Increasing the resilience of atomic commit, at no additional cost

This paper presents a new atomic commitment protocol, Enhanced Three Phase Commit (E3PC), that always allows a quorum in the system to make progress. Previously suggested quorum-based protocols (e.g. the quorum-based Three Phase Commit (3PC) [Ske82]) allow a quorum to make progress in case of one failure. If failures cascade, however, and the quorum in the system is 'lost' (i.e. at a given time no quorum component exists, e.g. because of a total crash), a quorum can later become connected and still remain blocked. With our protocol, a connected quorum never blocks. E3PC is based on the quorum-based 3PC [Ske82], and it does not require more time or communication than 3PC. The principles demonstrated in this paper can be used to increase the resilience of a variety of distributed services, e.g. replicated database systems, by ensuring that a quorum will always be able to make progress.