OptaPlanner - Explain score of non-optimal solutions - optaplanner

Question

We have a use-case where we want to present the user with some human-readable message with why an "assignment" was rejected based on the score of the constraints.
For e.g. in the CloudBalancing problem with 3 computers (Computer-1,2,3) and 1 process (Process-1) we ended up with the below result:
Computer-1 broke a hard constraint (requiredCpu)
Computer-2 lost due to a soft constraint (min cost)
Computer-3 assigned to Process-1 --> (Optimal solution)
We had implemented the BestSolutionChanged listener where we used solution.explainScore() to get some info and enabled DEBUG logging which provided us the OptaPlanner internal logs for intermediate moves and their scores. But the requirement is to provide some custom human readable information on why all the non-optimal solutions (Computer-1, Computer-2) were rejected even if they were infeasible (basically explanation of scores of these two solutions).
So wanted to know how can we achieve the above ?
We did not want to rely on listening to BestSolutionChanged event as
it might not get triggered for other solutions if the LS/CH
phase starts with a solution which is already a "best solution"
(Computer-3). Is this a valid assumption ?
DEBUG logs do provide us with the
information but building a custom message from this log does not seem
like a good idea so was wondering if there is another
listener/OptaPlanner concept which can be used to achieve this.

Answer 1

By "all the non-optimal solutions", do you mean instead a particular non-optimal solution? Search space can get very large very quickly, and OptaPlanner itself probably won't evaluate the majority of those solutions (simply because the search space is so large).
You are correct that BestSolutionChanged event will not fire again if the problem/solution given to the Solver is already the optimal solution (since by definition, there are no solutions better than it).
Of particular interest is ScoreManager, which allows you to calculate and explain the score of any problem/solution:
(Examples taken from https://www.optaplanner.org/docs/optaplanner/latest/score-calculation/score-calculation.html#usingScoreCalculationOutsideTheSolver)
To create it and get a ScoreExplanation do:
ScoreManager<CloudBalance, HardSoftScore> scoreManager = ScoreManager.create(solverFactory);
ScoreExplanation<CloudBalance, HardSoftScore> scoreExplanation = scoreManager.explainScore(cloudBalance);
Where cloudBalance is the problem/solution you want to explain. With the
score explanation you can:
Get the score
HardSoftScore score = scoreExplanation.getScore();
Break down the score by constraint
Collection<ConstraintMatchTotal<HardSoftScore>> constraintMatchTotals = scoreExplanation.getConstraintMatchTotalMap().values();
for (ConstraintMatchTotal<HardSoftScore> constraintMatchTotal : constraintMatchTotals) {
String constraintName = constraintMatchTotal.getConstraintName();
// The score impact of that constraint
HardSoftScore totalScore = constraintMatchTotal.getScore();
for (ConstraintMatch<HardSoftScore> constraintMatch : constraintMatchTotal.getConstraintMatchSet()) {
List<Object> justificationList = constraintMatch.getJustificationList();
HardSoftScore score = constraintMatch.getScore();
...
}
}
and get the impact of individual entities and problem facts:
Map<Object, Indictment<HardSoftScore>> indictmentMap = scoreExplanation.getIndictmentMap();
for (CloudProcess process : cloudBalance.getProcessList()) {
Indictment<HardSoftScore> indictment = indictmentMap.get(process);
if (indictment == null) {
continue;
}
// The score impact of that planning entity
HardSoftScore totalScore = indictment.getScore();
for (ConstraintMatch<HardSoftScore> constraintMatch : indictment.getConstraintMatchSet()) {
String constraintName = constraintMatch.getConstraintName();
HardSoftScore score = constraintMatch.getScore();
...
}
}