Abstract: A task queue is structured as a single-keyed indexed file in which the key has a most significant portion indicating a priority level and a less significant portion that is ordered with the loading of the tasks into the queue. For any given task record in the queue, the less significant portion of the key is determinable from a respective task identifier. Preferably the less significant portion of the key is a "time stamp" including the current date when the task was created and a representation of the data processor's internal 24 hour time clock, and a task identification number is formed by appending a node number to the time stamp in the event that the system has multiple processors capable of creating different tasks at the same time. This format of the single key causes an internal ordering of the records in the queue that is sequential with respect to the less significant portion of the key within blocks of records having the same priority.

Abstract: Exactly-once semantics in a transaction processing system are provided by queuing requests for data base transactions on a "task queue" and assigning a unique identification number to each request or "task" placed on the queue. When a requested transaction is performed, the task identification number is written into the data base when the data base is updated at the end of the transaction. To ensure that the updates are made exactly once in the event of a partial system failure or "crash", the data base is read to obtain any preexisting task identification number before any updates are made. If a match occurs between the identification number of the current task and any preexisting task identification number, the requested transaction is terminated without performing any current updates. Automatic recovery with exactly-once semantics therefore occurs as the result of resumed servicing of the task queue.