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J2EE servers are required only to implement the flat transaction model. This model is sufficient in situations where the transactional resources are centralized and under the control of one party. The ACID properties of such transactions can be maintained without compromise. This transactional model is adequate for Web service applications with short-running transactions and those in which transactions do not span organizational boundaries.

The flat transaction mode is inadequate to address the transactional needs of some business transactions in Web services, which frequently involve the assembly of disparate services. These disparate services may have been written with their own, autonomous transaction boundaries and may control their own resources. Web services for business-to-business integration can also be long-running, causing inefficiency as a result of the locking mechanism used for achieving isolation in traditional transaction models.

Relaxing the isolation property of transactions and allowing subtransactions to make visible state-change effects before the larger transaction completes solves the problem of inefficient resource locking in long-running transactions. Allowing a transaction to complete even if only a subset of subtransactions completes (i.e., relaxing the atomic property) allows a business transaction to be composed based on application needs.

The BTP, WS-TX, and Activity Service specifications address the needs of coordinating long-running business transactions that span autonomous, loosely coupled services while accommodating the needs of short-running, centralized transactions. BTP specifies the working of atomic and cohesive transactions, and WS-TX describes the atomic and business activity patterns for coordination. The Activity Service submission to OMG also provides a specification for business transactions but does not reference XML-based messages specifically. (Because Activity Service is based on OMG OTS, it does provide IDL specifications.)

Two JSRs, JSR-95 and JSR-156 propose an extended transaction model for the J2EE platform. As of now, no implementations of BTP or Activity Service transaction managers work seamlessly with J2EE servers. It will be some time before the relevant JSRs are fleshed out and we see servers with embedded, standards-based transaction support for extended transaction models. Until then, our current JTA- and JTS-based flat transaction managers will have to suffice.

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