The main logging mechanism in Drupal is a database log by which client code can use an API to save messages into watchdog table. The messages in there are cleared after they reach a certain number, but meanwhile they can be viewed in the browser via a handy interface (at admin/reports/dblog):
Alternatively, a core module that is disabled by default, Syslog, can be used to complement/replace this logging mechanism with the Syslog of the server the site is running on. For the purpose of this book, we will focus on how logging works with any mechanism, but we will also take a look at how we can implement our own logging system in Drupal ...
