JultikaUniversity of Oulu repository

This thesis presents the UbiBroker, an event-based communication middleware for ubiquitous computing environments. The experimental computing environment used in the thesis is provided by the UBI-hotspots, a network of pervasive displays deployed around Oulu. The functional requirements for the UbiBroker are elicited from the long-term experience in developing and maintaining the middleware software layer for the UBI-hotspots, including support for distributed ad hoc applications triggered by user input and context events, interoperability with a wide range of computing platforms, stability, and simplicity to reduce the burden of application developers. The UbiBroker enables ad hoc composition of distributed applications by referential decoupling of communicating processes with a communication model based on events and topic-based publish/subscribe messaging paradigm. The UbiBroker is implemented using the open source RabbitMQ message broker that provides interoperability with a wide range of operating systems and programming languages. The UbiBroker is subjected to comprehensive evaluation, including heuristic assessment with the distributed systems taxonomy of Tanenbaum and Steen, experimental performance evaluations in a lab network and in the UBIhotspot network, long-term production deployment in the UBI-hotspot network, and the development of several prototype applications such as the distributed Ubidoku game deployed on the UBI-hotspots. The evaluations show that the UbiBroker enables easy development of distributed applications for the UBI-hotspots. Stability is demonstrated by the fact that a single broker instance has been serving the UBI-hotspot network for five months and so far there has not been a single incident of the broker crashing or becoming unstable. The performance evaluations reveal hard upper limits for the message delivery capacity of the proposed broker architecture and provide insight into optimizing the configuration of the broker for different operating environments.