JDO: Is the PersistenceManager a singleton?

Question

Just the basics: I'm using DataNucleus backed with an embedded DB4O database.

If I do this simple test:

    PersistenceManager pm1 = persistenceManagerFactory.getPersistenceManager();
    PersistenceManager pm2 = persistenceManagerFactory.getPersistenceManager();

    pm1.makePersistent(t1);
    pm2.makePersistent(t2);

I get a file locked exception:

com.db4o.ext.DatabaseFileLockedException: C:\<path>\primary_datastore.data

Which tells me I don't know how the PersistenceManager is supposed to work. I thought I just called PersistenceManagerFactory whenever I needed a PersistenceManager to query or persist data and I would get something thread safe.

• Do I need to make PersistenceManager a singleton across my entire application?
• How do multiple threads, performing queries and updates work in JDO/DataNucleus?

Answer 1

• You don't need to create a singleton instance. Instead, DataNucleus recommends using a Persistence Manager Proxy.

So, following the guide, your code should work with this change:

PersistenceManager pm1 = persistenceManagerFactory.getPersistenceManager();
PersistenceManager pm2 = persistenceManagerFactory.getPersistenceManagerProxy();

pm1.makePersistent(t1);
pm2.makePersistent(t2);

The second instance is a Proxy referring the first PersistenceManager instantiated.

• To make your PersistenceManager thread safe, you have to set the datanucleus.ConnectionFactory (or its alias javax.jdo.option.Multithreaded) property at your PersistenceManagerFactory.

For instance, setting it programatically:

Properties properties = new Properties();
properties.setProperty("javax.jdo.PersistenceManagerFactoryClass",
                "org.datanucleus.jdo.JDOPersistenceManagerFactory");
//configure connection, etc...
properties.setProperty("javax.jdo.option.Multithreaded", "true");
PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(properties);

Answer 2

Do I need to make PersistenceManager a singleton across my entire application?

It depends on you application. If you developing a desktop-application you probably only need only one persistence manager. This persistence-manager represent the state of the database for you desktop-app. However for other scenarios this isn't the case. For example in web application you want to isolate the requests or sessions from each other. Therefore you use multiple PersistenceManager. For example a PersistenceManager per request. Each PersistenceManager hold the state and transaction for the current request.

So a PersistenceManager-instance represents a unit work / transaction.

Answer 3

How exactly do you expect db4o to support concurrent requests when you operate in "file" mode ? Would have thought server mode is a prerequisite

Answer 4

Incidentally, I ripped out DB4O and popped in NeoDatis (thank you DN for making that a 5 min task) and each of a half dozen test cases that had me baffled and throwing up my hands magically worked. Concurrent transactions behaved as I assumed they should, I could suddenly persist collections of serializable objects (a separate but equally frustrating issue), and at least 4 others that were derivatives of these.

Perhaps my fault for mis-configuring DB4O (though I had as vanilla an install as I could possibly contemplate), but NeoDatis got major bonus points in the "It just works" category. Both vanilla embedded installs, both create a file, both respond to JDO via DataNucleus.

I can't imagine switching back to DB4O after 3 days of hell that were erased with 5 minutes of NeoDatis bliss. :)