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Understanding transactions
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Isolation levels
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The concept of isolation as it pertains to databases is not as cut and dried as we just suggested. As you might imagine, making transactions wait for one another s data locks limits the number of concurrent transactions that can run on a system. However, different isolation strategies allow for a balance between concurrency and locking, primarily by sacrificing lock acquisition strictness. Each isolation strategy corresponds to an isolation level. Here are the four most common isolation levels, from the highest level of concurrency to the lowest:
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Read uncommitted At this isolation level, your transaction can read the uncommitted data of other transactions, also known as a dirty read. You should not use this level in a multithreaded environment. Read committed Your transaction will never read uncommitted changes from another transaction. This is the default level for most databases. Repeatable read The transaction is guaranteed to get the same data on multiple reads of the same rows until the transaction ends. Serializable This is the highest isolation level and guarantees that none of the tables you touch will change during the transaction, including adding new rows. This isolation level is very likely to cause performance bottlenecks.
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A good rule of thumb is to use the highest isolation level that yields an acceptable performance level. Generally, you do not directly control isolation levels from EJBs the isolation level is set at the database resource level instead.
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error changes can be reverted to be properly reapplied (think of untangling a cassette tape and rewinding it to where the tape started tangling). Changes made during the transaction are applied again by executing the appropriate entries from the transaction log (replaying the rewound tape to finish). This property is the muscle behind transactions ensuring that commit really does mean commit. In the next section, we ll examine the internals of transaction management and define such concepts as distributed transactions, transaction managers, and resource managers.
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6.1.3 Transaction management internals
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As you have probably already guessed, application servers and enterprise resources like the database management system do most of the heavy lifting in transaction management. Ultimately, everything that you do in code translates into low-level database operations such as locking and unlocking rows or tables in a database,
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Transactions and security
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beginning a transaction log, committing a transaction by applying log entries, or rolling back a transaction by abandoning the transaction log. In enterprise transaction management, the component that takes care of transactions for a particular resource is called a resource manager. Remember that a resource need not just be a database like Oracle. It could be a message server like IBM MQSeries or an enterprise information system (EIS) like PeopleSoft CRM. Most enterprise applications involve only a single resource. A transaction that uses a single resource is called a local transaction. However, many enterprise applications use more than one resource. If you look carefully at our Snag-It order example, you ll see that it most definitely involves more than one database: the credit card provider s database used to charge the customer, as well as the ActionBazaar database to manage bids, items, and ordering. It is fairly apparent that for sane business application development some kind of abstraction is needed to manage multiple resources in a single transaction. This is exactly what the transaction manager is a component that, under the hood, coordinates a transaction over multiple distributed resources. From an application s view, the transaction manager is the application server or some other external component that provides simplified transaction services. As figure 6.2 shows, the application program (ActionBazaar) asks the transaction
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Figure 6.2 Distributed transaction management. The application program delegates transaction operations to the transaction manager, which coordinates between resource managers.
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