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Understanding transactions

Understanding transactions

Application programmers benefit from developing their applications on platforms such as Java 2Enterprise Edition (J2EE) that support transactions. A transaction-based system simplifies application development because it frees the developer from the complex issues of failure recovery and multi-user programming. Transactions are not limited to single databases or single sites. Distributed transactions can simultaneously update multiple databases across multiple sites.

A programmer typically divides the total work of an application into a series of units. Each unit of work is a separate transaction. As the application progresses, the underlying system ensures that each unit of work, each transaction, fully completes without interference from other processes. If not, it rolls back the transaction and completely undoes whatever work the transaction had performed.
Characteristics of transactions
Typically, transactions refer to operations that access a shared resource like a database. All access to a database is performed in the context of a transaction. All transactions share the following characteristics:
• Atomicity
• Consistency
• Isolation
• Durability
These characteristics are denoted by the acronym ACID.
A transaction often consists of more than a single operation. Atomicity requires that either all or none of the operations of a transaction are performed for the transaction to be considered complete. If any of a transaction’s operations cannot be performed, then none of them can be performed.
Consistency refers to resource consistency. A transaction must transition the database from one consistent state to another. The transaction must preserve the database’s semantic and physical integrity.
Isolation requires that each transaction appear to be the only transaction currently manipulating the database. Other transactions can run concurrently. However, a transaction must not see the intermediate data manipulations of other transactions until and unless they successfully complete and commit their work. Because of interdependencies among updates, a transaction can get an inconsistent view of the database were it to see just a subset of another transaction’s updates. Isolation protects a transaction from this sort of data inconsistency.
Transaction isolation is qualified by varying levels of concurrency permitted by the database. The higher the isolation level, the more limited the concurrency extent. The highest level of isolation occurs when all transactions can be serialized. That is, the database contents look as if each transaction ran by itself to completion before the next transaction started. However, some applications can tolerate a reduced level of isolation for a higher degree of concurrency. Typically, these applications run a greater number of concurrent transactions even if transactions are reading data that may be partially updated and perhaps inconsistent.
Lastly, durability means that updates made by committed transactions persist in the database regardless of failure conditions. Durability guarantees that committed updates remain in the database despite failures that occur after the commit operation and that databases can be recovered after a system or media failure.
Transaction attributes
EJBs that use bean-managed transaction have transaction attributes associated with each method of the bean. The attribute value tells the container how it must manage the transactions that involve this bean. There are six different transaction attributes that can be associated with each method of a bean. This association is done at deployment time by the Application Assembler or Deployer.
These attributes are:
• Required—This attribute guarantees that the work performed by the associated method is within a global transaction context. If the caller already has a transaction context, then the container uses the same context. If not, the container begins a new transaction automatically. This attribute permits easy composition of multiple beans and co-ordination of the work of all the beans using the same global transaction.
• RequiresNew—This attribute is used when the method does not want to be associated with an existing transaction. It ensures that the container begins a new transaction.
• Supports—This attribute permits the method to avoid using a global transaction. This must only be used when a bean’s method only accesses one transaction resource—or no transaction resources—and does not invoke another enterprise bean. It is used solely for optimization, because it avoids the cost associated with global transactions. When this attribute is set and there is already a global transaction, the EJB Container invokes the method and have it join the existing global transaction. However, if this attribute is set, but there is no existing global transaction, the Container starts a local transaction for the method, and that local transaction completes at the end of the method.
• NotSupported—This attribute also permits the bean to avoid using a global transaction. When this attribute is set, the method must not be in a global transaction. Instead, the EJB Container suspends any existing global transaction and starts a local transaction for the method, and the local transaction completes at the conclusion of the method.
• Mandatory—It is recommended that this attribute not be used. Its behavior is similar to Requires, but the caller must already have an associated transaction. If not, the container throws a javax.transaction.TransactionRequiredException. This attribute makes the bean less flexible for composition because it makes assumptions about the caller’s transaction.
• Never—It is recommended that this attribute not be used. However, if used, the EJB Container starts a local transaction for the method. The local transaction completes at the conclusion of the method.