The first thing you need to do is scan your log output for the very first error message and stack dump, which will be the root cause of the failure. Subsequent error message are likely to be related to this initial fault.
Once you've found the first error then you can start digging. This is done by first splitting the text of the message into two parts: what TDI is reporting, and what the underlying library or system is complaining about. For example, TDI could be giving you a standard message like:
Here the entire prose of the error message comes from TDI. In this case there is no data source involved (yet) since the Connector itself can't be initialized without the JDBC Driver parameter being set correctly. These are also the types of errors that Google may be less than helpful for, and your best bet may be the discussion forum, the community website or the online docs.
No default JDBC driver. The 'jdbcDriver' parameter must be set to use the JDBC Connector.
To illustrate the type of message that a net search can help you decipher, let's look at a message I helped debug last week (formatted in bold and italics to illustrate my point):
10:12:03 [DB2_Update] CTGDIS810E handleException - cannot handle exception , initialize
Unable to obtain schema: com.ibm.db2.jcc.c.SqlException: DB2 SQL error: SQLCODE: -443, SQLSTATE: 38553, SQLERRMC: SYSIBM.SQLCOLUMNS; COLUMNS;SYSIBM:CLI:-805
In a case like this where the exception originated in a call to some driver or API, the first part of the message (italicized above) will help you find the point in your AssemblyLine where the failure occurred.
At the start of the error message is the name of an AssemblyLine component in brackets: [DB2_Update]. This tells you where the error occured in your AL. Immediately following the component name we can furthermore see TDI telling us that this unhandled exception occurred during initialization of the component. So far so good: we know where and when now. The next step is to discover why we are getting this exception.
That is where the text that I've formatted as bold comes in: this is the error that TDI received from the underlying RDBMS. Since it is a DB2 error (not specific to TDI) there is a much greater chance that someone else has fixed and documented this already.
So when I did a Google search with tidbits gleaned from this part of the message (e.g. "sqlcode -443 sqlstate 38553 cli -805") I uncovered plenty of relevant links, eventually leading me to a newgroup post that described a bind problem easily fixed with "bind db2schema.bnd". The search was over, the problem solved and I am ready for new adventures.