1. For use in a computer system capable of executing multiple transactions concurrently and including a repository of information relating to such transactions, a computer-implemented method of controlling the execution of individual instances of said transactions, said method comprising: retrieving stored information from the repository identifying a transaction instance to be executed either as non-interruptible or as interruptible; in response to an identification of the transaction instance as non-interruptible, initiating execution of said transaction instance; in response to an identification of a task instance as interruptible, retrieving stored information from the repository defining breakpoint conditions under which execution of said transaction instance is to be interrupted, storing said retrieved information in a cache associated to said transaction instance, initiating execution of said transaction instance, monitoring the execution of said transaction instance for the occurrence of a defined breakpoint condition defined in the associated cache, upon occurrence of a defined breakpoint condition, interrupting execution of said transaction instance to permit user-controlled operations to be performed, upon conclusion of the user-controlled operations, updating the associated cache and the repository in accordance with the user-controlled operations, and resuming execution of said transaction instance.