From static distributed systems to dynamic systems: an approach for a first step

A noteworthy advance in distributed computing is due to the recent development of peer-to-peer systems. These systems are essentially dynamic in the sense that no process can get a global knowledge on the system structure. They mainly allow processes to look up for data that can be dynamically added/suppressed in a permanently evolving set of nodes. Although protocols have been developed for such dynamic systems, to our knowledge, up to date no computation model for dynamic systems has been proposed. Nevertheless, there is a strong demand for the definition of such models as soon as one wants to develop provably correct protocols suited to dynamic systems. The talk will present an approach aiming at defining an appropriate model for (a class of) dynamic systems. That dynamic model is defined by (1) a parameter (an integer denoted alpah) and (2) two basic communication abstractions (query-response and persistent reliable broadcast). The parameter alpha is a threshold value introduced to capture the liveness part of the system (it is the counterpart of the minimal number of processes that do not crash in a static system). To show the relevance of the model, the paper discusses several distributed computing issues in the context of dynamic systems, such as atomicity, quorums, and leader election.