CSL-89-1_Epidemic_Algorithms_for_Replicated_Database_Maintenance

EPIDEMIC ALGORITHMS FOR REPLICATED DATABASE MAINTENANCE 21traffic on certain critical links by a factor of 30 when compared with an algorithm using uniformselection of partners.The observation that anti-entropy behaves like a simple epidemic led us to consider otherepidemic-like algorithms such as rumor mongering, which shows promise as an efficient replacementfor the initial mailing step for distributing updates. A backup anti-entropy scheme easily spreadsthe updates to the few sites that do not receive them as a rumor. In fact, it is possible to combinea peel back version of anti-entropy with rumor mongering, so that rumor mongering never fails tospread an update completely.N either the epidemic algorithms nor the direct mail algorithms can correctly spread the absenceof an item without assistance from death certificates. There is a trade-off between the retentiontime for death certificates, the storage space consumed, and the likelihood of old data undesirablyreappearing in the database. By retaining dormant death certificates at a few sites we cansignificantly improve the network's immunity to obsolete data at a reasonable storage cost.There are more issues to be explored. Pathological network topologies present performanceproblems. One solution would be to find algorithms that work well with all topologies; failingthis, one would like to characterize the pathological topologies. Work still needs to be done on theanalysis and design of epidemics. So far we have avoided differentiating among the servers; betterperformance might be achieved by constructing a dynamic hierarchy, in which sites at high levelscontact other high level servers at long distances and lower level servers at short distances. (Thekey problem with such a mechanism is maintaining the hierarchical structure.)5. AcknowledgmentsWe would like to thank Mike Paterson for his help with parts of the analysis, and SubhanaMenis and Laurel Morgan for threading the arcane production maze required to produce the cameraready copy of this paper.References[Ab][Aw][Ba]Karl Abrahamson, Andrew Addler, Lisa Higham, David KirkpatrickProbabilistic Solitude Verification on a Ring.Proceedings of the Fifth Annual ACM Symposium on Principles of Distributed Computing.Calgary, Alberta, Canada. 1986, Pages 161-173.Baruch Awerbuch and Shimon Even.Efficient and Reliable Broadcast is Achievable in an Eventually Connected Network.Proceedings of the Third Annual ACM Symposium on Principles of Distributed Computing.Vancouver, B.C., Canada. 1984, Pages 278-281.Norman T. J. Bailey.The Mathematical Theory of Infectious Diseases and its Applications (second edition).Hafner Press, Second Edition, 1975.XEROX PARC, CSL-89-1, JANUARY 1989