A Replication Framework for Program-to-Program Interaction across Unreliable Networks and its Implementation in a Servlet Container

We propose a service replication framework for unreliable networks. The service exhibits the same consistency guarantees about the order of execution of operation requests as its non-replicated implementation. Such guarantees are preserved in spite of server replica failure, network failure (either between server replicas or between a client and a server replica) and irrespective of when the failure occurs. Moreover, the service guarantees that in case a client sends an “update” request multiple times, there is no risk that the request be executed multiple times. No hypothesis about the timing retransmission policy of clients are made, e.g., the very same request might even arrive at different server replicas simultaneously. All these features make the proposed framework particularly suitable for interaction between remote programs, a scenario that is gaining increasing importance. We discuss a prototype implementation of our replication framework based on Tomcat, a very popular Java-based web server. The prototype comes into two flavors: replication of HTTP client session data and replication of a counter accessed as a web service.