Rack::CommonLogger forwards every request to the given app, and logs a line in the Apache common log format to the logger.
If logger is nil, CommonLogger will fall back rack.errors, which is an instance of Rack::NullLogger.
logger can be any class, including the standard library Logger, and is expected to have a write method, which accepts the CommonLogger::FORMAT. According to the SPEC, the error stream must also respond to puts (which takes a single argument that responds to to_s), and flush (which is called without arguments in order to make the error appear for sure)
Common Log Format: httpd.apache.org/docs/1.3/logs.html#common
lilith.local - - [07/Aug/2006 23:58:02] "GET / HTTP/1.1" 500 - %{%s - %s [%s] "%s %s%s %s" %d %s\n} %
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