How should I log while using multiprocessing in Python?

Question

Right now I have a central module in a framework that spawns multiple processes using the Python 2.6 multiprocessing module. Because it uses multiprocessing, there is module-level multiprocessing-aware log, LOG = multiprocessing.get_logger(). Per the docs, this logger has process-shared locks so that you don't garble things up in sys.stderr (or whatever filehandle) by having multiple processes writing to it simultaneously.

The issue I have now is that the other modules in the framework are not multiprocessing-aware. The way I see it, I need to make all dependencies on this central module use multiprocessing-aware logging. That's annoying within the framework, let alone for all clients of the framework. Are there alternatives I'm not thinking of?

Answer 1

I just now wrote a log handler of my own that just feeds everything to the parent process via a pipe. I've only been testing it for ten minutes but it seems to work pretty well.

(Note: This is hardcoded to RotatingFileHandler, which is my own use case.)

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Update: @javier now maintains this approach as a package available on Pypi - see multiprocessing-logging on Pypi, github at https://github.com/jruere/multiprocessing-logging

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Update: Implementation!

This now uses a queue for correct handling of concurrency, and also recovers from errors correctly. I've now been using this in production for several months, and the current version below works without issue.

from logging.handlers import RotatingFileHandler
import multiprocessing, threading, logging, sys, traceback

class MultiProcessingLog(logging.Handler):
    def __init__(self, name, mode, maxsize, rotate):
        logging.Handler.__init__(self)

        self._handler = RotatingFileHandler(name, mode, maxsize, rotate)
        self.queue = multiprocessing.Queue(-1)

        t = threading.Thread(target=self.receive)
        t.daemon = True
        t.start()

    def setFormatter(self, fmt):
        logging.Handler.setFormatter(self, fmt)
        self._handler.setFormatter(fmt)

    def receive(self):
        while True:
            try:
                record = self.queue.get()
                self._handler.emit(record)
            except (KeyboardInterrupt, SystemExit):
                raise
            except EOFError:
                break
            except:
                traceback.print_exc(file=sys.stderr)

    def send(self, s):
        self.queue.put_nowait(s)

    def _format_record(self, record):
        # ensure that exc_info and args
        # have been stringified.  Removes any chance of
        # unpickleable things inside and possibly reduces
        # message size sent over the pipe
        if record.args:
            record.msg = record.msg % record.args
            record.args = None
        if record.exc_info:
            dummy = self.format(record)
            record.exc_info = None

        return record

    def emit(self, record):
        try:
            s = self._format_record(record)
            self.send(s)
        except (KeyboardInterrupt, SystemExit):
            raise
        except:
            self.handleError(record)

    def close(self):
        self._handler.close()
        logging.Handler.close(self)