Shows how to build a simple asynchronous pubsub message bus.
(Note that it is NOT aiming to be a reliable high performance msgbus
to compete with solutions such as zmq, rabbitmq, celery)
It uses a few Pyro features to achieve this:

- autoproxy  (for subscribers)
- instance_mode
- auto reconnect


Start the message bus server from this example's directory with:
   python -m messagebus.server   [options]


Use -h to get a help screen of available options.
You can run multiple publishers at the same time, make sure you give
a different location argument when you start each of them to see the results.
You can also run multiple subscribers at the same time, the published
messages will be delivered to each subscriber.

There are two kinds of subscribers:
- one that automatically consumes the messages as soon as they arrive on the bus,
- one that has a manual message processing loop


The messagebus is a bit simplistic if you use the in-memory storage:
it only keeps messages and subscribers in memory. If the message bus server
dies, everything is lost. If an error occurs when forwarding a message to subscribers,
the message is immediately discarded. If it was a communication error,
the subscriber is immediately removed from the topic as well.
The in-memory storage is very fast though, so if you're after a very high
message throughput, it may be the storage option for you.

However you can also use the SqliteStorage which uses a database on disk
to store topics, messages and subscriptions. If the message bus server dies,
it will simply continue where it was. No messages will get lost, and it
also remembers the subscribers. So simply restarting the message bus server
is enough to get everything back on track without lost data.
The sqlite storage is slower than the in-memory storage (and MUCH slower when
running on Windows), so if you need a high message throughput, it may not be suitable.


There's no queue mechanism, this is left as an exercise for the reader.
(A queue is 1-to-1 communication whereas pubsub is 1-to-many)
