A Refresh Scheme for Remote Snapshots

This article presents a scheme called “Difference Table” for maintaining database snapshots stored at sites remote from a central database which are refreshed only upon user request. Database snapshots are currently in widespread use where a subset of the central database is extracted and transmitted to a local workstation and utilized for decision support. The Difference Table method checks each update to a central database table against the definition of the snapshot. If the update is relevant, its effect is stored in a difference table. On receiving the refresh request, the contents of the difference table are transmitted to the remote site where they update the snapshot. The Difference Table scheme allows a selective refresh of the snapshot, in the sense that only the changes to a snapshot since the last refresh are transmitted. We discuss the additional database tables and processes required to support the Difference Table scheme. Performance measures are developed, and both quantitative and qualitative comparisons are made to alternative methods such as full regeneration and the approach used by System R^*. By most criteria and in many environments, the Difference Table scheme is preferable to these alternatives. It also has several attractive side benefits which are not available in alternative methods.