Hard constraints vs. soft constraints

What is the difference between "hard" and "soft" constraints?

Hard constraints

Hard constraints are constraints that are absolutely non-negotiable. The scheduling engine will always respect every hard constraint that you give it. When you specify a hard constraint, you're effectively telling the scheduler that "If I can't have this constraint met, then I don't want any schedule at all."

In many cases, hard constraints also make the scheduler run slower, which also decreases the likelihood that it will find any schedules for you within the its computation window.

Soft constraints

Soft constraints, on the other hand, are constraints that could be negotiated once in awhile. The scheduling engine still works hard to respect every soft constraint that you give it, but will make an exception on the constraint in the rare case that something must give. When you specify a soft constraint, you're effectively telling the scheduler that "If I can't have this constraint met, I suppose it's okay--just do the best you can. I'd rather have a near-perfect schedule than no schedule at all."

Soft constraints *never* slow the scheduler down, which also increases the likelihood that it will find schedules for you within its computation window.

Which type should I use, and when?

Try to minimize the number of hard constraints that you use. Hard constraints usually make the scheduler run slower, which also decreases the likelihood that it will find any schedules for you within the its computation window.

You're encouraged to use as many soft constraints as you like. There is no computational penalty for using more of them.

Comparison

Hard constraints	Soft constraints
Absolutely non-negotiable	Negotiable if necessary
Always respected by the scheduler	Usually respected by the scheduler
Exceptions are never made	Exceptions are made if it's the difference between generating a schedule and not generating one
Usually make the scheduler run slower	No impact on scheduler speed
Decrease likelihood that the scheduler will find any schedules	Likelihood of finding schedules not affected
Too many hard constraints may mean not getting any schedules	No penalty for having lots and lots of soft constraints
Examples: Student "Must Not" availability entries Therapist "Must Not" availability entries “Must” Groupings “Must Only” Groupings “Must Not” Groupings Therapist travel time between buildings Therapist Locked to Building Therapist Start Time Therapist End Time Maximum Group Size Maximum Grade Span Student Start Time Student End Time Buildings Categories Days Apart Delivery Model Exact Time Spans	Examples: Student "Prefer Not" availability entries Student "Prefer" availability entries “Prefer” Groupings “Prefer Only” Groupings “Prefer Not” Groupings Therapist Day Start Buffer Therapist Day End Buffer Therapist Lunchtime Therapist Post-Session Recovery Session Goals Anything else you set preferences for: Productivity Session Cost Billing Cost Behavior Cost Grade Span Cost Classroom Count Cost Goal Count Cost Travel Trip Cost Travel Time Cost

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