# `Runic.Workflow.Coordinator`
[🔗](https://github.com/zblanco/runic/blob/main/lib/workflow/coordinator.ex#L1)

Optional protocol for nodes that need post-fold coordination.

After `apply_event/2` has folded a runnable's events into the workflow,
some node types need to inspect the updated graph to determine if a
coordination step is complete (e.g. all Join branches satisfied, all
FanIn items collected).

Nodes that implement this protocol will have `finalize/3` called during
`apply_runnable/2`. The callback receives the node, the current workflow
(with events already folded), and the original runnable. It returns
`{workflow, derived_events}` where `derived_events` have been folded
into the returned workflow.

Nodes that do **not** implement this protocol are assumed to need no
coordination — `apply_runnable/2` skips the coordination step for them.

## Built-in Implementations

- `Runic.Workflow.Join` — rechecks branch satisfaction after folding
  `JoinFactReceived` events, emits `JoinCompleted` + `JoinEdgeRelabeled`
  events when all branches are satisfied.

- `Runic.Workflow.FanIn` — rechecks whether all fan-out items have been
  processed, emits `FanInCompleted` + sister `ActivationConsumed` events
  when ready.

## Example

    defimpl Runic.Workflow.Coordinator, for: MyApp.CustomCoordinationNode do
      def finalize(node, workflow, runnable) do
        # Check coordination condition, emit derived events if complete
        {workflow, []}
      end
    end

# `t`

```elixir
@type t() :: term()
```

All the types that implement this protocol.

# `finalize`

```elixir
@spec finalize(
  node :: struct(),
  workflow :: Runic.Workflow.t(),
  runnable :: Runic.Workflow.Runnable.t()
) :: {Runic.Workflow.t(), [struct()]}
```

Post-fold coordination finalization.

Called after all events from `execute/2` have been folded into the workflow.
Returns `{workflow, derived_events}` where `derived_events` have already
been folded into the returned workflow.

---

*Consult [api-reference.md](api-reference.md) for complete listing*