A business transaction is a self-contained business deal. Some transactions involve a short conversation (for example, an address change). Others involve multiple actions that take place over an extended period (for example, the booking of a trip, including car, hotel, and airline tickets).

A single transaction might consist of many application programs that carry out the processing needed. Large-scale transaction systems (such as the IBM® CICS® product) rely on the multitasking and multithreading capabilities of z/OS® to allow more than one task to be processed at the same time, with each task saving its specific variable data and keeping track of the instructions each user is executing.

Multitasking is essential in any environment in which thousands of users can be logged on at the same time. When a multitasking transaction system receives a request to run a transaction, it can start a new task that is associated with one instance of the execution of the transaction; that is, one execution of a transaction, with a particular set of data, usually on behalf of a particular user at a particular terminal. You might also consider a task to be analogous to a UNIX® thread. When the transaction completes, the task is ended.

Multithreading allows a single copy of an application program to be processed by several transactions concurrently. Multithreading requires that all transactional application programs be reentrant; that is, they must be serially reusable between entry and exit points. Among programming languages, reentrance is ensured by a fresh copy of working storage section being obtained each time the program is invoked.