1. Methodology overview

The diagram below explains the overall structure of the Timefold source code:


In the diagram above, it’s important to understand the clear separation between the configuration and runtime classes.

The development philosophy includes:

  • Reuse: The examples are reused as integration tests, stress tests and demos.

  • Consistent terminology: Each example has a class App (executable class) and Panel (swing UI).

  • Consistent structure: Each example has the same packages: domain, persistence, app, solver and swingui.

  • Real world usefulness: Every feature is used in an example. Most examples are real world use cases with real world constraints, often with real world data.

  • Automated testing: There are unit tests, integration tests, performance regressions tests and stress tests. The test coverage is high.

  • Fail fast with an understandable error message: Invalid states are checked as early as possible.

2. Development guidelines

2.1. Fail fast

There are several levels of fail fast, from better to worse:

  1. Fail Fast at compile time. For example: Don’t accept an Object as a parameter if it needs to be a String or an Integer.

  2. Fail Fast at startup time. For example: if the configuration parameter needs to be a positive int and it’s negative, fail fast

  3. Fail Fast at runtime. For example: if the request needs to contain a double between 0.0 and 1.0 and it’s bigger than 1.0, fail fast.

  4. Fail Fast at runtime in assertion mode if the detection performance cost is high. For example: If, after every low level iteration, the variable A needs to be equal to the square root of B, check it if and only if an assert flag is set to true (usually controlled by the EnvironmentMode).

2.2. Exception messages

  1. The Exception message must include the name and state of each relevant variable. For example:

    if (fooSize < 0) {
        throw new IllegalArgumentException("The fooSize (" + fooSize + ") of bar (" + this + ") must be positive.");

    Notice that the output clearly explains what’s wrong:

    Exception in thread "main" java.lang.IllegalArgumentException: The fooSize (-5) of bar (myBar) must be positive.
        at ...
  2. Whenever possible, the Exception message must include context.

  3. Whenever the fix is not obvious, the Exception message should include advice. Advice normally starts with the word maybe on a new line:

    Exception in thread "main" java.lang.IllegalStateException: The valueRangeDescriptor (fooRange) is nullable, but not countable (false).
    Maybe the member (getFooRange) should return CountableValueRange.
        at ...

    The word maybe is to indicate that the advice is not guaranteed to be right in all cases.

2.3. Generics

  1. The @PlanningSolution class is often passed as a generic type parameter to subsystems.

  2. The @PlanningEntity class(es) are rarely passed as a generic type parameter because there could be multiple planning entities.

2.4. Lifecycle

One of the biggest challenges in multi-algorithm implementations (such as Timefold) is the lifecycle management of internal subsystems. These guidelines avoid lifecycle complexity:

  1. The subsystems are called in the same order in *Started() and *Ended methods.

    1. This avoids cyclic subsystem dependencies.

  2. The *Scope class’s fields are filled in piecemeal by the subsystems as the algorithms discover more information about its current scope subject.

    1. Therefore, a *Scope has mutable fields. It’s not an Event.

    2. A subsystem can only depend on scope information provided by an earlier subsystem.

  3. Global variables are sorted:

    1. First by volatility

    2. Then by initialization time