Glossary · System coordination, integration and orchestration
Priority rule
Also known as: Dispatching rule, Scheduling rule
German: Prioritätsregel
In production scheduling and automation, a priority rule is a rule that decides which of several waiting jobs, requests or tasks is served next by a resource, for example first in, first out, earliest due date or shortest processing time.
- System integration
In one sentence
A priority rule decides which waiting job, request or task a resource serves next, e.g. FIFO, earliest due date or shortest processing time.
Example
The dispatcher of a machining cell uses earliest due date as its priority rule, but moves rush orders flagged by the MES to the front of the queue.
How it applies
- Engineering: Priority rules appear at several levels: order dispatching in the MES, request arbitration in a line controller and task scheduling in a real-time operating system. Choose rules that match the goal (throughput, due-date adherence, setup reduction) and document the tie-breakers.
- Operation: Simple rules are predictable and easy to explain to operators; combined or weighted rules can perform better but make decisions harder to follow. Log why a job was chosen when decisions are disputed.
- Risk: Strict priorities can starve low-priority requests. Add aging or a maximum waiting time where every request must eventually be served.
- Documentation: State the priority rules in the functional specification and, where operators can influence them, in the operating instructions (for example how to mark a rush order).
Priority rule vs. queue discipline
A Queue discipline describes the general order in which a queue is served. A priority rule is a specific criterion that implements such a discipline; FIFO is both a queue discipline and a very simple priority rule.