Events & Streams

Types behind Hooks and engine.stream().

EngineEvent

A discriminated union on type. Every variant carries runId and workflow.

type EngineEvent =
  | { type: 'runStart';     runId: string; workflow: string }
  | { type: 'stepStart';    runId: string; workflow: string; stepName: string }
  | { type: 'stepComplete'; runId: string; workflow: string; stepName: string; output: PersistedValue; attempts: number }
  | { type: 'runComplete';  runId: string; workflow: string; output: PersistedValue }
  | { type: 'runFailed';    runId: string; workflow: string; stepName: string; error: Error }

Variant	Fires
runStart	A run begins executing (also on crash-recovery resume)
stepStart	Before each step (or parallel branch) runs
stepComplete	After a step's result is persisted
runComplete	A run finishes successfully; output is the final result
runFailed	A run fails; stepName is the offending step, error the cause

Checking event.type narrows the rest of the fields:

for await (const event of engine.stream()) {
  if (event.type === 'runComplete') {
    event.output // narrowed — available here
  }
}

EngineEventOf<T>

Helper to extract a single variant by its type:

type Completed = EngineEventOf<'runComplete'>
// { type: 'runComplete'; runId: string; workflow: string; output: PersistedValue }

StreamOptions

interface StreamOptions {
  bufferSize?: number // default Infinity; 0 = rendezvous; n = up to n buffered before the engine pauses
}

ResultStream

interface ResultStream<E extends EngineEvent = EngineEvent>
  extends AsyncIterableIterator<E> {
  [Symbol.asyncDispose](): Promise<void>
}

An AsyncIterableIterator (works with for await) that is also AsyncDisposable (works with await using). Disposing — by breaking out of the loop, await using scope exit, or engine.stop() — unsubscribes the stream from the engine. Event payloads are defensive clones, isolated from engine state.

EngineHooks

The hook callback shapes, mirroring the events above. See createEngine.