实时Client/Server数据库并发控制和恢复机制研究

该文提出了实时Client／Server数据库系统多版本两阶段封锁并发控制协议和有效的恢复机制。协议区分只读事务和更新事务。只读事务在执行读操作时遵从多版本时间排序协议，更新事务执行强两阶段封锁协议，即持有全部锁直到事务结束。只读事务读请求从不失败，不必等待等特性。在典型数据库系统中，读操作比写操作频繁。这个特性对于实践来说至关重要。为了提高只读事务的响应时间，协议让每个客户端与一个一致数据库影子相联，只读事务在客户端处理。更新事务提交到服务端运行。服务端每个事务Ti在提交时系统必须向所有客户端广播信息。客户端根据得到的广播信息自动构造一致数据库影子。一致数据库影子还将用于系统恢复。通过仿真模拟。与2V2PL和OCC-TI-WAIT-50协议进行比较，结果表明：该并发控制协议不仅能有效降低事务延误截止时间率和重起动率，而且能改善只读事务的响应时间，减少优先级高事务的锁等待时间。协议性能优于2V2PL协议和OCC-TI-WAIT-50协议。

A Study of Concurrency Control and Recovery in Real-Time Client/Server Database Systems

This article presents multiversion two-phase locking concurrency control protocol and efficient recovery mechanism for real-time client/server database systems.The protocol differentiates between read-only transactions and update transaction.Read-only transactions follow the multiversion timestamp ordering protocol.Update transactions perform rigorous two-phase locking;that is,they hold all locks up to the end of the transaction.Thus,they can be serialized according to their commit order.The read request of read-only transaction never fails and is never made to wait.In typical database systems,where reading is a more frequent operation than writing is,this advantage may be of major practical significance.To boost the response time of read-only transactions,the protocol associates each client-side with a consistent database image and processes read-only transactions in client-side.All updating transactions are submitted to the server-side system for execution.Each server transaction T must broadcast message to all client-side system.Each of the client-side systems autonomously fabricates the consistent database image.The client-side database images will be used for efficient failure recovery.The protocol presented is compared with 2V2PL protocol and OCC-TI-WAIT-50 protocol by simulation experiments.The experiments show that the protocol can not only effectively reduce miss rate of transaction,but also significantly improve the response time of client-side read-only transactions and the blocking time of higher-priority.