The scheduler will play a central role in the NCCS. Its main role is to manage the observing system's resources in an optimum manner. Three different classes of observing programs have been defined in the Requirements & Specifications document. The so-called Master, Routine and Background Observing Programs (MOP, ROP and BOP) classes. The priority of each of these classes is well-defined in the Requirements & Specifications document. The scheduler has to make sure that only one observing program has control of critical resources (e.g. PC-STEER and the telescope) at a time and according to the priorities of each class.
The scheduler proposes to implement the priority in a device server. The fundamental unit of the scheduler is a scan. Different types of observation can have different definitions for a scan. Mapping could define a scan to be a real scan of the sky, spectral line observers would define a scan to be a single integration for example. All clients (observing programs) who want to do a scan or reserve a resource for their exclusive use e.g. a radiometer, must request authorisation from the scheduler. If the scheduler grants authorisation it will return a unique ID number to the client and inform the device server managing the resource which ID is current. The client can then use the resource until it decides to free it or the scheduler gives priority to another client. Clients can ask to be queued and be woken up at a later time. Queued clients are kept in batch file and are woken up using the Unix cron mechanism. This way in the event of a system crash or restart queued clients are automatically rescheduled. The data flow for the scheduler is presented in figure 2.
Tasks still to be done :