New in Symfony 7.4: Weighted Workflow Transitions

How Weighted Transitions Work

You can assign a weight to specify how many instances are produced or required:

• Weight on output (to): the transition places the object in the target
  place N times;
• Weight on input (from): the transition requires the object to be in the
  source place N times before it can fire.

Without weights, every transition produces or consumes exactly one instance per place.

Example: Building a Table

Consider a workflow that models table construction. A table needs four legs,
one tabletop, and we also want to track timing. Here’s how weighted transitions
handle this:

# config/packages/workflow.yaml
framework:
    workflows:
        make_table:
            transitions:
                start:
                    from: init
                    to:
                        - place: prepare_leg
                          weight: 4
                        - place: prepare_top
                          weight: 1
                        - place: stopwatch_running
                          weight: 1
                build_leg:
                    from: prepare_leg
                    to: leg_created
                build_top:
                    from: prepare_top
                    to: top_created
                join:
                    from:
                        - place: leg_created
                          weight: 4
                        - top_created
                        - stopwatch_running
                    to: finished

When using the following code:

$subject = new Subject();
$workflow = new Workflow($definition);

$workflow->apply($subject, 'start');

$workflow->apply($subject, 'build_leg');
$workflow->apply($subject, 'build_top');
$workflow->apply($subject, 'build_leg');
$workflow->apply($subject, 'build_leg');
$workflow->apply($subject, 'build_leg');
$workflow->apply($subject, 'join');

This is how the execution flow works:

1. Firing start marks the object as being in prepare_leg four times, in
   prepare_top once, and in stopwatch_running once;
2. Each time you fire build_leg, it consumes one instance from prepare_leg
   and adds one to leg_created (fire it four times to accumulate four instances);
3. Firing build_top creates the single tabletop;
4. The join transition requires the object to be in leg_created exactly
   four times, plus once in each of the other places, before it can fire.

This creates a synchronization point: you cannot proceed to assembly until
all required components reach their specified quantities. The workflow enforces
these structural constraints at the model level.