Mastering Perl, 2nd Edition

Chapter 12. Logging

Logging records messages from my program so I can watch its progress or later look at what happened. This is much more than warnings or errors from my program, since I can also emit messages when things are going well. Along with configuration, logging is one of the features most often missing from Perl applications, or bolted on as an afterthought. I can easily add logging from the start of development or retrofit a legacy application.

Recording Errors and Other Information

Web applications already have it made. They can send things to STDERR (through whichever mechanism or interface the program might use) and they show up in the web server error log. Other programs have to work harder. In general, logging is not as simple as opening a file, appending some information, and closing the file. That might work if the program won’t run more than once at the same time and definitely finishes before it will run again. For any other case, it’s possible that two runs of the program running at the same time will try to write to the same file. Output buffering and the race for control of the output file means that somebody is going to win and not all of the output may make it into the file.

Logging, however, doesn’t necessarily mean merely adding text to a file. Maybe I want to put the messages into a database, display them on the screen, send them to a system logger (such as syslogd), or more than one of those. I might want to send them directly to my email or pager. Indeed, ...