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  Oracle Database Tips by Donald Burleson

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

Concurrency and Consistency

Database systems provide data concurrency by enabling multiple users to access the same data without compromising data consistency. Data consistency means that each user sees a consistent view of the data, including the visible changes made by the users' own transactions, as well as the transactions of other users. Oracle automatically supplies a query with read-consistent data, so that all data that the query sees comes from a single point in time (statement-level read consistency).

Optionally, Oracle can provide read consistency to all queries in a transaction (transaction-level read consistency). Oracle maintains undo records to manage such consistent views. The undo segments contain the old data values that have been changed by uncommitted or recently committed transactions.

In a RAC system, users can connect with multiple instances to run database queries. Typically, users will be connected to different nodes but access the same set of data or data blocks. This situation demands that the data consistency, formerly confined to a single instance, be effectively extended to multiple instances. Therefore, buffer cache coherence from multiple instances must be maintained.

Instances require three main types of concurrency:

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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