Using a @PlanningId notation on an Instant @PlanningVariable

I am currently working on setting up a gantt-based planification problem, where a user can choose which tasks they want to plan, and OptaPlanner would do it for them.

I use incremental Java score calculation, and not the drools engine. My issue is that OptaPlanner won't take an Instant as a planning variable, as it isn't able to find a PlanningId for it.

I've been stuck on getting OptaPlanner to use multiple threads. My current model seems to be flawed, or I am not understanding how to use OptaPlanner properly.

I tried masking the Instant behind another class, but it still did not help.

My model uses only one PlanningEntity, which is a task.

Here's a simplified version of my @PlanningEntity :

@PlanningEntity(difficultyComparatorClass = TaskDifficultyComparator.class)
public class Task extends AbstractTask {
    private Machine machine;
    private Instant start;

    @PlanningId
    private Integer id;

    @PlanningVariable(valueRangeProviderRefs = {"machineRange"}, nullable = true, strengthComparatorClass = MachineStrengthComparator.class)
    public Machine getMachine() {
        return machine;
    }

    @PlanningVariable(valueRangeProviderRefs = {"timeRange"}, nullable = true, strengthComparatorClass = StartStengthComparator.class)
    public Instant getStart() {
        return start;
    }
}

In my config, I have this added to the solver tag :

<moveThreadCount>AUTO</moveThreadCount>

This gives me an exception:

Exception in thread "Thread-6" java.lang.IllegalStateException: The move thread with moveThreadIndex (0) has thrown an exception. Relayed here in the parent thread.
    at org.optaplanner.core.impl.heuristic.thread.OrderByMoveIndexBlockingQueue.take(OrderByMoveIndexBlockingQueue.java:142)
    at org.optaplanner.core.impl.localsearch.decider.MultiThreadedLocalSearchDecider.forageResult(MultiThreadedLocalSearchDecider.java:187)
    at org.optaplanner.core.impl.localsearch.decider.MultiThreadedLocalSearchDecider.decideNextStep(MultiThreadedLocalSearchDecider.java:157)
    at org.optaplanner.core.impl.localsearch.DefaultLocalSearchPhase.solve(DefaultLocalSearchPhase.java:70)
    at org.optaplanner.core.impl.solver.AbstractSolver.runPhases(AbstractSolver.java:87)
[...]
Caused by: java.lang.IllegalArgumentException: The externalObject (2019-04-16T20:31:17.162Z) cannot be looked up.
Maybe give the class (class java.time.Instant) a PlanningId annotation or change the PlanningSolution annotation's LookUpStrategyType or don't rely on functionality that depends on ScoreDirector.lookUpWorkingObject().
    at org.optaplanner.core.impl.domain.lookup.NoneLookUpStrategy.lookUpWorkingObject(NoneLookUpStrategy.java:47)
    at org.optaplanner.core.impl.domain.lookup.LookUpManager.lookUpWorkingObject(LookUpManager.java:74)
[...]

I expected OptaPlanner to use the tasks' ID, but it seems like it wants an id on each of the PlanningVariables. I am able to add an ID on the Machine, but not on the Instant.

Solution

A java.time.Instant is immutable, so any lookup can just return the same object instance. Just like Integer, Double, LocalDate, etc, there is no need for a @PlanningId to begin with. This exposes 3 issues in OptaPlanner:

LookUpStrategyResolver.java's build-in decision cache must also include Instant. I've fixed this issue in this PR for 7.20.
It should be possible to configure extra immutable classes.
It should be possible to configure @PlanningId externally on 3th party classes.

Please create a jira for 2. and 3. on issues.jboss.org project PLANNER.