Database Error Detection and Recovery

CowboyRobot writes "ACM Queue has an interview by Steve Bourne with Bruce Lindsay, responsible for a lot of the SQL and RDBMS we use today, in which they discuss error detection and recovery. My favorite part other than the photos is the definition of Heisenbugs - those problems that disappear only when you explicitly look for them."

Comments

Rite of Passage

[beldraen]

Heisenburg bugs are a rite of passage in the computer world. They result from the production environment being different from the development environment. For instance, a debugger may initialize all memory in the process space to zero. An errant loop control now happens to be set properly, so no error occurs; however, in the production environment, whatever is left over in memory is used, which means the loop wanders off into nomansland and crashes. Always initialize your variables, period! Even in languages that automatically do it for you so that you are aware to what they are initialized.

Re:Rite of Passage

[leoval]

Nothing like putting extra printf's to get rid of an error. Thankfully the universe is a better place since the invention of Purify, (since most of the Heisenbugs are memory problems --evil pointers--). However the most challenging heisenbugs are the timing related ones, specially in networked applications. Those @#%'s are really hard to debug.I remember a project in particular where the heisenbug will only occur in Windows (not flame intended), but it would go away whenever we put a fprintf just before sending any packets trhu the socket. I think that the developer could not figure out the problem on time for the production deadline, so the fprintf to a bogus file is still there (about 6 years after the fact).

Re:Rite of Passage

[mikael]

Oh man, I remember those. The worst case was when I was trying to fix a SLIP (the model protocol) bug in an Ethernet probe. You could step through the code and everything worked correctly. The Token Ring network version worked correctly without failing. But run the system normally, and it failed to connect. The quickest solution was to compare the two code segments of each system; the only difference. was a 15 millisecond wait. Once replaced the system worked correctly (This was a requirement specified in the back page appendix of the modem chip specification).

Exception Handling

[Joe U]

Not quite on topic, but, I once tried writing code in SQL (in this case for ColdFusion) by using stored procedures and exception handling.

What a nightmare.

Many people code unique inserts like this.

Check for duplicate record.
if not found, then insert.
else, prompt user.

Using exception handling, you code like this.

insert.
if error thrown, prompt user.

One less query, lots less code.

One problem, the web application language treated all db errors as fatal. When asked, I was told this was by design.

Thinking about it, I feel that Macromedia didn't want me to code efficiently. You don't sell extra ColdFusion servers if you can offload all your data logic to the SQL server. (Where it belongs)

Re:Has language in CS matured?

[zuzulo]

Strangely, this is a phenomenon i have noticed with experts in many different fields who *really* understand what they are doing. They really do have an internal model of what is happening that essentially boils down to this sort of simplicity.

In fact, the correlation is so strong that I am suspicious of folks who *cannot* boil an arbitrarily complex interaction into an easily understood metaphor.

Jargon does in no way denote true understanding.