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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."
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.
Bel, the mostly sane.. "Of course I can't see anything! I'm standing on the shoulders of idiots." -- Me
ignore this response.
*everything* is Orwellian to cats.
"Heisenbug as originally defined--and I was there when it happened--are bugs in which clearly the behavior of the system is incorrect, and when you try to look to see why it's incorrect, the problem goes away.
This is a really cool article, but it was especially fun to see the heisenbug mention. Years ago, some fellow CS people and myself conjectured a similar phenomenon that seemed to manifest once in a while, in which a computer malfunction goes away after one "proves" that there's no cause for the error to exist.
Here's a list of heisenbug anecdotes, but note that some of these submissions aren't strictly heisenbugs.
BL: In the heart monitor case, you better keep the heart going, whereas in the Microsoft Word case, you can just give them a blue screen and everybody is used to that.
SB: But also in the heart monitor case, it?s hard to ask users if they want to keep the heart going because the answer is pretty obvious, whereas in the Word case, you can ask the user in some cases what to do about it.
New Microsoft Pace - Heart Monitor and Pacemaker
STOP: 0x0000000A (0x0000015a, 0x0000001c, 0x00000000, 0x80116bf4)
IRQL_NOT_LESS_OR_EQUAL - Beat.exe
Please hold your breath while a dump file is created...
That picture is really something. I didn't know Gandalf wrote bsh.
A good design principle is: either do what you're told to do or tell us you didn't do it and why, but don't do something completely different.
Exactly. Compare and contrast with MySQL's behaviour.
That's why there are loads of people who point out that you can't trust MySQL for important data, or that it isn't a "real" database. A real database tells you when it fails, which is something that is necessary for trusting it with data integrity.
The key point here is that if you go to sea with only one clock, you can't tell whether it's telling you the right time.
Ahh... but a man with one clock always knows the time - but a man with two is never quite sure :).
The guy looks like he's covered in coke dust.
Web pages that disappear when you try to look at them....
One of the things that is addressed to some extent in the article is the need to make error messages meaningful! There is nothing more frustrating to me than to encounter an error message like "syntax error."
At a minimum, an error message should have a Unique ID of where in the code this message is coming from, what was expected, what was actually found, and the context where it was found.
EXAMPLE:
Which would you prefer:In my experience, meaningful error messages save more debugging time than it takes to put them in.
And /. needs better thread handling too. When there's to much posts, the thread is splited in 72 pages. Pages 1, 2, and 3 are the same post reappearing over and over, then page 4 skipped a couple messages. You have to manually change the startat= parameter in the URL to see thoses missing posts. This is nuts. I mean, how difficult is it to code message threading? It's not rocket science. This makes /. look like ass when they are complaining about Microsoft bugs.
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)
I bet he didn't look into Java. Java (at least) allows and enforces that. A method will only throw an exception if declares to do so. A caller is forced to provide appropriate handlers or to declare it throws the exceptions not handled at its level. If a method can throw A, B or C but gets D during its execution, it has to in some way map D to either A, B or C (or not throw an exception at all).
Of course, I am talking here about checked exceptions. Unchecked exceptions are supposed to represent *bugs*, and nobody should be trying to capture those.
The sad thing is that even seasoned Java programmers do not understand how to write code w.r.t. exception handling. And beginners are usually turned off by the verbosity required by exception handling, so it is usual to see code where people capture (because they are forced by the language) and ignore exceptions (because they are too lazy and/or stupid to understand the consequences).
I just found my new avatar picture. :)
ps: not a troll, this guy's a freakin genius. I hope I look like that in 20+ years.
while (!asleep()) sheep++
I couldn't help noticing Mr. Lindsay explanations of what a process would or could do. He kept describing it in the first person:
- "You asked me to do X, I didn't do it."
- "Aha, this seems like I should go further."
- "Oh, I see this as one of those really bad ones."
- "I'm going to initiate the massive dumping now."
Obviously he is an expert in his field but I'm not sure if he talks this way because of his personality or because there isn't a vocabulary big enough to describe it.
Would you imagine a medical doctor talking this way?
- "So the white blood cells fight with the cancer cells: die evil cell, die!!"
Or an engineer:
- "The little peg ask it's big brother : can you help me convert this energy into circular motion?"