Ciprian Tutu

From total order to database replication

This paper presents in detail an efficient and provably correct algorithm for database replication over partitionable networks. Our algorithm avoids the need for end-to-end acknowledgments for each action while supporting network partitions and merges and allowing dynamic instantiation of new replicas. One round of end-to-end acknowledgments is required only upon a membership change event such as a network partition. New actions may be introduced to the system at any point, not only while in a primary component. We show how performance can be further improved for applications that allow relaxation of consistency requirements. We provide experimental results that demonstrate the efficiency of our approach.

N-way fail-over infrastructure for reliable servers and routers

Maintaining the availability of critical servers and routers is an important concern for many organizations. At the lowest level, IP addresses represent the global namespace by which services are accessible on the Internet. We introduce Wackamole, a completely distributed software solution based on a provably correct algorithm that negotiates the assignment of IP addresses among the currently available servers upon detection of faults. This reallocation ensures that at any given time any public IP address of the server cluster is covered exactly once, as long as at least one physical server survives the network fault. The same technique is extended to support highly available routers. The paper presents the design considerations, algorithm specification and correctness proof, discusses the practical usage for server clusters and for routers, and evaluates the performance of the system.

The Wackamole approach to fault tolerant networks

We present Wackamole, a high availability tool for clusters of servers. Wackamole ensures that a server handles the requests that arrive on any of the services public IP addresses. Wackamole is a completely distributed software solution based on a provably correct algorithm that negotiates the assignment of IP addresses among the available servers upon detection of faults and recoveries, and provides N-way fail-over, so that any one of a number of servers can cover for any other. Using a simple algorithm that utilizes strong group communication semantics, Wackamole demonstrates the application of group communication to address a critical availability problem at the core of the system, even in the presence of cascading network or server faults and recoveries. The same architecture is extended to provide a similar service for highly available routers.