Notes on a Message Passing design

I am an avid supporter of "message based" systems. I took the time to perform a quick "brain dump" of some of my thoughts on the subject.



Follows a simple Python based implementation.

	"""
	Simple "Bus" based message publish/subscribe
	Created on 2010-01-28
	@author: jldupont
	"""
	__all__=["Bus"]
	class Bus(object):
	"""
	Simple publish/subscribe "message bus"
	with configurable error reporting
	Message delivery is "synchronous i.e. the caller of
	the "publish" method blocks until all the subscribers
	of the message have been "called-back".
	Any "callback" can return "True" for stopping the
	calling chain.
	"""
	logger=None
	ftable={}
	@classmethod
	def subscribe(cls, msgType, callback):
	"""
	Subscribe to a Message Type
	@param msgType: string, unique message-type identifier
	@param callback: callable instance which will be called upon message delivery
	"""
	subs=cls.ftable.get(msgType, [])
	subs.append(callback)
	cls.ftable[msgType]=subs
	@classmethod
	def publish(cls, msgType, *pa, **kwa):
	"""
	Publish a message from a specific type on the bus
	@param msgType: string, message-type
	@param *pa: positional arguments
	@param **kwa: keyword based arguments
	"""
	subs=cls.ftable.get(msgType, [])
	for sub_cb in subs:
	try:
	stop_chain=sub_cb(msgType, *pa, **kwa)
	except Exception, e:
	stop_chain=True
	if cls.logger:
	cls.logger(msgType, e)
	if stop_chain:
	return
	## =========================================================== TESTS
	if __name__ == '__main__':
	class Logger(object):
	def __call__(self, msgType, e):
	print "Logger: Error: msgType(%s) Exception(%s)" % (msgType, str(e))
	def callback(msgType, *pa, **kwa ):
	print "cb: msgType(%s) pa(%s) kwa(%s)" % (msgType, pa, kwa)
	Bus.logger=Logger()
	Bus.subscribe("test", callback)
	Bus.subscribe("test", callback)
	Bus.publish("test", p1="v1", p2="v2")
	Bus.publish("test2", p1="v1", p2="v2")
	Bus.subscribe("test", None)
	Bus.publish("test", p2="v2", p3="v3")
	Bus.logger=None
	Bus.publish("test", p2="v2", p3="v3")

view raw Simple_Message_Bus.py hosted with ❤ by GitHub

I will very likely make other contributions to my blog along the same lines.