Workers

Conduit orchestrates; workers execute. A serviceTask carries a <conduit:taskTopic> that the engine uses purely as a routing label — it knows nothing about HTTP, gRPC, Kafka, or any other transport. Workers poll the engine, run the side effect, and report the outcome back.

Reference SDKs ship in the engine's repo at workers/, one independent build per language. Each SDK conforms to a single language-agnostic wire contract (PROTOCOL) and exposes registration sugar native to its host language.

What is an external task?

An external task is a regular <bpmn:serviceTask> with a <conduit:taskTopic> extension element. The element type is ordinary BPMN; the topic is what tells the engine "don't run this yourself — hand it off to a worker." There is no separate ExternalTask element kind in Conduit.

<bpmn:serviceTask id="ping" name="Ping URL">
  <bpmn:extensionElements>
    <conduit:taskTopic>http.call</conduit:taskTopic>
  </bpmn:extensionElements>
</bpmn:serviceTask>

The engine writes the task into its job queue, workers poll /api/v1/external-tasks/fetch-and-lock for tasks matching their topic, run the side effect, and report back via /complete, /failure, or /bpmn-error. The "external task" name refers to the runtime mode and the API path, not a BPMN element. See the Service Task element page for the full element reference, including the alternative HTTP-push mode (deprecated; replaced by the reference HTTP worker).

Pick your language

#[handler(topic = "http.call")]
async fn http_call(task: &ExternalTask)
    -> Result<HandlerResult, HandlerError> { /* ... */ }

@TaskHandler(topic = "http.call")
public class HttpCallHandler implements Handler {
  public HandlerResult handle(ExternalTask t) { /* ... */ }
}

@handler(topic="http.call")
async def http_call(task: ExternalTask) -> Complete:
    return Complete([Variable.string("status", "ok")])

export const httpCall = defineHandler({
  topic: "http.call",
  async handle(task) { return HandlerResult.complete([]); },
});

runner.Register("http.call", func(ctx context.Context, t *cw.ExternalTask)
    (*cw.Result, error) { return cw.Complete(), nil })

Cross-cutting