One of the challenges in implementing a distributed system which supports a social network is how to efficiently compute a time ordered stream that is the combination of a time ordered substream for each person of interest. A related challenge is how to identify how many changes of interest have happened to a particular stream since it was last viewed.
For example, a user may have a set of friends that they follow and each of those friends may, over time, upload posts, comments, pictures and the like. A challenge exists because individuals whom a user follows may have their respective information or streams distributed across a number of servers.