Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/api/DataEngine.java
/**
	 * Provides a hint to DtE that the consumer is done with the given 
	 * data source, and 
	 * that its resources can be safely released as appropriate.
	 * This tells DtE that there is to be no more query
	 * that uses such data source.
	 * @param	dataSourceName	The name of a data source. The named data source
	 *          must have been previously defined.
	 */
	abstract public void closeDataSource( String dataSourceName )
			throws BirtException;
	
	/**
	 * Prepare the cube query definition, return an IPreparedCubeQuery instance 
	 * @param query
	 * @param appContext
	 * @return
	 * @throws BirtException
	 */
	abstract public IPreparedCubeQuery prepare( ICubeQueryDefinition query,
			Map appContext ) throws BirtException;
	
	/**
	 * Shuts down this instance of data engine, and releases all associated resources.
	 * This method should be called when the caller is done with an instance of the data engine.
	 */
	abstract public void shutdown();
	
}