Prevayler is an open-source (BSD) system-prevalence layer for Java: it transparently persists plain old Java objects. It is an in-RAM database backed by snapshots of the system via object serialization, which are loaded after a system crash to restore state. Changes to data happen via transaction operations on objects made from serializable classes. Prevayler's development was started by Klaus Wuestefeld.^[1]

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Read operations are three to four orders of magnitude faster with Prevayler when compared to traditional database systems since all objects are always in RAM and in-process.

Prevayler requires enough RAM to keep the entire system state.