116 Primary interfaces and classes interfaces of components,
Transaction management Effective transaction management is a critical aspect of object persistence. This chapter discusses JDO transactions in the context of the non-managed environment. Chapter 11, contains in-depth discussions of transaction management in the managed environment. 7.1 Transactions A transaction is a grouping together of work typically changes to persistent data that must be completed in its entirety or not at all. The fundamental aim of a transaction is to ensure that partial completion, where some data changes persist in the data store and others in the same transaction don t, does not occur. This so-called atomic property of transactions is integral to the maintenance of data integrity. Another fundamental property of transactions is that they must be isolated, to a given degree, from each other. Isolation levels dictate the consistency with which data, being manipulated in one transaction, is presented, in the context of another transaction. Experienced database programmers will be familiar with the four transaction isolation levels that are recognized by JDBC: Read Uncommitted, Read Committed, Repeatable Read, and Read Serializable. Some database products, particularly object databases, define much richer sets. JDO does not explicitly specify the isolation level that will be applied. instances may have fields read at different times and, conceivably, from different data sources. Developers should not rely on any isolation level greater than Read Committed. For reference, Read Committed is defined as: This isolation level does not allow other transactions to see state changes made by this transaction until a commit has been issued. Nothing in JDO actually specifies that the locking of data in the data store should take place. Locking strategies do, however, account for the most common method of implementing the Read Committed level of isolation.
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