In a new algorithm for maintaining replicated data, every copy of a replicated file is assigned some number of votes. Every transaction collects a read quorum of r votes to read a file, and a write quorum of w votes to write a file, such that r+w is greater than the total number number of votes assigned to the file. This ensures that there is a non-null intersection between every read quorum and every write quorum. Version numbers make it possible to determine which copies are current. The reliability and performance characteristics of a replicated file can be controlled by appropriately choosing r, w, and the file's voting configuration. The algorithm guarantees serial consistency, admits temporary copies in a natural way by the introduction of copies of an application system called Violet.

In addition to maintaining database consistency as in conventional databases, real-time database systems must also handle transactions with timing constraints. While transaction response time and throughput are usually used to measure a conventional database system, the percentage of transactions satisfying the deadlines or a time-critical value function is often used to evaluate a real-time database system. Scheduling real-time transactions is far more complex than traditional real-time scheduling in the sense that (1) worst-case execution times are typically hard to estimate, since not only CPU but also I/O requirement isinvolved � and (2) certain aspects of concurrency control may not integrate well with real-time scheduling. In this paper, we rst develop a taxonomy of the underlying design space of concurrency control including the various techniques for achieving serializability and improving performance. This taxonomy provides us with a foundation for addressing the real-time issues. We then consider the integration of concurrency control with realtime requirements. The implications of using run policies to better utilize real-time scheduling in a database environment are examined. Finally, as timing constraints may be more important than data consistency in certain hard real-time database applications, we also discuss several approaches that explore the non-serializable semantics of real-time transactions to meet the hard deadlines. Index terms: concurrency control, real-time databases, real-time scheduling, real-time transactions, serializability, schedulability.

Immediate materialized view maintenance with transactional consistency is highly desirable to support real-time decision making. Nevertheless, due to high deadlock rates, such maintenance can cause significant performance degradation in the database system. To increase concurrency during such maintenance, we previously proposed the V locking protocol for materialized aggregate join views and showed how to implement it on hash indices. In this paper, we address the thorny problem of implementing the V locking protocol on B-tree indices. We also formally prove that our techniques are both necessary and sufficient to ensure correctness (serializability).