Distributed publish/subscribe system

[Kaiduan Xie]

Hi,

Any one has experience to build distributed publish/subscribe system?
The system has the following requirements,

1. Subscriber subscribes to the interested topic
2. Publisher publishes event related to the topic
3. System delivers the event to all the users subscribed to this
topic. If subscriber is connected, the event is delivered immediately.
    If the subscriber is not connected, system should persist the
event. When the user next time becomes online, system delivers the
event to the user.

Can you share the experience?

Best regards,

/Kaiduan