RQL: Retrospective Computations over Snapshot Sets

Applications need to analyze the past state of their data to provide auditing and other forms of fact checking. Retrospective snapshot systems that support computations over data store snapshots, allow applications using simple data stores like Berkeley DB or SQLite, to provide past state analysis in a convenient way. Current snapshot systems however, offer no satisfactory support for computations that analyze multiple snapshots. We have developed a Retrospective Query Language (RQL), a simple declarative extension to SQL that allows to specify and run multi-snapshot computations conveniently in a snapshot system, using a small number of simple mechanisms defined in terms of relational constructs familiar to programmers. We describe RQL mechanisms, explain how they translate into SQL computations in a snapshot system, and show how to express a number of common analysis patterns with illustrative examples. We also describe how we implemented RQL in a simple way utilizing SQLite UDF framework in a Berkeley DB data store using Retro page-level incremental snapshot system. Multi-snapshot computations running over page-level incremental snapshots bring up interesting performance issues that have not been studied before. We present the first study defining a performance envelope for multi-snapshot computations over page-level incremental snapshots.