The design of 'cancellable' data acquisition environments

This paper presents a discussion of the architectural issues resulting when software systems need to cancel operations once they have been initiated. This may seem a minor issue, but our experience is that this requirement can have a huge effect on the design of instrumental software environments. A number of major constraints on the structure of command-based environments such as the AAO's DRAMA system can be traced to the perceived need to be able to cancel any operation cleanly. This becomes particularly difficult to implement if these operations involve significant amounts of time or even potentially indefinite amounts of time, such as operations involving blocking I/O. In general, the cleanest results come from having a process or thread cancel itself, rather than relying on the ability to cancel it externally, but this turns the problem into one of finding mechanisms whereby processes can discover, reliably, that they need to cancel themselves. As system architectures are considered for the next generation of telescopes, it seems timely to consider these design problems and even to what extent the ideal requirement of cleanly cancellable operations may have been reduced by the move towards queue-scheduled operations and away from traditional interactive observing.