A core principle of error and warning messages

February 24, 2009

Here is a core principle of how and when programs should show error and warning messages of all sorts:

Error messages should only go to the people who can do something about them.

This goes double for warning messages.

(Sometimes you don't have a choice, at least in theory; if your program suffers a fatal error and you have nowhere else to log it, dumping it on the user does some moderate amount of good. Maybe.)

There's two reasons to be careful where your error messages go (or maybe three). First, it's annoying to people to get messages that they can't do anything about, and if you are dumping warning messages, you are also training people to ignore all messages from your program. Second, it's not effective in letting you know about problems, since most bystanders, confronted with a warning or an error message, will simply throw it away.

Or in short: dumping internal messages on bystanders annoys them and doesn't help you. Presumably these are not your goals.

(I find this especially inexplicable in web applications. Anything that is coherent enough to produce a nice HTML-formatted stack backtrace is certainly coherent enough to log that backtrace somewhere where the developers might actually see it, especially since most web servers have error logs.)

This doesn't mean that you need a hideously complex logging system just in order to print internal warnings or detailed error reports for your own use during development. What you should do is make those things conditional (often on something in the environment, such as a debug variable), and default them to off. Then you can turn on all of the debugging you want when you run the program, and everyone else isn't subjected to it.

(And yes, I turn pretty HTML-formatted stack backtraces on while I'm developing CGI programs. But when I put those programs into production I turn those backtraces back off or have them logged to a file.)

Written on 24 February 2009.
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Last modified: Tue Feb 24 00:24:46 2009
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