The very last phase of the transaction before the cleanup at the end is the logging phase. At this point, the request record contains everything there is to know about the transaction, including the content handler's final status code and the number of bytes transferred from the server to the client.
Apache's built-in logging module mod_log_config ordinarily handles this phase by writing a line of summary information to the transfer log. As its name implies, this module is highly configurable. You can give it printf() -like format strings to customize the appearance of the transfer log to your requirements, have it open multiple log files, or even have it pipe the log information to an external process for special processing.
By handling the logging phase yourself, you can perform special processing at the end of each transaction. For example, you can update a database of cumulative hits, bump up a set of hit count files, or notify the owner of a document that his page has been viewed. There are a number of log handlers on CPAN, including Apache::DBILogger , which sends log information to a relational database, and Apache::Traffic, which keeps summaries of bytes transferred on a per-user basis.
The first example of a log handler that we'll show is Apache::LogMail. It sends email to a designated address whenever a particular page is hit and can be used in low-volume applications, such as the vanity home pages ...