In contrast to extended transaction models, TP monitors supporting ACID transactions are a well established technology that has been around for almost 20 years. They provide a general framework for transaction processing, supplying the ``glue'' to bind together the many functional components of a transaction processing system through services like multithreaded processes, interprocess communication, queue management, and system management [Ber90]. Early TP monitors, e.g. IMS/DC, CICS, were single monolithic systems used in proprietary mainframe environments to achieve high transaction rates in large-scale online transaction processing (OLTP) systems, such as airline reservation, electronic banking, or securities trading.
While early TP monitors were proprietary and constructed from single monolithic proprietary systems, modern TP monitors, such as Transarc's Encina, DEC's ACMS, and IBM's CICS/6000, are more modular and constructed from open transaction middleware. These middleware modules provide the basic functional building blocks that a TP monitor requires for transaction processing, such as a Transaction Manager, a Lock Manager, a Log Manager and a Resource Manager (i.e., DBMS). Each module provides its transaction services through a relatively simple and uniform application programming interface (API). The relationships among a transactional application and these basic functional components are depicted in Figure 1. In a commercial setting, we might find a TP Monitor such as Transarc Encina or DEC ACMS providing access to various resource managers, such as an Oracle or Informix relational DBMS.
Although the functional components of modern TP monitors allow users access to system behaviors and transaction services via the API, the size and complexity of the system forms a barrier to users wishing to extend the behavior of the TP monitor. Rather than a smooth progression in the level of complexity and degree of expertise required to perform customization with the available API, there is often a steep learning curve involved in learning all the functions and understanding how they can be combined.
