Open Multithreaded Transactions: Keeping Threads and Exceptions under Control

Although transactional models have proved to be very useful for numerous applications, the development of new models to reflect the ever-increasing complexity and diversity of modem applications is a very active area of research. Analysis of the existing models of multithreaded transactions shows that they either give too much fi-eedom to threads und do not control their participation in transactions, or unnecessarily restrict the computational model by assuming that only one thread can enter a transaction. Another important issue, which many models do not address properly, is providing adequate exception handling features. In this paper a new model of multithreaded transactions is proposed. Its detailed description is given, including rules of thread behaviour when transactions start, commit and abort, and rules of exception raising, propagation and handling. This model is supported by enhanced error detection techniques to allow for earlier error detection and for localised recovery. General approaches to implementing transaction support a~ discussed and a detailed description of an Ada implementation is given. Special attention is paid to outlining typical applications for which this model is suitable and to comparing it with several known approaches (Coordinated Atomic actions, CORBA, and Argus).