jdbc: oracle database change notification & duplicate events

Question

I need some Listener for any change(update, insert, delete) of Oracle database table.

Problem: I get many detection by single update on my table.

I think its

Answer 1

You will get an event for each commit that modifies one of the tables you're using in your query (in your code sample only one table called "EXAMPLE"). Think of it as "Table Change Notification" TCN. In other words you may get a lot of false positives because you're only interested in one row but you'll be notified if other rows are changed. It's then up to you to filter the events. This is why this feature should only be used for read mostly tables.

In 11gR2, Oracle improved this notification mechanism to allow a finer notification called "Query Change Notification". This time you will only be notified by changes that affect your query results. There is an option that needs to be turned on to enable QCN instead of TCN. Note that the server may not always be able to enable QCN. If the query is too complex it will fall back to TCN.

Answer 2

The point is that according to https://docs.oracle.com/cd/E18283_01/appdev.112/e13995/oracle/jdbc/OracleConnection.html you are responsible for releasing your DatabaseChangeNotification by unregister them at your Connection. Otherwise as described "The registration will continue to live after this connection is closed. You need to explicitly unregister it to destroy it in the server and release the resources in the driver."

Meaning if you test your sample code and you kill it, your registrations remains living on the server and you receive extra notifications after registering next DatabaseChangeNotification. Unfortunately I haven't found out yet hot to reconnect with living registration - I would need regId for that, but I have no clue yet how to obtain it from the OracleConnection.

Answer 3

Correcting typo error:

Please modify the event handler as follows:

public void onDatabaseChangeNotification(DatabaseChangeEvent dce) {
   if (dce.getRegId() == dcr.getRegId()) 
                    System.out.println("Changed row id : "+dce.getTableChangeDescription()[0].getRowChangeDescription()[0].getRowid().stringValue());
                }

Answer 4

From what I've read so far, I'm a bit confused as well.

Have you tried printing out the transactionId to see if it is, in fact, a unique event being generated?

System.out.println("DCE : regId="+dce.getRegristrationId()+"; transactionId="+dce.getTransactionId());

The listeners that I've been working with for JMS require an acknowledgment, but I don't see anything of that sort in the Oracle docs (http://docs.oracle.com/cd/B28359_01/java.111/b31224/dbmgmnt.htm).

I also found that it might disturb the oracle cache if the event treatment is unthreaded, but it looks you're already doing that too...

Good luck!

Answer 5

As already said you are responsible for releasing your DatabaseChangeNotification by unregistering them at your connection. In my case for some reason I am doing this when application server starts on tomcat. First I am releasing old registrations and then create new. I think I can reuse the old registration but anyway i am not.

I am doing something like this:

This query returns existing registrations. You must login as the same user that registered the notification.

SELECT REGID, CALLBACK FROM USER_CHANGE_NOTIFICATION_REGS

then code like this pseudo code unregisters the notification.

connection.unregisterDatabaseChangeNotification(REGID, CALLBACK) this is called for every row returned by the query.

Something else I found is that when updating a row using PL/SQL Developer using the included editor I receive multiple notifications for the same update. When using sql to update the row I receive one notification as expected. I don't know why is this happening but it is happening.

Answer 6

Please modify the event handler as follows:

public void onDatabaseChangeNotification(DatabaseChangeEvent dce) {
    if (e.getRegId() == dcr.getRegId()) 
        System.out.println("Changed row id : "+dce.getTableChangeDescription()[0].getRowChangeDescription()[0].getRowid().stringValue());
}