A Transactional Model for Data Warehouse Maintenance

A Data Warehouse Management System (DWMS) incrementally maintains materialized views by issuing maintenance queries to the data sources. To address erroneous query results caused by concurrent source updates, state-of-the-art maintenance strategies typically apply compensations to resolve the conflicts. For this, they assume however that the source schema are not updated and remain stable over time. However, if schema changes occur in any of the sources, then an anomaly may arise, namely, the maintenance or the compensation queries may be broken. We now tackle this open problem by modeling the complete maintenance process as a special transaction, called a DWMS_Transaction. The anomaly problem can now be rephrased as the serializability of DWMS_Transactions. This allows us to apply well-established transaction theory to address this new anomaly problem. To achieve such serializability, we propose a multiversion concurrency control technique appropriate for loosely-coupled environments with autonomous sources. TxnWrap is complementary to maintenance algorithms from the literature by removing concurrency issues from their consideration. The experimental results confirm that TxnWrap achieves predictable steady performance even under a varying rate of concurrency.