em.persist(new A(..));
em.persist(new Z(..));
// insert A into db1; insert Z into db2:
// the two different data bases possibly from different vendors
em.flush();

/**
* Indicates if it's a broker persistence unit ("true").
* The property must be specified in persistence.xml of a broker persistence unit.
* The property passed to createEntityManagerFactory method or in system properties is ignored.
* Broker persistence unit would contain all persistence units found in jar files specified by <jar-file> elements in persistence.xml. 
*    <jar-file>contained1.jar</jar-file>
*    <jar-file>contained2.jar</jar-file>
*    <properties>
*        <property name="eclipselink.is-broker-persistence-unit" value="true"/>
*    </properties>
* Currently broker persistence unit cannot itself be contained in another broker persistence unit.
* Contained persistence units could be independent or contained-only.
* @see #IS_CONTAINED_PERSISTENCE_UNIT_ONLY
* @see #CONTAINED_PERSISTENCE_UNITS_PROPERTIES
*/
public static final String IS_BROKER_PERSISTENCE_UNIT = "eclipselink.is-broker-persistence-unit";

/**
* Indicates if the persistence unit must be contained in a broker persistence unit ("true"),
* can't be used as an independent persistence unit.
* That happens if contained persistence unit has dependencies on other contained persistence unit(s).
* The property may be specified in persistence.xml of contained persistence unit.
* The property passed to createEntityManagerFactory method or in system properties is ignored.
* Though EntityManagerFactory for contained-only persistence unit could be created,
* it can't be connected: an attempt to create entity manager would cause an exception.
* @see #IS_BROKER_PERSISTENCE_UNIT
* @see #CONTAINED_PERSISTENCE_UNITS_PROPERTIES
*/
public static final String IS_CONTAINED_PERSISTENCE_UNIT_ONLY = "eclipselink.is-contained-persistence-unit-only";

/**
* The property may be passed to createEntityManagerFactory method of a broker persistence unit
* to pass properties to contained persistence units.
* The value is a map: 
* the key is contained persistence unit's name,
* the value is a map of properties to be passed to this persistence unit. 
* "eclipselink.containes-persistence-units-properties" -> (
*   ("containedPu1" -> (   "javax.persistence.jdbc.user" -> "user1", 
*                          "javax.persistence.jdbc.password" -> "password1",
*                          "javax.persistence.jdbc.driver" -> "oracle.jdbc.OracleDriver",
*                          "javax.persistence.jdbc.url" -> "jdbc:oracle:thin:@oracle_db_url:1521:db",
*                      ) , 
*   ("containedPu2" -> (   "javax.persistence.jdbc.user" -> "user2",
*                          "javax.persistence.jdbc.password" -> "password2"
*                          "javax.persistence.jdbc.driver" -> "com.mysql.jdbc.Driver",
*                          "javax.persistence.jdbc.url" -> "jdbc:mysql://my_sql_db_url:3306/user2",
*                      )
* )
* @see #IS_BROKER_PERSISTENCE_UNIT
*/
public static final String CONTAINED_PERSISTENCE_UNITS_PROPERTIES = "eclipselink.contained-persistence-units-properties";

@PersistenceUnit(unitName="brokerPu")
EntityManager entityManager;
@PersistenceUnit(unitName="brokerPu")
private EntityManagerFactory entityManagerFactory;

EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("brokerPu", properties);