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.. PyZMQ logging doc, by Min Ragan-Kelley, 2011
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Asynchronous Logging via PyZMQ
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==============================
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* The ØMQ guide `coverage <http://zguide.zeromq.org/chapter:all#toc7>`_ of PUB/SUB
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* Python logging module `documentation <http://docs.python.org/library/logging.html>`_
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Python provides extensible logging facilities through its :py:mod:`logging` module. This
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module allows for easily extensible logging functionality through the use of
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:py:class:`~logging.Handler` objects. The most obvious case for hooking up pyzmq to
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logging would be to broadcast log messages over a PUB socket, so we have provides a
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:class:`.PUBHandler` class for doing just that.
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The ØMQ PUB/SUB pattern consists of a PUB socket broadcasting messages, and a collection
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of SUB sockets that receive those messages. Each PUB message is a multipart-message, where
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the first part is interpreted as a topic. SUB sockets can subscribe to topics by setting
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their ``SUBSCRIBE`` sockopt, e.g.::
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sub = ctx.socket(zmq.SUB)
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sub.setsockopt(zmq.SUBSCRIBE, 'topic1')
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sub.setsockopt(zmq.SUBSCRIBE, 'topic2')
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When subscribed, the SUB socket will only receive messages where the first part *starts
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with* one of the topics set via ``SUBSCRIBE``. The default behavior is to exclude all
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messages, and subscribing to the empty string '' will receive all messages.
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The :class:`.PUBHandler` object is created for allowing the python logging to be emitted
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on a PUB socket. The main difference between a PUBHandler and a regular logging Handler is
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the inclusion of topics. For the most basic logging, you can simply create a PUBHandler
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with an interface or a configured PUB socket, and just let it go::
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pub = context.socket(zmq.PUB)
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pub.bind('tcp://*:12345')
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handler = PUBHandler(pub)
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logger = logging.getLogger()
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logger.addHandler(handler)
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At this point, all messages logged with the default logger will be broadcast on the pub
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the PUBHandler does work with topics, and the handler has an attribute ``root_topic``::
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handler.root_topic = 'myprogram'
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Python loggers also have loglevels. The base topic of messages emitted by the PUBHandler
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will be of the form: ``<handler.root_topic>.<loglevel>``, e.g. 'myprogram.INFO' or
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'whatever.ERROR'. This way, subscribers can easily subscribe to subsets of the logging
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messages. Log messages are always two-part, where the first part is the topic tree, and
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the second part is the actual log message.
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>>> logger.info('hello there')
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>>> print sub.recv_multipart()
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['myprogram.INFO', 'hello there']
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You can also add to the topic tree below the loglevel on an individual message basis.
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Assuming your logger is connected to a PUBHandler, you can add as many additional topics
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on the front of the message, which will be added always after the loglevel. A special
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delimiter defined at ``zmq.log.handlers.TOPIC_DELIM`` is scanned by the PUBHandler, so if
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you pass your own subtopics prior to that symbol, they will be stripped from the message
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and added to the topic tree::
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>>> log_msg = "hello there"
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>>> subtopic = "sub.topic"
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>>> msg = zmq.log.handlers.TOPIC_DELIM.join([subtopic, log_msg])
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>>> print sub.recv_multipart()
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['myprogram.WARN.sub.topic', 'hello there']
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docs/source/logging.rst
