Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Performance Tuning Guide

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#### Isolation Level Settings

Two settings control the connection pool’s transaction isolation level on the database server:

- Transaction Isolation Level: specifies the transaction isolation level of the pooled database connections. If this parameter is unspecified, the pool uses the default isolation level provided by the JDBC Driver.

- Isolation Level Guaranteed: Guarantees that every connection obtained from the pool has the isolation specified by the Transaction Isolation Level parameter. Applicable only when the Transaction Isolation Level is specified. The default value is true.

  This setting can have some performance impact on some JDBC drivers. Set to false when certain that the application does not change the isolation level before returning the connection.

Avoid specifying Transaction Isolation Level. If that is not possible, consider setting Isolation Level Guaranteed to false and make sure applications do not programmatically alter the connections’ isolation level.

If you must specify isolation level, specify the best-performing level possible. The isolation levels listed from best performance to worst are:

1. READ_UNCOMMITTED

2. READ_COMMITTED

3. REPEATABLE_READ

4. SERIALIZABLE

Choose the isolation level that provides the best performance, yet still meets the concurrency and consistency needs of the application.

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