Transactions and Code Testing

A little while ago I worked with a customer to migrate their DB from using MyISAM to InnoDB (something I definitely don’t mind doing!) I set up a smaller test instance with all tables using the InnoDB engine as part of the testing. I instructed them to thoroughly test their application against this test instance and let me know if they identified any issues.

They reported back that everything seemed fine, and we went off to do the actual migration. Everything went according to plan and things seemed well. After a while they started seeing some discrepancies in the stock portion of their application. The data didn’t add up with what they expected and stock levels seemed surprisingly high. A crontabbed program was responsible for periodically updating the stock count of products, so this was of course the first place I looked. I ran it manually and looked at its output; it was very verbose and reported some 2000 products had been updated. But looking at the actual DB, this was far from the case.

Still having the test environment available, I ran it a few times against that and could see the com_update and com_insert counters being incremented, so I knew the queries were making it there. But the data remained intact. At this point, I had a gut feeling what was going on.. so to confirm this, I enabled query logging to see what was actually going on. It didn’t take me long to spot the problem. On the second line of the log, I saw this:

       40 Query set autocommit=0

The program responsible for updating the stock levels was a python script using MySQLDB. I couldn’t see any traces of autocommit being set explicitly, so I went on assuming that it was off by default (which turned out to be correct). After adding cursor.commit()* after the relevant queries had been sent to the server, everything was back to normal as far as stock levels were concerned. Since the code itself was seeing its own transaction, calls such as cursor.rowcount which the testers had relied on were all correct.

But the lesson here; when testing your software from a database point of view, don’t blindly trust what your code tells you it’s done, make sure it’s actually done it by verifying the data! A lot of things can happen to data between your program and the platters. Its transaction can deadlock and be rolled back, it can be reading cached data, it can get lost in a crashing message queue, etc.

As a rule of thumb, I’m rather against setting a blanket autocommit=1 in code, I’ve seen that come back to haunt developers in the past. I’m a strong advocate for explicit transaction handling.

Aug 18th, 2011