End-to-End Latency of a Fault-Tolerant CORBA Infrastructure

This paper presents measured probability density functions (pdfs) for the end-to-end latency of two-way remote method invocations from a CORBA client to a replicated CORBA server in a fault-tolerance infrastructure. The infrastructure uses a multicast group-communication protocol based on a logical token-passing ring imposed on a single local-area network. The measurements show that the peaks of the pdfs for the latency are affected by the presence of duplicate messages for active replication, and by the position of the primary server replica on the ring for semi-active and passive replication. Because a node cannot broadcast a user message until it receives the token, up to two complete token rotations can contribute to the end-to-end latency seen by the client for synchronous remote method invocations, depending on the server processing time and the interval between two consecutive client invocations. For semi-active and passive replication, careful placement of the primary server replica is necessary to alleviate this broadcast delay to achieve the best possible end-to-end latency. The client invocation patterns and the server processing time must be considered together to determine the most favorable position for the primary replica. Assuming that an effective sending-side duplicate suppression mechanism is implemented, active replication can be more advantageous than semi-active and passive replication because all replicas compete for sending and, therefore, the replica at the most favorable position will have the opportunity to send first.