Everybody knows the web,snd it is everywhere. All the platforms and programing languages have tools to make request to it.
RESTful architecture
Restful architecture is the simplest and more portable way to request information. Http is a standard and it provides as a good bearer for information.
Websockets
For asynchronous notifications we have choose Socketio instead plain websocket. Some platforms like Android doesn't like websocket over ssl with self-signed certificates. Socketio use long polling and if it's posible it upgrades the transport to websockets. You can find implementations for most popular languages and platforms.