File Replication systems are really just one manifestation of replicated persistent data. We will talk about replicated files in the rest of this section, but the concepts are generally applicable to other objects, including parts of files. There are a number of techniques that are used to provide replication, and they each have their costs and benefits. Replication is used to provide a number of transparencies: Replication carries with it several overheads. These include: Replication techniques to provide consistency can be divided into two main classes: