JPA: skipped updates

Question

Hello!
I have an issue and I don’t understand the reasons of it.
In my system I have a stateless session bean that every N seconds retrieves entity list from persistence context:
@PersistenceContext 
private EntityManager em; 
public List<Item> getItemsToProcess() { 
        String sql = "select t " + 
                "from Item t join ...."; 
        return em.createQuery(sql).getResultList(); 
}Then all items are updated to “Busy” state and sent to MDB queue for processing. After all processing is done MDB updates item’s state back to “Normal”.
businessBean.setInProcess(itemPK, true); // busy (called from session bean)
businessBean.setInProcess(itemPK, false); // normal (called from MDB)
public void setInProcess(ItemPK itemPK, Boolean value) {
        Item item = em.find(Item.class, itemPK);
        item.setInProcess(value); // field is mapped to DB column
        em.merge(item); 
        //em.flush(); // does not helps
}So far everything is working, but when I increase load to system (decrease N period) then some items (randomly) are not updated back to “Normal” state. All beans are deployed as single EJB module on the same server, using same persistence context (as I think :). No special transactions are used (only default CMP).
I have done trace over called methods and found out, that there is (!) a call of function to set “Normal” state, but somehow it does not reach database (SQL logs does not show any update). I mean I set entity field to correct value, and merge it to persistence context, but context does not forwards it to database.
Is it possible that while MDB tries to update item’s state first select … statement is run and it somehow locks that record? That SQL returns only items with “Normal” state.
Thanks!

Answer 1

I have found temporal solution for this issue. I rewrote function for setting items state like this:
public void setInProcess(ItemPK itemPK, Boolean value) {
        String sql = "update Item t " +
                "set inProcess = …”;
        Query q = em.createQuery(sql);
        q.setParameter(…);
        q.executeUpdate();
}{code}
Now there are no skipped items, but sometimes I receive “java.sql.SQLException: Some non-transactional changed tables couldn't be rolled back”. This exception is specific for MySQL database (MyISAM), but I use InnoDB storage engine for tables, that supports transactions.
But anyway I do not understand why em.merge() skips updates, but JPQL queries not.
Any ideas?