QoS-Aware Middleware for Scalable Replication

When using replication as a technique for achieving high performance and scalability, optimistic replication is the natural choice. But optimistic replication introduces uncertainty that may lead to confusion in the clients. For example consecutive requests from the same client may be processed out of order, or the answer from a query might be too old to be useful to the client, or an operation may cause unwanted side-effect in the system when the client times out due to long response time. In such situations the clients can benefit from being allowed to specify their tolerable levels of uncertainty, in other words their desired quality-of-service (QoS) from the system. This paper introduces a novel optimistic replication protocol QRep with its QoS-aware middleware that allows the clients´ to specify the following three QoS parameters: (1) session guarantee, (2) freshness and (3) deadline. QRep attempts to fulfill the specified QoS while processing the requests. QRep supports both synchronous and asynchronous clients. We carried out experiments to demonstrate the effectiveness of the protocol in terms of both QoS fulfillment and scalability. We also demonstrated that there is no significant performance penalty for providing the QoS.