Oracle Parallel Processing

Lock Types in OPS

Oracle Parallel Server uses different types of locks to control access to database resources by an instance. The following sections briefly describe the lock types used in a parallel server environment and contrast them with the types used in a standalone database.

Transaction Locks

An Oracle database (whether standalone or parallel server) allows multiple transactions to access the same data concurrently. Without proper control, this access would result in improper modifications and might destroy data consistency. Oracle maintains data concurrency and consistency using locks. Some locks prevent destructive interaction between simultaneous transactions that are accessing the same data. These locks are called transaction locks. Transaction locks are row-level locks, and they protect rows of data from being concurrently modified by more than one transaction. Row-level locking works identically in standalone Oracle (single-instance) and Oracle Parallel Server (multiple-instance) configurations.

Instance Locks

Parallel server environments have an additional need for locks to maintain cache coherency. However, the locks used to maintain cache coherency are entirely different from transaction locks and are known as instance locks. Because instance locks are managed globally across all instances, some people refer to them as global locks. As discussed in Chapter 6, instance locks and inter-instance locking activities are coordinated by the Integrated Distributed Lock Manager ...