Prevayler is a free, mainly written in Java persistence layer for Plain Old Java Objects ( POJOs ). Prevayler is considered very fast, fault-tolerant and provides load balancing.

From other persistence layers such as Hibernate or TopLink to Prevayler differs in that the objects are not stored in a relational database, but are held in memory. It is only at certain intervals to a dump, a nonvolatile medium such as a hard disk.

The problem lies in the permanent keeping of all objects in memory. For some applications, this is a knock-out criterion ( because: the amount of data may can cope with rapidly larger than the main memory). Prevayler does not support explicit resetting of the stored data.