Test DB2 JDBC Data source failed in WebSphere 7 cluster

As far as how to create a new DB2 datasource in WebSphere 7 is pretty straightforward, it contains two steps:
1. Create a new JAAS J2C data entry to hold your username and password.
2. Create a new DataSource by using the newly created JAAS J2C data entry.
That's all you need to do. But where may be a catch when you try to do it in a cluster environment, you may run into the following exepction when you try to test your newly created Data source:
java.lang.Exception: java.sql.SQLException: null userid not supported

My solution is to restart the nodeagents on the custer nodes.

How to setup and use DB2Mon

I used to use TOAD to monitor the connection to Oracle, but when I moving to DB2 world, I am struggling to find out a similar tool to look at what kind connections/activities happen on the Database. One of my collegue suggested DB2Mon, I set off and decide to give it a try, here I record what I need to do:
1. Start IBM DB2 Configuration Assistant, right click, Add Database Using Wizard, Manually configure a connection to a database; setup your database connection; Test your connection by right click the newly created connection->TestConnection->CLI
2. Start your DB2Mon, you should see your newly created Database connection, select it and input your user/password.

Decoding IBM DB2 error message

Ever get an IBM DB2 error message like "SQL0407N Assignment of a NULL value to a NOT NULL column TBSPACEID=n1, TABLEID=n2, COLNO=n3 is not allowed. What actually does "TBSPACEID=n1, TABLEID=n2, COLNO=n3" mean? Don't ask me, you should ask IBM DB2, but how?

SELECT C.TABSCHEMA, C.TABNAME,
C.COLNAME
FROM SYSCAT.TABLES AS T,
SYSCAT.COLUMNS AS C
WHERE T.TBSPACEID = n1
AND T.TABLEID = n2
AND C.COLNO = n3
AND C.TABSCHEMA = T.TABSCHEMA
AND C.TABNAME = T.TABNAME