Concurrency on high-traffic data elements

摘要

In many large database applications there are certain elements mostly containing aggregate information, which are very frequently referred to (read and modified) by many transactions. If access to such fields has to obey to conventional two-phase lock protocols (1,2), transactions will be serialized in front of these "hot spots", i.e. the degree of parallelism is reduced. To avoid this kind of lock contention some improved lock protocols have been proposed, the most interesting of which is the one implemented in IMS Fast Path (3,4), where add and subtract may be performed concurrently on numerical fields, since backout is always possible with the unique inverse of each operand. A similar scheme is proposed in (10). We expand this idea to parallel readers and writers on numerical data types, proving that under certain conditions the result of such concurrent operations is consistent in the sense that it is equal to some serial schedule (2,5).