AW: [OJB-developers] Re: Named Root Objects
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AW: [OJB-developers] Re: Named Root Objects
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From: Mahler T. <tho...@it...> - 2002-01-15 09:29:38
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Hi Michael,
There will be a lot of people using OJB who do not use named roots.
Just imagine their faces when the GC starts and deletes all their objects
because they where unreferenced >:-() !!!
Thus Having the GC as a separate tool is a good idea, not to say a MUST!
Implementing a GC that will reclaim everything that can not be reached from
a named root will not be difficult.
The algorithm you describe is a simple "mark and sweep" algorithm. Would be
fun to implement such a thing!
cheers,
Thomas
>
> Hi Thomas!
>
> I've been thinking about the GC process over the last few days.
>
> > Currently there is no GC implemented.
> > IMHO it will be VERY difficult to have a real GC as it
> would be possible
> > with an OODBMS.
> > It will be very difficult to decide if objects can safely
> be deleted.
> >
> > BUT: we have been thinking of changing the semantics of
> DList.remove()
> > an similar methods as name roots unbind so that removed objects are
> > deleted from the DB.
>
> A first step towards true persistence by reachability could
> be to offer
> offline GC. This way you don't have the to deal with objects which not
> reachable from any named root object any more but are still
> referenced under
> another transaction that has not yet committed. The algorithm
> could look
> like this:
>
> 1. Make a list of all objects stored in the database
> 2. from every named root object traverse the object graph and mark any
> object you come across as being persistent
> 3. delete all the objects from the database which are
> currently not marked
> as persistent.
>
> The algorithm can be implemented using recursion. In order to avoid
> indefinite loops caused by cyclic references, the algorithm
> has to check
> whether an object under investigation is already marked.
>
> Another advantage is that people who do not want seamless
> persistence the
> "Java Way" but instead want to have a more basic level
> control over their
> data still have the option not to execute the garbage collector.
>
> What do you think about it?
>
> Greetings Michael
> ----- Original Message -----
> From: "Thomas Mahler" <tho...@ho...>
> To: "Michael Nowotny" <mic...@gm...>; "ojb"
> <obj...@li...>
> Sent: Wednesday, January 09, 2002 6:51 PM
> Subject: Re: Named Root Objects
>
>
> > Hi again,
> >
> >
> > Michael Nowotny wrote:
> >
> > > Hi Thomas!
> > >
> > > Thanks for the illustration. Binding collections works
> fine for me now!
> > > The reason why my earlier attempt failed was that I was
> still using
> > > InstantDB.It seems that some transactions got only
> halfway commited due
> to
> > > program termination following db.close(). Now I'm using
> Hypersonic and
> > > everything works.
> > >
> >
> > Fine !
> >
> >
> > > During the course of setting up a test procedure I came
> across some
> strange
> > > behaviour. The following code snipped throws an
> ObjectNameNotBoundException.
> > >
> > > // fails:
> > > tx = odmg.newTransaction();
> > > tx.begin();
> > > db.bind(odmg.newDList(), "MyList");
> > > db.unbind("MyList");
> > > db.bind(odmg.newDList(), "MyList");
> > > // DList list = (DList)db.lookup("MyList");
> > > tx.commit();
> > >
> > > tx = odmg.newTransaction();
> > > tx.begin();
> > > DList newList = (DList)db.lookup("MyList");
> > > tx.commit();
> > >
> > > Binding the same name twice within the same transaction
> seems to cause
> an
> > > error.
> >
> >
> > Oops, This is a bug. I'll add it to my todo list!
> >
> >
> > This is not very important though because usually one wouldn't want
> > > to bind an object to a some name and thereafter unbinding
> that name
> withing
> > > the same transaction.
> > >
> > > Another question arose when I had a look at the contents
> of my database.
> If
> > > I add content to my list which is a named root object
> this content gets
> > > persistent as well. That's part of persistence by
> reachability. But when
> I
> > > unbind the root object and the content is therefore no
> longer reachable,
> it
> > > doesn't get removed from the database but is still
> present. Is there any
> way
> > > of performing some kind of garbage collection?
> > >
> >
> >
> >
> > Currently there is no GC implemented.
> > IMHO it will be VERY difficult to have a real GC as it
> would be possible
> > with an OODBMS.
> > It will be very difficult to decide if objects can safely
> be deleted.
> >
> > BUT: we have been thinking of changing the semantics of
> DList.remove()
> > an similar methods as name roots unbind so that removed objects are
> > deleted from the DB.
> >
> > Any ideas?
> >
> > -- Thomas
> >
> >
> >
> >
> > > Keep up the good work!
> > >
> > > Thanks, Michael
> > >
> > > ----- Original Message -----
> > > From: "Thomas Mahler" <tho...@ho...>
> > > To: "Michael Nowotny" <mic...@gm...>; "ojb"
> > > <obj...@li...>
> > > Sent: Tuesday, January 08, 2002 8:20 PM
> > > Subject: Re: Named Root Objects
> > >
> > >
> > >
> > >>Hi Michael,
> > >>
> > >>please see the attached Java class.
> > >>
> > >>You can replace the original version of
> test.ojb.odmg.OdmgExamples with
> > >>this new version to have it executed with the OJB JUnit tests.
> > >>
> > >>There is a new testMethod testNrmAndDlists that tests
> your scenario.
> > >>Works great!
> > >>
> > >>HTH,
> > >>
> > >>Thomas
> > >>
> > >>Michael Nowotny wrote:
> > >>
> > >>
> > >>>Hi Thomas!
> > >>>
> > >>>Just found your OJB project on sourceforge. I think it's
> great to have
> > >>>
> > > an
> > >
> > >>>open source ODMG implementation around.
> > >>>Now I've got a question concerning the ODMG implemetation.
> > >>>Is it possible to use a DList for example as a named root object.
> > >>>
> > > Binding
> > >
> > >>>that object to a name works but looking it up again causes an
> > >>>IllegalArgumentException.
> > >>>
> > >>>Have you ever tried to bind a Collection. If so, I would
> be grateful if
> > >>>
> > > you
> > >
> > >>>could drop me some lines of source code.
> > >>>
> > >>>Thanks Michael
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>
> > >>
> > >>
> > >
> > >
> >
> >
> --------------------------------------------------------------
> ------------
> --
> > > ----
> > >
> > >
> > >
> > >>package test.ojb.odmg;
> > >>
> > >>
> > >>import junit.framework.TestCase;
> > >>import ojb.broker.Identity;
> > >>import ojb.broker.PersistenceBroker;
> > >>import ojb.broker.PersistenceBrokerFactory;
> > >>import ojb.broker.cache.ObjectCacheFactory;
> > >>import ojb.odmg.OJB;
> > >>import ojb.odmg.states.ModificationState;
> > >>import ojb.odmg.states.StateNewClean;
> > >>import ojb.odmg.states.StateNewDirty;
> > >>import org.odmg.*;
> > >>
> > >>import java.util.List;
> > >>
> > >>/** Demo Application that shows basic concepts for
> Applications using
> the
> > >>
> > > OJB ODMG
> > >
> > >> * implementation as an transactional object server.
> > >> */
> > >>public class OdmgExamples extends TestCase
> > >>{
> > >> public static void main(String[] args)
> > >> {
> > >> String[] arr = {CLASS.getName()};
> > >> junit.textui.TestRunner.main(arr);
> > >> }
> > >>
> > >> private static Class CLASS = OdmgExamples.class;
> > >> private String databaseName;
> > >>
> > >> /**
> > >> * Insert the method's description here.
> > >> * Creation date: (24.12.2000 00:33:40)
> > >> */
> > >> public OdmgExamples(String name)
> > >> {
> > >> super(name);
> > >> }
> > >>
> > >> /**
> > >> * Insert the method's description here.
> > >> * Creation date: (06.12.2000 21:58:53)
> > >> */
> > >> public void setUp()
> > >> {
> > >> databaseName =
> > >>
> > >
> PersistenceBrokerFactory.getConfiguration().getRepositoryFilename();
> > >
> > >>
> > >> }
> > >>
> > >> /**
> > >> * Insert the method's description here.
> > >> * Creation date: (06.12.2000 21:59:14)
> > >> */
> > >> public void tearDown()
> > >> {
> > >> databaseName = null;
> > >> }
> > >>
> > >> /**TestThreadsNLocks state transition of modification states*/
> > >> public void testModificationStates()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> ModificationState oldState = StateNewClean.getInstance();
> > >> ModificationState newState = oldState.markDirty();
> > >> assertEquals(StateNewDirty.getInstance(), newState);
> > >>
> > >> oldState = newState;
> > >> newState = oldState.markDirty();
> > >> assertEquals(oldState, newState);
> > >>
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >>
> > >> }
> > >>
> > >> public void testOdmgSession()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> Transaction tx = odmg.newTransaction();
> > >>
> > >> //perform transaction
> > >> try
> > >> {
> > >> tx.begin();
> > >>
> > >> Article example = new Article();
> > >> example.setArticleId(777);
> > >> db.makePersistent(example);
> > >>
> > >>
> > >>
> > >> // modify Object
> > >> example.setStock(333);
> > >> example.addToStock(47);
> > >> example.addToStock(7);
> > >> example.addToStock(4);
> > >>
> > >>
> > >> //System.out.println("now commit all changes...");
> > >> tx.commit();
> > >> }
> > >> catch (Exception ex)
> > >> {
> > >> tx.abort();
> > >> }
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> }
> > >>
> > >> public void testOQLQuery()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> Transaction tx = odmg.newTransaction();
> > >>
> > >> //perform transaction
> > >> try
> > >> {
> > >> tx.begin();
> > >>
> > >>
> > >> OQLQuery query = odmg.newOQLQuery();
> > >> query.create("select anArticle from " +
> > >>
> > > Article.class.getName() + " where articleId = 7");
> > >
> > >> List results = (List) query.execute();
> > >>
> > >> Article a = (Article) results.get(0);
> > >>
> > >>
> > >> // cross check with PersistenceBroker lookup
> > >> // 1. get OID
> > >> Article example = new Article();
> > >> example.setArticleId(7);
> > >> Identity oid = new Identity(example);
> > >> // 2. lookup object by OID
> > >> PersistenceBroker broker =
> OJB.getInstance().getBroker();
> > >> broker.clearCache();
> > >> Article b = (Article) broker.getObjectByIdentity(oid);
> > >>
> > >> assertEquals("should be same object", a, b);
> > >>
> > >> //System.out.println("now commit all changes...");
> > >> tx.commit();
> > >> }
> > >> catch (Exception ex)
> > >> {
> > >> tx.abort();
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> }
> > >>
> > >> public void testNrmAndDlists()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> Transaction tx = odmg.newTransaction();
> > >>
> > >> //perform transaction
> > >> try
> > >> {
> > >> tx.begin();
> > >>
> > >>
> > >> OQLQuery query = odmg.newOQLQuery();
> > >> query.create("select x from " +
> Article.class.getName() + "
> > >>
> > > where productGroupId = 7");
> > >
> > >> DList results = (DList) query.execute();
> > >>
> > >>
> > >> int originalSize = results.size();
> > >>
> > >> OJB.getLogger().info(results);
> > >>
> > >> String name = "gimme fruits !";
> > >>
> > >> db.bind(results, name);
> > >> tx.commit();
> > >>
> > >> // clear the cache to make sure things are
> loaded from RDBMS
> !
> > >> OJB.getInstance().getBroker().clearCache();
> > >>
> > >> tx = odmg.newTransaction();
> > >> tx.begin();
> > >>
> > >> // look it up again
> > >> DList newResults = (DList) db.lookup(name);
> > >>
> > >> assertEquals(originalSize,newResults.size());
> > >> OJB.getLogger().info(results);
> > >>
> > >> tx.commit();
> > >>
> > >>
> > >> }
> > >> catch (Throwable t)
> > >> {
> > >> tx.abort();
> > >> fail("ODMGException: " + t.getMessage());
> > >> }
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> }
> > >>
> > >>
> > >>
> > >> public void testOQLQueryBind()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> Transaction tx = odmg.newTransaction();
> > >>
> > >> //perform transaction
> > >> try
> > >> {
> > >> tx.begin();
> > >>
> > >>
> > >> OQLQuery query = odmg.newOQLQuery();
> > >> query.create("select anArticle from " +
> > >>
> > > Article.class.getName() + " where articleId = $678");
> > >
> > >> query.bind(new Integer(7));
> > >>
> > >> List results = (List) query.execute();
> > >>
> > >> Article a = (Article) results.get(0);
> > >>
> > >> //crosscheck with PersistenceBroker lookup
> > >> // 1. get OID
> > >> Article example = new Article();
> > >> example.setArticleId(7);
> > >> Identity oid = new Identity(example);
> > >>
> > >> ObjectCacheFactory.getObjectCache().clear();
> > >> // 2. lookup object by OID
> > >> PersistenceBroker broker =
> OJB.getInstance().getBroker();
> > >> broker.clearCache();
> > >> Article b = (Article) broker.getObjectByIdentity(oid);
> > >>
> > >> assertEquals("should be same object", a, b);
> > >>
> > >> //System.out.println("now commit all changes...");
> > >> tx.commit();
> > >> }
> > >> catch (Exception ex)
> > >> {
> > >> tx.abort();
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> }
> > >>
> > >> public void tesOQLQueryOnCollections()
> > >> {
> > >> // get facade instance
> > >> Implementation odmg = OJB.getInstance();
> > >> Database db = odmg.newDatabase();
> > >> //open database
> > >> try
> > >> {
> > >> db.open(databaseName, Database.OPEN_READ_WRITE);
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> Transaction tx = odmg.newTransaction();
> > >>
> > >> //perform transaction
> > >> try
> > >> {
> > >> tx.begin();
> > >> OQLQuery query = odmg.newOQLQuery();
> > >> query.create("select aLotOfArticles from " +
> > >>
> > > Article.class.getName() + " where productGroupId = 4");
> > >
> > >> DCollection results = (DCollection) query.execute();
> > >> results = results.query("price > 35");
> > >>
> > >> // now perform control query
> > >> query = odmg.newOQLQuery();
> > >> query.create("select aLotOfArticles from " +
> > >>
> > > Article.class.getName() + " where productGroupId = 4 and
> price > 35");
> > >
> > >> DCollection check = (DCollection) query.execute();
> > >>
> > >> assertEquals(results, check);
> > >>
> > >>
> > >> tx.commit();
> > >> }
> > >> catch (Exception ex)
> > >> {
> > >> tx.abort();
> > >> ex.printStackTrace();
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >>
> > >> // close database
> > >> try
> > >> {
> > >> db.close();
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> fail("ODMGException: " + ex.getMessage());
> > >> }
> > >> }
> > >>
> > >>
> > >> /**try to open non-existing db*/
> > >> public void tesWrongDbName()
> > >> {
> > >> // get facade instance
> > >> OJB objectserver = OJB.getInstance();
> > >> Database db = objectserver.newDatabase();
> > >>
> > >> //try open database with non existing repository file:
> > >> String wrongDatabaseName = "ThereIsNoSuchFile";
> > >> try
> > >> {
> > >> db.open(wrongDatabaseName, Database.OPEN_READ_WRITE);
> > >> fail("should not be able to open database " +
> > >>
> > > wrongDatabaseName);
> > >
> > >> }
> > >> catch (ODMGException ex)
> > >> {
> > >> return;
> > >> }
> > >> }
> > >>}
> > >>
> > >>
> > >
> > >
> > >
> > >
> >
> >
> >
>
>
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