An exception "The source path (previousJob.endTime) starting from root entity class (JobForward) does not end on a variable

[kentbill]

Hi, Teams.

I'm using the DSV in my program and got an exception below, the timefold version 1.27.0.
What is the cause for this? What does "The source path does not end on a variable." mean?
And what direction should I think to solve it?

java.lang.IllegalArgumentException: The source path (previousJob.endTime) starting from root entity class (JobForward) does not end on a variable.
at ai.timefold.solver.core.impl.domain.variable.declarative.RootVariableSource.from(RootVariableSource.java:192)
at ai.timefold.solver.core.impl.domain.variable.declarative.DeclarativeShadowVariableDescriptor.linkVariableDescriptors(DeclarativeShadowVariableDescriptor.java:114)
at ai.timefold.solver.core.impl.domain.entity.descriptor.EntityDescriptor.linkVariableDescriptors(EntityDescriptor.java:596)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.afterAnnotationsProcessed(SolutionDescriptor.java:675)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.buildSolutionDescriptor(SolutionDescriptor.java:159)
at ai.timefold.solver.core.impl.solver.DefaultSolverFactory.buildSolutionDescriptor(DefaultSolverFactory.java:181)
at ai.timefold.solver.core.impl.solver.DefaultSolverFactory.(DefaultSolverFactory.java:72)
at ai.timefold.solver.core.api.solver.SolverFactory.create(SolverFactory.java:107)
at com.easyplan.scheduling.facade.SolverService.initSolver(SolverService.java:95)

The following is a small code snippet of the Planning Entity class.

`@PlanningEntity
public class JobForward extends Job{

@InverseRelationShadowVariable(sourceVariableName = "jobList")
protected PrimaryResource resource;

@PreviousElementShadowVariable(sourceVariableName = "jobList")
protected Job previousJob;

@NextElementShadowVariable(sourceVariableName = "jobList")
protected Job nextJob;

@ShadowVariable(supplierName = "startTimeSupplier")
protected LocalDateTime startTime;

@ShadowVariable(supplierName = "endTimeSupplier")
protected LocalDateTime endTime;

@ShadowVariablesInconsistent
public boolean isInconsistent;


public JobForward() {
}


@SuppressWarnings("unused")
@ShadowSources({"previousJob.endTime",
        "predecessorJobList[].endTime",
        "resource"})
public LocalDateTime startTimeSupplier() {

    LocalDateTime startTime = null;

    if (this.resource == null) {
        return null;
    }

...
...
...

    return startTime;
}

...
...
...
}`

Many thank you!

[Christopher-Chianelli]

The issue is Job.endTime is not a variable, whereas JobForward.endTime is. You might be able to move the @ShadowVariable and @ShadowSources to Job, which would fix the issue.

[kentbill]

After modifying the program, the Source Path can be found now, but it doesn't seem to meet my design intention. In my scenario, there are different calculation methods for the start and end times of jobs; in other words, xxxTimestampupplier is different.

One of the situations is that the start time depends on its predecessor jobs, and then the end time is calculated based on the start time. I define it as JobForward; Another scenario is that its end time depends on the start time of the successor jobs, and then the start time is calculated based on the end time, I define it as JobBackward.

That is to say, the xxxTimestampupplier for these two types of jobs is different, their Source Path as detailed below:
The startTimestampupplier of JobForward: previousJob.endTime, predecessorJobList []. endTime, resource;
The endTimestampupplier of JobForward: startTime;
The startTimestampupplier of JobBackward: endTime;
The endTimestampupper of JobBackward: nextJob.startTime, successorJobList []. startTime, resource;

So, if only Job is used as the Planning Entity, it is impossible to simultaneously consider the Source Path of JobForward and JobBackward. Should I define JobForward and JobBackward as planning variables so that they can define their own xxxTimestampupplier's Source Path? But if yes, how to avoid the exception "The source path does not end on a variable."? Or is there any other way to achieve it?
Thanks!

[Christopher-Chianelli]

As of right now, it is impossible. You would need to include all the shadow sources on Job, but in your case, that will cause a loop and throw an exception (since you cannot depend on nextJob and previousJob at the same time). If a planning entity can only have JobForward or JobBackward (and not both at the same time), you might be able to do this by having two seperate planning entity classes (one where the planning variable is JobForward, and another where it is JobBackward; you will need to use a chained model), which would in turn allow you to use JobPrevious/JobForward inside your NextElementShadowVariable/PreviousElementShadowVariable.

[kentbill]

Well, thank you for your suggestion @Christopher-Chianelli. I never want to switch back to the Chained Through Time pattern, ever since the DSV feature came along. :&)

For scenarios like this with predecessor/successor jobs, do you know how I implemented with Chained Through Time pattern? I added a base planning variable — delay in the Job class. A job's start time base on the later of its previous job and its predecessor jobs's end time, and plus delay's value. Then, using a minimize make span constraint to minimize the delay to find a more compact solution. Otherwise, a job might never have a start time that later than its predecessor's end time without delay.

This model is quite inefficient and more complex than DSV, especially when the durations of the jobs vary significantly. I need to set a quite large value range for delay, which makes the search space huge.

[Christopher-Chianelli]

To be clear, by chained, I mean chained planning variable (https://docs.timefold.ai/timefold-solver/latest/using-timefold-solver/modeling-planning-problems#chainedPlanningVariable). The reason why PlanningListVariable won't work is because you would need two separate PlanningListVariable, which is currently unsupported.

[kentbill]

Understood, I will try again with your suggestions in mind. Thank you so much for your help!