Repairing Optimistic Concurrency Control

Abstract

Optimistic concurrency control means end-of-transaction validation of read- and write-sets or of timestamps attached to database items. Optimistic concurrency control suffers from multiple problems the present chapter attempts to remedy. First, concurrent validation of multiple transactions requires communication among them in order to detect conflicting database accesses. Second, two-phase commit requires that all local participants prepare themselves to abide by the global coordinator’s commit decisions, which means that other transactions must not read or write database items in conflict with a local participant of a committing distributed transaction. Third, the danger of premature publication of updates committed but not yet durable requires that the atomic phase at the end of each optimistic transaction must include not only validation and propagation of updates from the transaction-private update buffer to the shared buffer pool but also forcing the transaction’s commit log record to stable storage.

The remedies proposed for the problems above are perhaps radical but definitely simple. They employ locks and a lock manager for commit processing in optimistic concurrency control. The remedy for the last problem relies on controlled lock violation, which prior research has introduced and considered only for pessimistic concurrency control.