1. A server system, comprising: a computer including one or more microprocessors; a composite persistence unit, executing on the computer, that includes a plurality of member persistence units, wherein each of the plurality of member persistence units includes a different set of objects and is associated with a different data source, wherein states of the different set of objects are persisted to the different data source, and wherein the composite persistence unit provides a persistence context which configures queries and transactions to be performed across a combined set of objects from the plurality of member persistence units; a configuration file for the composite persistence unit, wherein the configuration file defines properties of each of the plurality of member persistence units, including a property for determining whether a persistence unit is a member persistence unit of the composite persistence unit, and specifies a dependency or reference relationship between one or more objects in a first member persistence unit of the composite persistence unit and one or more objects in a second member persistence unit of the composite persistence unit, wherein the one or more objects in the first member persistence unit include one or more references to, or depend on data received from, the one or more objects in the second member persistence units; and an entity manager created using the composites persistence unit, wherein the entity manager serves as an interface to the composite persistence unit, operates to manage the member persistence units therein, and maps a request received by the entity manager to an appropriate member persistence unit.