he purpose of this article is to provide a high-level overview of the Java Transaction API (JTA), and how it is related to distributed transactions. A transaction defines a logical unit of work that either completely succeeds or produces no result at all. A distributed transaction is simply a transaction that accesses and updates data on two or more networked resources, and therefore must be coordinated among those resources. In this paper, we are concerned primarily with transactions that involve relational database systems.

The components involved in the distributed transaction processing (DTP) model that are relevant to our discussion are:

• The application
  
  
• The application server
  
  
• The transaction manager
  
  
• The resource adapter
  
  
• The resource manager

In the following sections, we describe these components and their relationship to JTA and database access.

Accessing Databases
It is best to think of the components involved in distributed transactions as independent processes, rather than thinking of them in terms of location on a particular computer. Several of the components may reside on one machine, or they may be spread among several machines. The diagrams in the following examples may show a component on a particular computer, but the relationship among the processes is the primary consideration.

The Simplest Case: Application to Database Local Transactions
The simplest form of relational database access involves only the application, a resource manager, and a resource adapter. The application is simply the end-user access point to send requests to, and obtain data from, a database.

The resource manager in our discussion is a relational database management system (RDBMS), such as Oracle or SQL Server. All of the actual database management is handled by this component.

The resource adapter is the component that is the communications channel, or request translator, between the "outside world," in this case the application, and the resource manager. For our discussion, this is a JDBC driver.

The following description is of a resource manager local transaction, that is, one transaction that is confined to a single, specific enterprise database.

The application sends a request for data to the JDBC driver, which then translates the request and sends it across the network to the database. The database returns the data to the driver, which then translates the result to the application, as illustrated in the following diagram:

This example illustrates the basic flow of information in a simplified system; however, the enterprise of today uses application servers, which adds another component to the process.

Application Servers
The application server is another component of the transaction process that is addressed by the JTA. Application servers handle the bulk of application operations and take some of the load off of the end-user application. Building on the preceding example, we see that the application server adds another process tier to the transaction:

Up to this point, our examples illustrate a single, local transaction and describe four out of the five components of the distributed transaction model. The fifth component, the transaction manager, comes into consideration only when transactions are to be distributed.

Distributed Transactions and the Transaction Manager

In this Article
Introduction	Distributed Transactions and the Transaction Manager