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  Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Row-Level Locks

These row locks or row-level locks are stored in the block, and each lock refers to the global transaction lock. As in the case of a single instance Oracle, the RAC controls concurrency down to the row level. The finest lock granularity is at the row level.

However, to keep the cache coherent, access to the data blocks is controlled by the GCS. This has no effect on the row-level lock. GCS resources and row locks operate independently of the GCS. An instance can request or ship the data block to another instance in the cluster without affecting the row-level locks that are held inside the data block. The row-level lock is fully controlled by the transaction that causes the row-level lock. When the transaction commits or rolls back, the row-level lock is released. In the meantime, if another transaction intends to update the same row, it has to wait until the initial transaction commits or rolls back.

The row lock method has an important advantage in maintaining data consistency, even if there are multiple instances, as in the RAC system. The behavior of the row lock and the release is the same, whether it is a single stand-alone database or a multi-instance RAC system. During the row lock period, even if the data block gets transferred to another instance, the row lock remains intact until released.

The above text is an excerpt from:

Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
ISBN 0-9744355-4-6

by Mike Ault, Madhu Tumma


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