A Mediator for Approximate Consistency: Supporting “GoodEnough” Materialized Views

Abstract

This paper addresses the needs of application designers who would like to tell an automated assistant the following: “Here is a query that defines a view I want to materialize within my application. I need this view to remain approximately consistent with the state of the data sources from which the view is derived, in accordance with declaratively specified staleness predicates. When the view becomes stale, follow the refresh strategy I specify (e.g., eager, lazy, hybrid). You must do this in heterogeneous environments containing both active and passive data sources”. This paper describes an architecture that realizes this vision. The approach supports materialized, object-based views, called quasi-views , defined over shared databases. Quasi-views are refreshed according to the consistency conditions and refresh strategies specified declaratively by application designers. These conditions allow for the deviation of quasi-views from their database counterparts according to well-defined and monitored approximate consistency predicates. A layer of software called a Mediator\ for\ Approximate\ Consistency automatically generates the database objects necessary to enforce these consistency conditions, shielding the application developer from the implementation details of consistency maintenance. In addition, it does this for both active and passive (e.g., legacy) data sources. This paper formalizes quasi-views, presents a declarative quasi-view specification language, and describes an architecture and implementation of a Mediator for Approximate Consistency.