Extending Logging for Database Snapshot Refresh

A database snapshot mechanism rcprcsents a cost effectivc substitute for replicated data in a distributed database. The contents of a database snapshot can be periodically refreshed to reflect the current state of the database. In a distributed database system it is sign& icant to reduce the cost of snapshot refresh. This can be obtained by a differential refresh strategy in which modifications to the base tables involved are detected. The paper proposes two methods based on using a separate table for logging the modifications made to a base table; a sequential and a condensed logging approach. The methods have been compared for various update frequency and composition. The sequential log performs well for single snapshots if the modification set is small relative to the base table size, or if the snapshot is restrictive. In the case of large modification sets and replicated snapshots, the condensed logging method is to be preferred.