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See also 
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Developer's Bible, 
Technical Overview
  internal package Foswiki::Logger::Compatibility  is a Foswiki::Logger 
Compatibility with old 
LocalSite.cfg settings, if user has not run
configure yet. This logger is automatically used if Foswiki senses
that the 
LocalSite.cfg hasn't been modified for 1.1 (configure has
not been run yet). It may also be explicitly selected in
configure.
Plain file implementation of the Foswiki Logger interface. Mostly
compatible with 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{DebugFileName}
-  infomessages are output to $Foswiki::cfg{LogFileName}
-  warning,error,critical,alert,emergencymessages are     output to $Foswiki::cfg{WarningFileName}.
-  error,critical,alert, andemergencymessages are also     written to standard error (the webserver log file, usually)
This is a copy of the Foswiki 1.0 code.
 ObjectMethod log($level, @fields) 
See 
Foswiki::Logger for the interface.
 StaticMethod eachEventSince($time, $level) → $iterator 
See 
Foswiki::Logger for the interface.
This logger implementation maps groups of levels to a single logfile, viz. 
-  infomessages are output together.
-  warning,error,critical,alert,emergencymessages are     output together.
-  $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.