In some visualization projects, it can be convenient to have the application state shared among users so that they can collaborate and explore the visualization as a group.

Adding this feature would usually imply the installation and configuration of a server and the use of WebSockets or a similar technology to manage the communication between the server and client-side code in the browser. Firebase is a service that provides real-time data storage and synchronization between application instances using the client-side code. If the data changes at one location, Firebase will notify the connected clients so that they can update the state of the application. It has libraries for several platforms, including OS X, iOS, Java, and JavaScript. In this section, we will use Firebase's JavaScript library to add real-time synchronization to the HDI Explorer application.

Adding synchronization to HDI Explorer doesn't make sense in most cases; it would be weird...