Publisher Summary This chapter focuses on the transaction management. The notion of a transaction is fundamental to business systems architectures. With the advent of Web service architectures, distributed applications (macroservices) are being built by assembling existing, smaller services (microservices). The resulting complex architectures introduce new challenges to existing transaction models. A transaction may be thought of an interaction with the system, resulting in a change to the system state. While the interaction is in the process of changing system state, any number of events can interrupt the interaction, leaving the state change incomplete and the system state in an inconsistent, undesirable form. Any change to system state within a transaction boundary is to ensure that the change leaves the system in a stable and consistent state. 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 flat transaction mode is inadequate to address the transactional needs of some business transactions in Web services that involve the assembly of disparate services. 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..