> The hardest part I have found with queue design is task cancellation. How does one 'cancel' tasks that are already in the queue or being processed?
That really depends on the queue implementation. If your queue is a DB table and you have identifiers for the items added to the queue that are mapped to a column in the DB, cancelling a queued item is just updating the appropriate column to flag the item as cancelled or deleting it outright (cancelling an in process item is usually more problematic, because its no longer on the queue, its with a worker. More robust engines that include more than just queue functionality may handle this -- but in general it requires that workers hand something back to the engine which then applies any permanent state changes, so that if the engine gets a cancellation it can just not make those changes; it could work if all changes also had a corresponding compensation mechanism so that if a work item was cancelled after some work was committed related to it, the appropriate compensation could be done.)
That really depends on the queue implementation. If your queue is a DB table and you have identifiers for the items added to the queue that are mapped to a column in the DB, cancelling a queued item is just updating the appropriate column to flag the item as cancelled or deleting it outright (cancelling an in process item is usually more problematic, because its no longer on the queue, its with a worker. More robust engines that include more than just queue functionality may handle this -- but in general it requires that workers hand something back to the engine which then applies any permanent state changes, so that if the engine gets a cancellation it can just not make those changes; it could work if all changes also had a corresponding compensation mechanism so that if a work item was cancelled after some work was committed related to it, the appropriate compensation could be done.)