Distributed data is split according to a distribution algorithm. When you decide which distribution algorithm to use, bear in mind that the volume of data might change, and that your distribution strategy must be scalable. Do not use an algorithm that necessitates complete redistribution of data.
A numeric distribution algorithm based on uid, for example, can be scaled fairly easily. If you start with two data segments of uid=0-999 and uid=1000–1999, it is easy to add third segment of uid=2000–2999 at a later stage.