Understanding Non-Blocking Atomic Commitment

In distributed database systems, an atomic commitment protocol ensures that transactions terminate consistently at all participating sites even in the presence of failures. An atomic commitment protocol is said to be non-blocking if it permits transaction termination to proceed at correct participants despite failures of others. Protocols that have this property are desirable since they limit the time intervals during which transactions may be holding valuable resources. In this paper, we show how non-blocking atomic commitment protocols can be obtained through slight modifications of the well-known Two-Phase Commit (2PC) protocol, which is known to be blocking. Our approach is modular in the sense that both the protocols and their proofs of correctness are obtained by plugging in the appropriate reliable broadcast algorithms as the basic communication primitives in the original 2PC protocol. The resulting protocols are not only conceptually simple, they are also efficient in terms of time and message complexity. 1. Department of Mathematics, University of Bologna,Piazza Porta S. Donato 5,40127 Bologna Italy. This author was supported in part by the Commission of European Communities under ESPRIT Programme Basic Research Project Number 6360 (BROADCAST), the United States Office of Naval Research under contract N00014-91-J1219, IBM Corporation, Hewlett-Packard of Italy and the Italian Ministry of University, Research and Technology. 2. Department of Computer Science, Cornell University, 4130 Upson Hall, Ithaca, New York 14583 USA. This author was supported in part by the National Science Foundation under Grant Number CCR-9102231, IBM Corporation (Endicott Programming Laboratory) and the Italian National Research Council (CNR-GNIM) through a visiting professor grant.