Python Cookbook

Using Publish/Subscribe in a Distributed Middleware Architecture

Credit: Graham Dumpleton

Problem

You need to allow distributed services to set themselves up as publishers of information and/or subscribers to that information by writing a suitable central exchange (middleware) server.

Solution

The OSE package supports a publisher/subscriber programming model through its netsvc module. To exploit it, we first need a central middleware process to which all others connect:

# The central.py script -- needs the OSE package from http://ose.sourceforge.net

import netsvc
import signal

dispatcher = netsvc.Dispatcher(  )
dispatcher.monitor(signal.SIGINT)

exchange = netsvc.Exchange(netsvc.EXCHANGE_SERVER)
exchange.listen(11111)

dispatcher.run(  )

Then, we need service processes that periodically publish information to the central middleware process, such as:

# The publish.py script -- needs the OSE package from http://ose.sourceforge.net import netsvc import signal import random class Publisher(netsvc.Service): def _ _init_ _(self): netsvc.Service._ _init_ _(self,"SEED") self._count = 0 time = netsvc.DateTime( ) data = { "time": time } self.publishReport("init", data, -1) self.startTimer(self.publish, 1, "1") def publish(self,name): self._count = self._count + 1 time = netsvc.DateTime( ) value = int(0xFFFF*random.random( )) data = { "time": time, "count": self._count, "value": value } self.publishReport("next", data) self.startTimer(self.publish, 1, "1") dispatcher = netsvc.Dispatcher( ) dispatcher.monitor(signal.SIGINT) ...