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See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
  internal package Foswiki::Logger::PlainFile  is a Foswiki::Logger 
Plain file implementation of the Foswiki Logger interface. Mostly
compatible with TWiki (and Foswiki 1.0.0) log files, except that dates
are recorded using ISO format, and include the time, and it dies when
a log can't be written (rather than printing a warning).
This logger implementation maps groups of levels to a single logfile, viz. 
-  debugmessages are output to $Foswiki::cfg{Log}{Dir}/debug.log
-  infomessages are output to $Foswiki::cfg{Log}{Dir}/events.log
-  warning,error,critical,alert,emergencymessages are     output to $Foswiki::cfg{Log}{Dir}/error.log.
-  error,critical,alert, andemergencymessages are also     written to standard error (the webserver log file, usually)
 ObjectMethod log($level, @fields) 
See 
Foswiki::Logger for the interface.
 ObjectMethod eachEventSince($time, \@levels, $version) → $iterator 
 
-  $time- a time in the past
-  \@levels- log levels to return events for.  Individual level or array reference.
-  $version- Version 1 of API returns a hash instead of an array.
-  $lock- boolean switch to enable locking, off by default
See 
Foswiki::Logger for the interface.