On Deferred Constraints in Distributed Database Systems

An atomic commit protocol (ACP) is a distributed algorithm used to ensure the atomicity property of transactions in distributed database systems. Although ACPs are designed to guarantee atomicity, they add a significant extra cost to each transaction execution time. This added cost is due to the overhead of the required coordination messages and log writes at each involved database site to achieve atomicity. For this reason, the continuing research efforts led to a number of optimizations that reduce the aforementioned cost. The most commonly adopted optimizations in the database standards and commercial database management systems are those designed around the early release of read locks of transactions. In this type of optimizations, certain participating sites may start releasing the read locks held by transactions before they are fully terminated across all participants. Hence, greatly enhancing concurrency among executing transactions and, consequently, the overall system performance. However, this type of optimizations introduces possible “execution infections” in the presence of deferred consistency constraints; a devastating complication that may lead to non-serializable executions of transactions. Thus, this type of optimizations could be considered useless, given the importance of preserving the consistency of the database in presence of deferred constraints, unless this complication is resolved in a practical and efficient manner. This is the essence of the “unsolicited deferred consistency constraints validation” mechanism presented in this paper.