Google App Engine PersistenceManager can process multiple objects ?

Question

I have some code like this :

        PersistenceManager pm=PMF.get().getPersistenceManager();
        String query="select from "+PayPal_Message.class.getName()+" where processed == false order by time desc";
        List<PayPal_Message> messages=(List<PayPal_Message>)pm.newQuery(query).execute();
        if (messages.isEmpty())
        {
        }
        else
        {
          for (PayPal_Message g : messages)
          {
            Contact_Info_Entry A_Contact_Entry=Process_PayPal_Message_To_Get_A_License(g.getContent().getValue());
            pm=PMF.get().getPersistenceManager();
            try
            {
              pm.makePersistent(A_Contact_Entry);
              g.setProcessed(true);
              pm.makePersistent(g);
            }
            catch (Exception e)
            {
              Send_Email(Email_From,"[email protected]","Servlet Error Message [ "+time+" ]",new Text(e.toString()));
            }
//            finally { pm.close(); }

          }
        }
        pm.close();

I wonder if it's ok to use the pm above to process multiple objects before closing it. Or do I have to get and close pm for processing each object ?

Answer 1

You should try to use the same PersistenceManager for as much work as possible.

Getting a new one all the time probably results in unnecessary overhead. If you want to use transactions, I believe you have to use the same manager for all activities in there anyway.

Answer 2

Try this ...

PersistenceManager pm = null;
try
{
            pm=PMF.get().getPersistenceManager();
            String query="select from "+PayPal_Message.class.getName()+" where processed == false order by time desc";
            List<PayPal_Message> messages=(List<PayPal_Message>)pm.newQuery(query).execute();
            if (!messages.isEmpty())
            {
              for (PayPal_Message g : messages)
              {
                Contact_Info_Entry A_Contact_Entry=Process_PayPal_Message_To_Get_A_License(g.getContent().getValue());
                try
                {
                  pm.makePersistent(A_Contact_Entry);
                  g.setProcessed(true);
                  pm.makePersistent(g);
                }
                catch (Exception e)
                {
                  Send_Email(Email_From,"[email protected]","Servlet Error Message [ "+time+" ]",new Text(e.toString()));
                }
              }
            }
}
finally
{
 if(pm != null) pm.close();
}

Answer 3

Hi Frank,

• PersistentManagerFactory will give you a new PersistenceManager every time you ask for it.
• If the app doesn't deal with too many requests, you're okay.
• If the app deals with lots of requests, you might either get:
  1. Some sort of Exception (PMF refuses to give more PM)
  2. Billed more by Google (inefficient)

There are 2 ways to use PM to perform operations to multiple objects:

• Batch processing (Create, Update, Delete multiple objects)
• Transaction (Execute one or more business logics/rules and persist)

Batch processing is limited to objects with the same type while transaction is limited to entities of the same group.

Some advises:

• Always close your PM
• Alternatively you can use the detach mechanism if you want to use the objects after you close the PM (i.e. you want JSP to render a list of objects, but your servlet already close your PM)

I hope this help.