Commit_LSN: A Novel and Simple Method for Reducing Locking and Latching in Transaction Processing Systems

摘要

This paper presents a novel and simple method, called Commit_LSN, for determining if a piece of data is in the committed state in a transaction processing system. This method is a much cheaper alternative to the locking approach used by the prior art for this purpose. The method takes advantage of the concept of a log sequence number (LSN). In many systems, an LSN is recorded in each page of the data base to relate the state of the page to the log of update actions for that page. Our method uses information about the LSN of the first log record (call it CommitLSN) of the oldest update transaction still executing in the system to infer that all the updates in pages with pageLSN less than Commit_LSN have been committed. This reduces locking and latching. In addition, the method may also increase the level of concurrency that could be supported. The Commit_LSN method makes it possible to use fine-granularity locking without unduly penalizing transactions which read numerous records. It also benefits update transactions by reducing the cost of fine-granularity locking when contention is not present for data on a page. We discuss in detail many applications of this method and illustrate its potential benefits for various environments. In order to apply the Commit_LSN method, extensions are also proposed for those systems in which (1) LSNs are not associated with pages (AS/400,SOL/DS, System R), (2) LSNs are used only partially (IMS), and/or (3) not all objects' changes are logged (AS/400, SOL/DS, System R).