Strong interaction fairness via randomization

We present Multi, a symmetric, fully distributed, randomized algorithm that, with probability 1, schedules multiparty interactions in a strongly fair manner. To our knowledge, Multi is the first algorithm for strong interaction fairness to appear in the literature. Moreover, the expected time taken by Multi to establish an interaction is a constant not depending on the total number of processes in the system. In this sense, Multi guarantees real-time response. Multi makes no assumptions (other than boundedness) about the time it takes processes to communicate. It thus offers an appealing tonic to the impossibility results of Tsay&Bagrodia and Joung concerning strong interaction fairness in an environment, shared-memory or message-passing, in which processes are deterministic and the communication time is nonnegligible. Because strong interaction fairness is as strong a fairness condition that one might actually want to impose in practice, our results indicate that randomization may also prove fruitful for other notions of fairness lacking deterministic realizations and requiring real-time response.