CS262A Reading Summary 14

On Optimistic Methods for Concurrency Control

H.T. Kung and John T. Robinson
Summary by Feng Zhou
10/6/2002

1. Optimistic concurrency control is a clean and attractive idea. It tries to optimise for the common case, in that most xacts do not conflict. It has the potential to dramatically improve concurrency and elliminate locking overhead for a lot of DB applications.
2. The paper convincingly states the disadvantages of the locking approach of concurrency control. The disadvantages are: large overhead, complexities about deadlocks, low concurrency in case of congested locking, low concurrency by using strict 2PL and locking actually needed in very rare cases.
3. The protocols are presented very clearly and discussed thoroughly. Two versions of validation, serial and parallel, are discussed. Its application in B-Tree insertion and performance analysis is also presented, predicting that optimistic concurrency control will perform well for this particular application.

One major flaw:

Although on average conflicts between xacts are rare. There may be a lot of cases when there are "hot spots" in the database. They didn't mention this kind of situations.