Crossbar.io is is a router for the open WAMP protocol, and it is open source. Together with the open source WAMP client libraries it is a connection fabric for components in distributed applications.
Crossbar.io connects an application with a temperature sensor, backend, alarm lamp, mobile frontend and a fan
Components can e.g. be a Web page/Web app frontend, a classic application backend running on a server, an individual microservice, a mobile client, or an IoT device.
Whatever they are, all components connect to Crossbar.io and Crossbar.io enables them to communicate with each other as equals.