“On Facebook, people can keep up with their family and friends through reading status updates and viewing photos. In our backend, we store all the data that makes up the social graph of these connections. On mobile clients, we can’t download the entire graph, so we download a node and some of its connections as a local tree structure.
The image below illustrates how this works for a story with picture attachments. In this example, John creates the story, and then his friends like it and comment on it. On the left-hand side of the image is the social graph, to describe relations in the Facebook backend. When the Android app queries for the story, we get a tree structure starting with the story, including information about actor, feedback, and attachments (shown on the right-hand side of the image)…”