A Pretty Flexible API for Generic Peer-to-Peer Programming

We propose and motivate an API for programming distributed applications using a structured overlay network of peers as infrastructure. The API offers simple primitives and powerful mechanisms, in a way that is independent from the underlying overlay. The dynamic set of participants is abstracted by providing a flat space of keys, transparently scattered across all participants in the overlay. The API primitives allow application instances to send messages towards individual keys. Two different kinds of messages can be exchanged, namely, unidirectional and request-response; the latter takes place in a split-phase non-blocking way, so that the application can be made latency-tolerant and thus more performing. The request-response pattern is also shown to be crucial for those applications demanding a degree of user anonymity. The semantics of messages is not defined by the API itself. Rather, the API offers a mechanism to allow the application to set up handlers, which are upcalls to run upon message arrivals at each peer. The overall behaviour of the application is thus shaped by the handlers. The API also allows to define application-level handlers for other two typical tasks of any dynamic peer-to-peer system, namely, the migration of keys across peers after new peer arrivals, and the regeneration of missing keys after peer departures.