Home / softtor / nestjs-hexagonal · skills/event-listeners/SKILL.md · GitHub

event-listeners skillA

event-listeners is agent-read markdown (skill) from softtor/nestjs-hexagonal: Use when creating event listeners that react to domain events — same-BC side effects (projections, audit, cache), cross-BC reactions (another bounded context consuming events), or bridge listeners (WebSocket broadcast, RabbitMQ publish, email). Covers listener taxonomy, placement rules, Strategy+Gateway pattern for fan-out, and anti-over-engineering criteria..

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What the file says

# Event Listeners

Domain events emitted via `entity.commit()` flow through the NestJS CQRS `EventBus`. Listeners are `@EventsHandler` classes that react to these events. Multiple handlers for the same event run in parallel via `Promise.allSettled` — if one fails, others continue.

This skill covers WHERE listeners live, WHAT they do, and WHEN you actually need one.

---

## Decision Tree

```
Do you need to react to a domain event?
│
├─ Is it a side effect within the SAME bounded context?
│  └─ YES → Same-BC Listener (see Section 1)
│     Examples: update Redis projection, write audit log, invalidate cache
│
├─ Does ANOTHER bounded context need to react?
│  └─ YES → Cross-BC Listener (see Section 2)
│     Examples: Billing creates invoice when Order is created
│
├─ Does the event need to leave the process?
│  └─ YES → Bridge Listener (see Section 3)
│     Examples: WebSocket broadcast, RabbitMQ publish, send email, call webhook
│
└─ Is the side effect simple and only 1 consumer exists?
   └─ YES → Consider putting it in the command handler directly (no listener needed)
```

---

## When NOT to Create a Listener (anti-over-engineering)

| Situation | Do this instead |
|---|---|
…

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softtor/nestjs-hexagonal · 5 stars · license MIT · pushed 2026-09-23 · branch main

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