Oracle Concepts - 9 Data Concurrency and Consistency

Transactions and Concurrency

- Multiuser database must provide the following:

• Data concurrency, which ensures that users can access data at the same time
• Data consistency, which ensures that each user sees a consistent view of the data, including visible changes made by the user's own transactions and committed transactions of other users

- Serializability transaction isolation model

- Oracle Database never permits dirty reads, which occur when a transaction reads uncommitted data in another transaction.

- Read consistency is guaranteed in single-instance and Oracle Real Application Clusters (Oracle RAC) environments. Oracle RAC uses a cache-to-cache block transfer mechanism known as Cache Fusion to transfer read-consistent images of data blocks from one database instance to another.

- Read Consistency in the Read Committed Isolation Level

- Locks are mechanisms that prevent destructive interaction between transactions accessing the same resource.

Preventable Read Phenomena by Isolation Level

Isolation Level	Dirty Read	Nonrepeatable Read	Phantom Read
Read uncommitted	Possible	Possible	Possible
Read committed	Not possible	Possible	Possible
Repeatable read	Not possible	Not possible	Possible
Serializable	Not possible	Not possible	Not possible