Examples

Event Streaming

What this example shows

Every transition emits a typed event. This example shows how to subscribe, persist an audit stream, and reconstruct a timeline from transition history for replay and debugging.

Loop diagram

1START ----> TRANSITION_1 ----> TRANSITION_2 ----> TERMINAL
2  |              |                  |
3  +---- emit events on every transition ----+

Actors

| Actor | Type | Transitions | Guards | |---|---|---|---| | runtime emitter | automation | all runtime transitions | loop-defined guards | | subscribers | automation | none (read-only stream consumers) | n/a |

Key annotated snippet

1"cmt">// @no-typecheck
2import { LOOP_EVENT_TYPES } from "@loop-engine/events";
3import { buildTimeline, replayLoop } from "@loop-engine/observability";
4import { aggregateId } from "@loop-engine/core";
5 
6const audit: Array<{ at: string; type: string; aggregateId: string }> = [];
7 
8eventBus.subscribe(async (event) => {
9  if (
10    event.type === "loop.started" ||
11    event.type === "loop.transition.executed" ||
12    event.type === "loop.completed"
13  ) {
14    audit.push({
15      at: event.occurredAt,
16      type: event.type,
17      aggregateId: String(event.aggregateId)
18    });
19  }
20});
21 
22const id = aggregateId("EXP-1");
23const instance = await engine.getState(id);
24const history = await engine.getHistory(id);
25if (instance) {
26  const timeline = buildTimeline(instance, history);
27  const replay = replayLoop(definition, history);
28  console.log(timeline.durationMs, replay.valid);
29}

What emitted events look like

1{
2  type: "loop.transition.executed",
3  loopId: "expense.approval",
4  aggregateId: "EXP-1",
5  transitionId: "approve",
6  fromState: "PENDING_APPROVAL",
7  toState: "APPROVED",
8  actor: { type: "human", id: "manager@acme.com" },
9  occurredAt: "2026-03-13T12:57:01.440Z"
10}

Try it yourself

1cd loop-examples/event-streaming
2pnpm install
3pnpm dev

Examples

Event Streaming

Edit this page

What this example shows

Every transition emits a typed event. This example shows how to subscribe, persist an audit stream, and reconstruct a timeline from transition history for replay and debugging.

Loop diagram

1START ----> TRANSITION_1 ----> TRANSITION_2 ----> TERMINAL
2  |              |                  |
3  +---- emit events on every transition ----+

Actors

Key annotated snippet

1"cmt">// @no-typecheck
2import { LOOP_EVENT_TYPES } from "@loop-engine/events";
3import { buildTimeline, replayLoop } from "@loop-engine/observability";
4import { aggregateId } from "@loop-engine/core";
5 
6const audit: Array<{ at: string; type: string; aggregateId: string }> = [];
7 
8eventBus.subscribe(async (event) => {
9  if (
10    event.type === "loop.started" ||
11    event.type === "loop.transition.executed" ||
12    event.type === "loop.completed"
13  ) {
14    audit.push({
15      at: event.occurredAt,
16      type: event.type,
17      aggregateId: String(event.aggregateId)
18    });
19  }
20});
21 
22const id = aggregateId("EXP-1");
23const instance = await engine.getState(id);
24const history = await engine.getHistory(id);
25if (instance) {
26  const timeline = buildTimeline(instance, history);
27  const replay = replayLoop(definition, history);
28  console.log(timeline.durationMs, replay.valid);
29}

What emitted events look like

1{
2  type: "loop.transition.executed",
3  loopId: "expense.approval",
4  aggregateId: "EXP-1",
5  transitionId: "approve",
6  fromState: "PENDING_APPROVAL",
7  toState: "APPROVED",
8  actor: { type: "human", id: "manager@acme.com" },
9  occurredAt: "2026-03-13T12:57:01.440Z"
10}

Try it yourself

1cd loop-examples/event-streaming
2pnpm install
3pnpm dev