Scalability:

In the context of this paper, scalability refers to the ability to grow services in a manner that is transparent to end users. Typically this involves growing services beyond a single chassis. There are a variety of methods for doing this and while it is very difficult to construct a generic, protocol-independent method for achieving this either DNS based or layer 4 switching based technologies often form the core of a solution.

The biggest problem with scaling across multiple machines is data replication. It is important that the data source is as reliable as possible. However, asynchronously replicating data between multiple machines is difficult at best.