A main concern rule is an algorithm that evaluates a priority number PNj for all operation j which enters the queue in front of a station (processing, cell, workplace, machine unit). This computation can be based on any of the subsequent quantities:
taj : arrival time in the queue;
oj : duration of the operation;
tdj : due date of the operation (static);
tdj* : due date of the corresponding job;
nj* : number of future operations in the job; and
wj* : remaining work content of the job.
Now, tdj is computed via backwards scheduling from the due date of the job along with constant flow factor.
Priority rules can be categorized into:
- Static or Dynamic Rules, and
- A Priori or a Posteriori Rules.
For a static priori rule, the value of PN is fixed when the job is initiating upto a linear shift consequent to the clock of increment of systems. Dynamic priority rules depend upon the actual evolution of the system and cannot be calculated beforehand with no any simulation. An easy dynamic priority rule is FIFO: PNj = taj; an easy static priority rule is SPT: PNj = oj. But also the slack rule as:
PNj = tdj* - t - wj*,
Here, t "absolute time", is static as per to our definition.
A priori priority rules compute PNj from the information that is available concerning the operation j alone. Absolutely not, a posteriori rules identify the situation that would arise if operation j would be scheduled like the next operation. They produce a comparison of damage that is done to the operations that have to wait additionally, while priority rules only compare the advantage acquired for operation j. Clearly, a posteriori rules cannot be static.
The subsequent rules have been proposed and all were reported to be successful in the literature as: