Replica Voting: a Distributed Middleware Service for Real-time Dependable Systems

In information assurance settings, majority voting among replica processes enhances the trust-worthiness of data collected from a hostile external environment. It allows a correct data fusion and dissemination by the end-users, in the presence of content corruptions and/or timing failures that may possibly occur during data collection. Two key elements are required of the voting functionality: i) fielding asynchronously generated real-time or near-real-time data, and ii) handling large sized non-numeric data sets with problem-specific interpretations-e.g., terrain images from radar stations. Under the constraints (i) and (ii), we describe a highly asynchronous voting service to effect a timely and low-overhead delivery of data to the users. The paper analyzes the service-level properties of a voting machinery to meet the stringent needs of IA applications. The externally visible properties are prescribed in terms of `safety´ and `liveness´ requirements of the underlying voting protocols that reflect the application-specific data integrity and availability constraints. When voting is provided as a middleware service, an application designer may prescribe the necessary service-level parameters: the level of resiliency and the performance aspects of data delivery