Application Drivers

A vital goal of MPA is to allow for the easy integration of new applications. To achieve this goal, we limit application-specific knowledge to small, modular blocks called Application Drivers. All other parts of MPA have to know only what types of application-specific protocols there are, not the details of each protocol.

Ideally, a driver should be able to convert any communication to any other format; however, in practice it is doubtful whether this is practical or desirable. In some situations the best a driver can do is to embed one format within another (e.g., embed a voicemail message as a sound file within an email message). Generally we can distinguish four types of drivers:

• Stream-to-stream drivers transform one live stream into another. This is currently most likely to be of interest for voice-based applications. Since these drivers must perform complex operations under stringent real-time constraints, they are the most difficult to design.

• Stream-to-message drivers convert a live stream into a message. If a user is contacted with an interactive streaming format but is only reachable by non-interactive message types, the Personal Proxy can use this type of driver. This is similar to the functionality of a voicemail system.

• Message-to-message drivers convert one message format to another, a process which need not be performed in real time.

• Filter drivers enable one of the key features of MPA: enabling users to direct incoming communications based on application-specific properties, such as keywords.

It is important to note that while the Application Drivers are shown in Figure 4 as being completely contained within the Personal Proxy, a driver can be built using a client-server model. Most of the real work could be accomplished by a server on a different machine; the local driver would be a simple stub that communicates with the server. The Berkeley NINJA Project [GWBC99] demonstrates the potential of such a system (see Section VI).

Message storage, performed in the figure by the calendar server and the POP3 server, is outside the scope of the Personal Proxy. In our design, we leverage the support for message storage already provided by applications.