Tuesday, September 15, 2009

PL/SQL: Exceptions

I'm not really sure what I learned today, yet. Here's what I went through though.

I'm rebuilding/redesigning/refactoring a payment processing platform. It's complete with WHEN others...there is logging after the WHEN others, but no RAISE.

I was taught to use exceptions, which to me, meant using
raise_application_error( -20001, 'something went wrong' )
which meant that my calling PL/SQL had to use the PRAGMA EXCEPTION_INIT declaration. Not a big deal when it's 1 or 2 layers deep, but that's part of today's lesson (for me).

Exceptions were used in the code, they were slightly different though, just the
some_exception EXCEPTION;
variety. It's way better than nothing and I believe they were headed in the right direction.

As I peel away the layers though, far too many errors are being caught with OTHERS. Bad. Bad. Bad.

Payment processing, being at the center of most everything, should, ney, must, blow up loudly if something unknown goes wrong. Before that ever goes live you should know about the vast majority of exceptions. Vast Majority to me means 99.9%.

By blowing up loudly, you don't have to rely on looking through error logs and you are far less likely to encounter strange behavior. If one pops up that you didn't account for, it's a quick code change to add that handling.

Of course much of this is predicated on having unit tests or other testing means available. Once of the first things I did was build about 80 test cases with SQLUnit. So I am fairly confident when I make changes that I haven't affected (much) the underlying code.

Finally, on to the exceptions.

There were 4 or 5 generic exceptions (other than OTHERS) defined. I wanted more though. So I began adding them in. Currently the code travels through about 5 levels of the candy cane forest, I mean, PL/SQL. In the lower most level, I used
raise_application_error( -20001, 'invalid card number (gateway)' );
Reran the tests and nothing showed up. I added an internal function to capture the error stack.
l_error_stack VARCHAR2(32767);
l_error_stack := dbms_utility.format_call_stack;
l_error_stack := l_error_stack || dbms_utility.format_error_backtrace;
l_error_stack := l_error_stack || dbms_utility.format_error_stack;

RETURN l_error_stack;
END error_stack;
so I wouldn't have to rewrite those 3 (long) lines over and over. I realize that you get an extra line in there, but I'll know to ignore it.

Rerun the tests and I can see the call stack with a reference to ORA-20001. I'm getting somewhere. That's when I realized that even if you throw an exception in that manner, if you have an exception block in the same block of code and a WHEN others, WHEN others will catch it. For some reason, I always thought it bypassed that current block of code, but then again, I've rarely used WHEN others.

One by one I began to remove the WHEN others from the calling layers. I created global exceptions:
invalid_card EXCEPTION;
PRAGMA EXCEPTION_INIT( invalid_card, -20001 );
, removed WHEN others and created a new exception block in the top-most procedure. Perfect!

I rerun the tests and the error propogates all the way to the top (as it should, I just wasn't used to it). Tests begin to work again and I'm all set to go. Win!

For more on exception handling in PL/SQL, go here for 10gR2, here for 11gR1, and here for 11gR2.

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