This appendix demonstrates a hands-on approach to Reactive system design, illustrating the tenets of the Manifesto through concrete examples. We encounter these while solving issues that come up when building applications that are scalable to a global user base. Globally distributed interaction is also at the heart of the example problem we will tackle here: you will build an app with which users can share their location in real time and in which they can watch others move on a map. An extension to this core function is that users can exchange text messages with other users in their vicinity.
To be more precise, the location of each individual is used for two purposes: