I recently explained how changes in SQL Server 2005 may break older applications and databases, especially where deprecated functions are used. Now I'm going to discuss how to use stored procedures in your migrated databases to take advantage of newer functionality in SQL Server 2005 without breaking existing functionality.
Let's say you have a front-end application that's being written or managed by a different team and it isn't upgraded as often as the back-end database. You still want to make changes in your SQL Server application's stored procedures to take advantage of newer features. You also want to do this as gracefully as possible so the database and front-end application don't have a falling out.
For example, you may want to update a stored procedure to use some new functions available to T-SQL in SQL Server 2005, such as the TRY and CATCH functions, which make sophisticated error trapping a lot easier. You'd normally copy the existing stored procedure into a new, parallel stored procedure, update it to use the new functions and then test the behavior of the two side-by-side through either a modified version of the front-end application (if it's available) or Query Analyzer.
When you're confident the new stored procedure is behaving as it should and it is ready to be used in production, you can swap it in seamlessly either by renaming the two stored procedures or by copying in the code. I personally prefer doing a rename, since that allows you to have a backup copy of the old code -- backed up with your database like everything else in case you run into an unexpected wrinkle.
This is a fairly standard technique, but one of the more interesting wrinkles I've seen on it involves the passing of an optional parameter:
CREATE PROCEDURE my_procedure
{other parameters go here},
@optionalparameter Boolean=FALSE
AS
If @optionalparameter=TRUE
Begin
{NEW version of stored procedure with SQL Server 2005-specific commands goes here}
End
Else
Begin
{OLD version of stored procedure goes here}
End
There is another useful way to selectively test the code. Existing front-end calls to the stored procedure will not use the optional parameter and will execute old code. You can test the stored procedure in place using new front-end code and then gracefully upgrade existing references to the stored procedure as you go. Since the parameter is optional all the existing calls to the stored procedure (i.e., those without the parameter) will go through exactly as before.
If you aren't allowed to add new stored procedures but are allowed to modify existing ones, this is a good way to work around such a limitation. When no legacy code remains, you can then phase out the IF in the stored procedure. Lastly, deprecate the optional parameter in both the stored procedure and your front-end code.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators -- and please share your thoughts as well!
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