FAS - A Freshness-Sensitive Coordination Middleware for a Cluster of OLAP Components

Data warehouses offer a compromise between freshness of data and query evaluation times. However, a fixed preference ratio between these two variables is too undifferentiated. With our approach, clients submit a query together with an explicit freshness limit as a new Quality-of-Service parameter. Our architecture is a cluster of databases. The contribution of this article is the design, implementation, and evaluation of a coordination middleware. It schedules and routes updates and queries to cluster nodes, aiming at a high throughput of OLAP queries. The core of the middleware is a new protocol called FAS (FreshnessAware Scheduling) with the following qualitative characteristics: (1) The requested freshness limit of queries is always met, and (2) data accessed within a transaction is consistent, independent of its freshness. Our evaluation shows that FAS has the following nice properties: OLAP queryevaluation times are close (within 10%) to the ones of an idealistic setup with no updates. FAS allows to effectively trade ’upto-dateness’ for query performance. Even when all queries request fresh data, FAS clearly outperforms synchronous replication. Finally, mean response times are independent of the cluster size (up to 128 nodes). ∗ Current affiliation: Otto-von-Guericke-Universität Magdeburg, Germany Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Very Large Data Base Endowment. To copy otherwise, or to republish, requires a fee and/or special permission from the Endowment. Proceedings of the 28th VLDB Conference, Hong Kong, China, 2002