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

Optional protocol for nodes with custom downstream activation patterns.

After coordination finalization (if any), `apply_runnable/2` needs to
activate downstream nodes. Most node types use the default pattern:
find `:flow` successors and draw activation edges from the result fact.

Some node types have non-standard activation patterns:

- **FanOut** emits multiple facts, each activating all downstream nodes
- **Condition/Conjunction** activate via
  `prepare_next_runnables` after a satisfied match

Nodes that implement this protocol override the default activation logic.
Nodes that do **not** implement it get the standard single-fact activation.

## Return Value

Implementations must return `{workflow, activation_events}` where
`activation_events` is a list of `RunnableActivated` events that were
folded into the workflow. This allows `apply_runnable/2` to capture
activation edges in the event stream for replay correctness.

## Example

    defimpl Runic.Workflow.Activator, for: MyApp.CustomBroadcastNode do
      def activate_downstream(node, workflow, runnable) do
        # Custom multi-target activation logic
        Runic.Workflow.Private.activate_downstream_with_events(workflow, node, fact)
      end
    end

# `t`

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

All the types that implement this protocol.

# `activate_downstream`

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

Activates downstream nodes after a runnable completes.

Receives the node, the current workflow, and the completed runnable.
Returns `{workflow, activation_events}` — the updated workflow with
activation edges drawn and the list of `RunnableActivated` events produced.

---

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