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 (Freshness-Aware 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 query-evaluation 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).