Re: [eclipselink-users] The ScrollableCursor's issues

[dmitryerkin <dmitryerkin@xxxxxxxxxxxxx>]

Thanks a lot for your answer, James Sutherland.
It is very useful for me.

Regards
Dmitry


James Sutherland wrote:
> 
> You are executing the query in the UnitOfWork so all of the returned
> objects will be managed (and not allowed to garbage collect).  If you
> execute it in the Session (uow.getParent()), then the objects will not be
> managed and free to gc, (but must be used as read-only).  You could also
> call clear() in between each page, or set the query to be read-only
> (setIsReadOnly(true) same as exec in session).
> 
> The rollback seems like it is cause because you have a transaction timeout
> set somewhere in WAS or your DB.  You will need to find and increase this
> timeout.
> 
> You could also potentially use JPA firstResult/maxResult to page the
> results.
> 
> 
> dmitryerkin wrote:
>> 
>> Hi
>> I am trying to use the ScrollableCursor for retrieving about 10.000
>> records.
>> I have to get them divided into small portions which include 100 records
>> each.
>> All works well except two issues.
>> 
>> Firstly I see that the size of memory which is used by JVM is being
>> increased while the sequence of invocations of the cursor's next(100)
>> method is happening although I want memory which was allocated for
>> previous portions to be released.
>> The total amount of used memory is about 1Gb for 10.000 records.
>> 
>> Secondly the transaction which is started to scroll through the cursor
>> switches to rollback state after 5 minutes although a lot of invocations
>> of the cursor's next(100) method are happening.
>> 
>> My questions are:
>> 1. How can I decrease the size of memory which is used by Eclipselink to
>> walk through the cursor?
>> 2. Can I set the period of transaction's timeout?
>> 
>> 
>> Here is my code:
>> 
>> 
>> 
>> The servlet:
>> 
>> @EJB ejb3.OebsInterface oebs;
>> 
>> ...
>> 
>> oebs.findOebsByTime(time);
>> 
>> int from = 10000;
>> int quantity = 100;
>> Vector<RootType> rootList = null;
>> while((rootList = oebs.getOebsByTime(from, quantity)) != null){
>> 	
>> 	int i = 0;
>> 	HeaderBaseType header;
>> 	
>> 	while(i < rootList.size()){
>> 		header = rootList.elementAt(i).getHeader();
>>                       ...
>> 		i++;
>> 	}
>> 		
>> 	from = from - quantity;
>> }
>> 			
>> oebs.releaseOebsByTime();
>> 
>> 
>> 
>> The session bean:
>> 
>> @Stateful
>> @TransactionManagement(TransactionManagementType.BEAN)
>> public class OebsInterfaceImpl implements OebsInterface {
>> 	
>> 	@PersistenceContext(name="oebsEJB")
>> 	EntityManager em;
>> 
>> 	@Resource javax.transaction.UserTransaction ut;
>> ...
>> 
>> 	protected JpaEntityManager jpaEM = null;
>> 	protected UnitOfWork uow = null;
>> 	protected ScrollableCursor cursorByTime = null;
>> 	
>> 
>> 	public void findOebsByTime(XMLGregorianCalendar time) throws
>> NotSupportedException, SystemException{
>> 		
>> 		ExpressionBuilder oebs = new ExpressionBuilder();
>> 	           Vector arguments = new Vector();
>> 	           arguments.addElement("/o:root/o:header[o:timeOfCreation=\"" +
>> time.toString() + "000+00:00\"]");
>> 	          
>> arguments.addElement("xmlns:o=\"http://www.rosbank.ru/oebs\"";);
>> 		Expression where =
>> oebs.getField("OBJECT_VALUE").getFunction(Oracle10Platform_Customizer.ExistsNodeWithNamespaces,
>> arguments).greaterThan(0);
>> 		
>> 		ReadAllQuery queryByTime = new ReadAllQuery(OebsView.class, where);
>> 		queryByTime.useScrollableCursor();
>> 		
>> 		ut.begin();
>> 		
>> 		jpaEM = org.eclipse.persistence.jpa.JpaHelper.getEntityManager(em);
>> 		uow = jpaEM.getActiveSession().getActiveUnitOfWork();
>> 		if(uow == null){
>>     		       uow = jpaEM.getActiveSession().acquireUnitOfWork();
>> 		}
>> 		cursorByTime = (ScrollableCursor) uow.executeQuery(queryByTime);		
>> 	}
>> 
>> 
>> 	public Vector<RootType> getOebsByTime(int from, int quantity) throws
>> JAXBException{
>> 		
>> 		Vector<RootType> rootList = null;
>> 
>> 		if(from <= cursorByTime.size()){
>> 
>> 			rootList = new Vector<RootType>();
>> 
>> 			cursorByTime.absolute(from);
>> 			
>> 			int q;
>> 			if((cursorByTime.size() - from) + 1 >= quantity){
>> 				q = quantity;
>> 			}
>> 			else{
>> 				q = cursorByTime.size() - from + 1;
>> 			}
>> 			
>> 			Vector<ejb3.OebsView> oebsList = new Vector<ejb3.OebsView>();
>> 			oebsList = cursorByTime.next(q);
>> 			System.out.println("oebsList.size is " + oebsList.size());    		
>> 			
>> 			JAXBContext jaxbContext =
>> JAXBContext.newInstance("ru.rosbank.oebs:ru.rosbank.oebs.header:ru.rosbank.oebs.content.assignment:ru.rosbank.oebs.content.organization:ru.rosbank.oebs.content.qualifications:com.ibm.xmlns.prod.websphere.j2ca.jdbc.appsxx_assignment_v:com.ibm.xmlns.prod.websphere.j2ca.jdbc.appsxx_organization_v:com.ibm.xmlns.prod.websphere.j2ca.jdbc.appsxx_qualifications_v");
>> 			Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
>> 			JAXBElement<RootType> jaxbElement;
>> 			RootType root;
>> 	    	
>> 			int i = 0;
>> 			while(i < oebsList.size()){
>> 				jaxbElement = (JAXBElement<RootType>) unmarshaller.unmarshal(new
>> InputSource(new StringReader(oebsList.elementAt(i).getObjectValue())));
>> 				root = (RootType) jaxbElement.getValue();
>> 				rootList.add(root);
>> 				i++;
>> 			}
>> 		}
>> 		
>> 		return rootList;
>> 	}
>> 
>> 	public void releaseOebsByTime() throws SecurityException,
>> IllegalStateException, RollbackException, HeuristicMixedException,
>> HeuristicRollbackException, SystemException{
>> 		uow.release();
>> 		
>> 		ut.commit();
>> 	} 
>> 
>> This code workes in the Websphere container.
>> 
>> Regards
>> Dmitry
>> 
> 
> 

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