Branching

Conditional routing and fan-out in Rust

Conditional Branching

Use #[step(emits = [...])] to declare multiple possible output types, then return a StepOutput::Single:

#[derive(Debug, Clone, Serialize, Deserialize, Event)]
struct PositiveEvent { text: String }

#[derive(Debug, Clone, Serialize, Deserialize, Event)]
struct NegativeEvent { text: String }

#[step(emits = [PositiveEvent, NegativeEvent])]
async fn classify(event: AnalyzeEvent, _ctx: Context) -> Result<StepOutput, WorkflowError> {
    if event.score > 0.5 {
        Ok(StepOutput::Single(Box::new(PositiveEvent { text: event.text })))
    } else {
        Ok(StepOutput::Single(Box::new(NegativeEvent { text: event.text })))
    }
}

Handling Branches

Each branch has its own step:

#[step]
async fn handle_positive(event: PositiveEvent, _ctx: Context) -> Result<StopEvent, WorkflowError> {
    Ok(StopEvent { result: serde_json::json!({"sentiment": "positive", "text": event.text}) })
}

#[step]
async fn handle_negative(event: NegativeEvent, _ctx: Context) -> Result<StopEvent, WorkflowError> {
    Ok(StopEvent { result: serde_json::json!({"sentiment": "negative", "text": event.text}) })
}

How It Works

• The router matches event types to steps
• Only one branch executes based on which event is emitted
• First StopEvent terminates the workflow