Diagnosing Dependency Cycles​#

A dependency cycle occurs when two or more models each need a new result from the others before they can run. For example, a leaf model needs water from a root model, but the root model needs carbon from the leaf model. Neither can go first, so PlantSimEngine reports the problem before running the equations. The error names the applications, objects, and variables involved.

Choose the solution that matches your equations. If the leaf should use the root's water from the previous step, mark that input with PreviousTimeStep(:water). For a daily simulation, this means yesterday's water. The leaf can then calculate today's carbon before the root calculates today's water:

ModelSpec(
    LeafModel();
    inputs=(
        PreviousTimeStep(:water) =>
            One(scale=:Root, application=:root, var=:water),
    ),
)

At the start, the leaf's initial water value is used until a result from a previous step is available.

If the two values must instead be solved together in the current step, use a controller model that calls both models repeatedly. Try values with publish=false and record each accepted result once with publish=true. The controller must decide when the result is close enough and what to do if the calculation does not converge. See Control Advanced Execution.

You may also need to rewrite the equations. Simply changing the order of the ModelSpec declarations does not resolve a cycle.