Load balancing and fairness

Load balancing is a common constraint for many Timefold Solver use cases. Especially when scheduling employees, the workload needs to be spread out fairly; there may even be legal requirements to that effect.

1. What is fair?

Fairness is a complex concept, and it is not always clear what is fair. For example, let’s assign 15 undesirable tasks to five employees. Each task takes a day, but the tasks have different skill requirements. In a perfectly fair schedule, each employee will get three tasks, because the average is 15 / 5 = 3.

Unfortunately, this doesn’t solve the entire problem, because there are other constraints. For example, there are seven tasks that require a skill which only two of the employees possess. One of them will have to do at least four tasks.

From the above, we can see that our schedule cannot be perfectly fair, but we can make it as fair as possible.

1.1. Which is fairer?

We can define fairness in two opposing ways:

  • A schedule is fair if most users think it is fair to them.

  • A schedule is fair if the employee with most tasks has as few tasks as possible.

Since we want to treat all employees equally, the second definition is correct. Besides, if we make almost everyone happy by letting one employee do all the work, that employee would probably quit and that doesn’t help.

Let’s look at a few different solutions of the same dataset, sorted from most to least fair. Each one has 15 tasks:

Ann Beth Carl Dan Ed Solution Quality

Schedule A

3

3

3

3

3

Most fair

Schedule B

4

4

3

2

2

Schedule C

5

3

3

2

2

Schedule D

5

5

2

2

1

Schedule E

6

3

3

2

1

Schedule F

5

6

2

1

1

Schedule G

11

1

1

1

1

Most unfair

Ann has the most tasks each time. How do we compare schedules in which Ann has the same number of tasks?

Ann Beth Carl Dan Ed Solution Quality

Schedule C

5

3

3

2

2

More fair

Schedule D

5

5

2

2

1

Less fair

We take a look at the second most loaded employee, Beth. In schedule D, she has five tasks, while in schedule C, she has less. This makes schedule C fairer overall.

This is the definition of fairness we use in the remainder of this section.

2. Fairness constraints

Timefold Solver supports fairness constraints out of the box through the use of grouping and the load balancing constraint collector. In your ConstraintProvider implementation, a load-balancing constraint may look like this:

Constraint fairAssignments(ConstraintFactory constraintFactory) {
    return constraintFactory.forEach(ShiftAssignment.class)
            .groupBy(ConstraintCollectors.loadBalance(ShiftAssignment::getEmployee))
            .penalizeBigDecimal(HardSoftBigDecimalScore.ONE_SOFT, LoadBalance::unfairness)
            .asConstraint("fairAssignments");
}

When using fairness constraints, we recommend that you use one of the score types based on BigDecimal, such as HardSoftBigDecimalScore. See below for a detailed rationale.

This constraint will penalize the solution based on its unfairness, defined in this case by how many ShiftAssignment are allocated to any particular Employee.

Unfairness is a dimensionless number that measures how fair the solution is. A lower value indicates a fairer solution. The smallest possible value is 0, indicating that the solution is perfectly fair. There is no upper limit to the value, and it is expected to scale linearly with the size of your dataset. The value is rounded to six decimal places.

Different unfairness values may only be compared for solutions coming from the same dataset. Unfairness values computed from different datasets are incomparable.

2.1. Why BigDecimal?

When using fairness constraints, we recommend that you use one of the score types based on BigDecimal. We base our recommendation on the following reasons:

  • The unfairness value is a rational number. The corresponding floating-point type (double) is not suitable for score calculations.

  • Rounding the unfairness value to the nearest integer would lose precision, causing a score trap. Even a small difference in unfairness (such as between 1.000001 and 1.000002) gives the solver crucial information to distinguish different solutions from each other.

If the somewhat worsened performance of a BigDecimal-based score calculation is a concern, instead, you may decide to multiply the unfairness value by some power of ten to preserve precision, and only then round it to the nearest integer. This allows you to use the simple integer-based score types.

Unfortunately, this also requires you to re-balance your constraint weights, multiplying them by the same power of ten as used for unfairness. Otherwise, the unfairness constraint will dominate the score, effectively becoming the most important constraint.

If you take this approach, we recommend that you preserve all six decimal places of the unfairness value to avoid the most severe score traps. This will leave considerably less space in the selected data type for actual values, which go from x to x × 1_000_000. This may entirely preclude you from using the 32-bit int, and reduce the usefulness of the 64-bit long.

We default to BigDecimal to help users avoid the pitfalls described above, while still allowing advanced users to choose other score types once they understand the risks.