Rendezvous-Notify 
                    Sasu Tarkoma
     Helsinki Institute for Information Technology
                  sasu.tarkoma@hiit.fi


Event-based computing is vital for future mobile services and
applications that need to meet user requirements irrespective of time and
location. The event paradigm supports asynchronous and anonymous
one-to-many communication and facilitates content-based information
dissemination, in which event filters define the set of communication
end-points. Rapid notification of changes is vital in order for the system
and the applications to adapt to changes in the communication and
computing context. The traditional client-server paradigm and RPC (Remote
Procedure Call) semantics are not sufficient alone, because of high response
times and possible disconnections. Event-based communication is a good
candidate for mobile computing, because it is asynchronous and supports
disconnected operation through message buffering. However, user and terminal
mobility present problems pertaining to event delivery that need to be
solved.

Rendezvous-Notify is a distributed event notification infrastructure especially for
mobile computing. The server-side part of the architecture focuses on the distribution
of event channels, and how the event servers that provide the notification service
communicate. This event-delivery architecture is based on two server roles: access
servers that maintain subscription information and buffered events, and resolution
servers that are responsible for event channels and routing events to access servers.
Access to event channels is done using a rendezvous mechanism.  The architecture
supports bounded event delivery times, terminal mobility, and disconnected operation.
Rendezvous-Notify supports filtering in several phases: first on the clients, then on
resolution servers (event channels), and finally at the access servers. Filtering is used to
realize content-based addressing of events, and maintain high precision in order to
avoid unnecessary communication.