Reliability in three-tier systems without application server coordination and persistent message queues

When dealing with fault tolerance in three-tier systems, two major problems need to be addressed, that is how to prevent duplicate transaction executions when classical timeout based retransmission logics are employed, and how to ensure the agreement among the back-end databases despite failures (a transaction needs to be aborted or committed at all the involved databases independently of the failure scenario). In this paper we address these problems by proposing a fault tolerant protocol that, unlike previous solutions, (i) avoids the additional phase of storing the client request into a persistent message queue and (ii) avoids explicit co-ordination of middle tier application servers (during both normal behavior and fail-over). Our protocol reduces therefore the overhead imposed on the end-to-end interaction, thus improving user perceived responsiveness, and provides better scalability. Copyright 2005 ACM.