CS262A Reading Summary 16

Granularity of Locks and Degrees of consistency in a Shared Data Base (Part 2)

J. N. Gray et al
Summary by Feng Zhou
10/10/2002

1. Four degrees of consistency in concurrency control are formally defined from the user point of view, which covers most of real-world DBMSes. They can be summarized as, from lowest degree (0) to highest degree (3), No-Overwrite, Atomic-Commit, No-Dirty-Read, Repeatable-Read.
2. These four degrees are then mapped to equivalent lock protocol definition, which is from the implementation point of view. They turn out to be four different combinations of well formed or not, two phase or not, regarding read and write respectively.
3. The third view of consistency is: a schedule is degree 1(2 or 3) consistent if and only if the relation <* (<<* or <<<*) is a partial order. This asserts that a consistent schedule of a certain degree should be serializable with transaction dependency corresponding to this consistency degree.

1 flaws:

The schedule model in the paper doesn't address consistency problem in multiprocessing/distributed environment, in which actions cannot be fit into a serial sequence.