From: "lroland@..." <lroland@...>
Date: 2005-11-10T02:17:13+09:00
Subject: Logging vs. Exceptions

Hi all

I am moving a rather big Perl application to Ruby, due to the nasty way
Perl (un)handles exceptions we originally used log4perl (a log4j/log4r
like logging facility) heavily in a 'log and recover' like manner.

Ruby however have a quite nice exception semantics and thus I am
playing with using exceptions where I used to use logging.  This brings
us to the dilemma of when to throw errors and when to log them. I read
on the net that a good policy is to either 'catch, wrap and throw', or
'log and recover', but never neither nor both.

Although this sounds like a reasonable plan I contiguously find myself
in limbo between doing the one or the other or mixing both.

i.e. if I have a class which contains a public method: 'x', and a
private method: 'y' (called by 'x'). Let's also put in as constraint
that it is vital that calling method: 'x' does not blow up the program
- the solution I ended up with is to catch the possible exceptions that
'y' may have thrown, log the errors and do something to recover (i.e.
thus letting the caller of 'x' handle whatever went wrong in 'x' by the
means of status codes).

-----------------
def x
  :
  begin
    res = y( args )
  rescue
    log("error calling method: y")
    # the method calling x should handle this
    # or maybe we should propagate the exception thrown from y
    # (or re throw a new one) for the caller to handle
    return nil
  end

  return res
end

def y
  :
  # do a lots of commands that may throws exceptions
  if error
    raise Exception.new("something went wrong")
  end
  :
  return res
end
-----------------

Although I normally follow the guidelines of not doing both I am
leaning towards letting the exception code also use the logging
facility that way I can propagate nested exceptions up through the code
and still have the all the errors logged into one place - would this be
ugly ? or bad design ?. I am a old school Haskell, C and Perl
programmer so I am not used to do much exception handling thus I would
like some opinions on the best practices of logging vs. exception
handling ?.



Regards.

Lars Roland