As of November 2017 Google deprecated the Realtime API, and it is unavailable for new projects. It will stop working on December 11th 2018. It provided collaboration as a service for files in Google Drive via the use of operational transforms. The API was a JavaScript library hosted by Google that provided collaborative objects, events, and methods for creating collaborative applications.
The Realtime API lets you, the developer, design a shared data model that looks just like a local, in-memory data model. You write code to manipulate maps, lists, and your own custom JavaScript data model objects. When your code makes a change to the data model on behalf of some user, the data model automatically changes for all the users on the document.
These collaborative data model objects work just like local in-memory data model objects, with one exception. Unlike a local JavaScript data model, a collaborative Realtime API data model may change as a result of edits that were made by someone other than the current user. As a result, applications need to attach listeners to a collaborative document so that the UI may be updated to reflect remote changes.
The Realtime API is based on the same collaboration technology used by Google Docs. Whenever a collaborative data model is modified, the change is applied immediately to the local, in-memory copy of the document. The API then silently sends a representation of the change to the server (called a "mutation") so that the change can be recorded in the document and communicated to other collaborators. The mutations have been carefully designed so that conflicts between collaborators can always be automatically resolved, so users will never have to see a message about edit conflicts.
Realtime data models are "eventually consistent." That means that if all collaborators stop editing, eventually everyone will see the same data model. However, we make no guarantees about when all the collaborators will see the changes or what order the changes will be delivered in.
