifications/HTTP calls/Timers/group management (see for instance https://platform.happening.im/docs/server-app.html#app-data, or the server.coffee code on the platform page). This server-side javascript runs in custom V8 instances and calls out to the core Happening backend (which is implemented in PHP/Go/C++).