Build collaborative apps with Google Drive Realtime API

Google Drive lets users create Docs, Sheets, and Slides, collaborate on
them in real time, and have their changes saved automatically. With the new
Google Drive Realtime API, you can now easily add some of the same
real-time collaboration that powers Google Drive to your own apps. This new
API handles network communication, storage, presence, conflict resolution,
and other collaborative details so you can focus on building great apps.

Developing for the Drive Realtime API is almost as simple as working with
local objects. The API provides collaborative versions of familiar data
objects such as maps, lists, strings, and JSON values and automatically
synchronizes and stores modifications to these objects. Your application
reads from and writes to these objects like any other local object. Change
event handlers can be added to collaborative objects so that your app can
react to changes from other collaborators.

http://googledevelopers.blogspot.ch/2013/03/build-collaborative-apps-with-google.html