2nd/10_Wiki/Topics/Backend/Django_Signals.md

---
id: wiki-2026-0508-django-signals
title: Django Signals
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Django Signal Framework, dispatch signals]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [django, python, observer-pattern, backend, decoupling]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: django-5
---

# Django Signals

## 매 한 줄
> **"매 in-process pub/sub for Django — observer pattern over the ORM lifecycle"**. Django signals 는 sender/receiver 의 decouple 하는 dispatch 메커니즘 — 2005 Django core 에 도입, 2026 현재 Django 5.1 LTS 까지 안정. 매 ORM hook (post_save, pre_delete) + custom signal 의 emit 의 standard way.

## 매 핵심

### 매 작동 원리
- **django.dispatch.Signal**: receiver list 의 weakref 보관 — 매 GC safe.
- **send() vs send_robust()**: send 의 raise on receiver error, send_robust 의 collect exceptions in result list — 매 production 의 send_robust 권장.
- **Synchronous**: 매 in-process, in-thread — 매 transaction.on_commit() 통해 post-commit 의 schedule.
- **Async receivers (5.0+)**: 매 async def receiver 의 native support.

### 매 built-in signals
- **Model**: pre_save, post_save, pre_delete, post_delete, m2m_changed, pre_init, post_init.
- **Request**: request_started, request_finished, got_request_exception.
- **Auth**: user_logged_in, user_logged_out, user_login_failed.
- **Migration**: pre_migrate, post_migrate.

### 매 응용
1. Audit log — 매 model save 의 log 기록.
2. Cache invalidation — 매 ORM update 시 cache key purge.
3. Side-effect dispatch — 매 user signup → email send.

## 💻 패턴

### Receiver registration with @receiver
```python
# myapp/signals.py
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.conf import settings

from .models import Profile

@receiver(post_save, sender=settings.AUTH_USER_MODEL)
def create_user_profile(sender, instance, created, **kwargs):
    if created:
        Profile.objects.create(user=instance)
```

### AppConfig.ready() 의 signal import
```python
# myapp/apps.py
from django.apps import AppConfig

class MyAppConfig(AppConfig):
    name = "myapp"

    def ready(self):
        from . import signals  # noqa: F401 — register receivers
```

### Custom signal
```python
# myapp/signals.py
from django.dispatch import Signal

order_paid = Signal()  # providing_args deprecated in 4.0+

# In a view/service after payment
order_paid.send(sender=Order, order=order, amount=order.total)

# Receiver
@receiver(order_paid)
def send_receipt(sender, order, amount, **kwargs):
    EmailService.send_receipt(order, amount)
```

### Transaction-safe side effects
```python
from django.db import transaction
from django.db.models.signals import post_save

@receiver(post_save, sender=Order)
def enqueue_fulfillment(sender, instance, created, **kwargs):
    if not created:
        return
    transaction.on_commit(
        lambda: fulfillment_queue.enqueue(instance.pk)
    )
```

### Async receiver (Django 5.0+)
```python
from asgiref.sync import sync_to_async

@receiver(post_save, sender=Comment)
async def notify_subscribers(sender, instance, **kwargs):
    await broadcast_to_channel(f"post-{instance.post_id}", {
        "event": "new_comment",
        "id": instance.pk,
    })
```

### Robust dispatch with error collection
```python
results = order_paid.send_robust(sender=Order, order=order)
for receiver_fn, response in results:
    if isinstance(response, Exception):
        logger.exception("receiver %s failed", receiver_fn, exc_info=response)
```

### Cache invalidation
```python
from django.core.cache import cache
from django.db.models.signals import post_save, post_delete

@receiver([post_save, post_delete], sender=Article)
def purge_article_cache(sender, instance, **kwargs):
    cache.delete(f"article:{instance.pk}")
    cache.delete_pattern("articles:list:*")  # if django-redis
```

### Disconnect for testing
```python
import pytest
from django.db.models.signals import post_save
from myapp.signals import create_user_profile

@pytest.fixture(autouse=True)
def _silence_profile_signal():
    post_save.disconnect(create_user_profile, sender=User)
    yield
    post_save.connect(create_user_profile, sender=User)
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Cross-app decouple side-effect | Signal ✅ |
| Same-app, deterministic flow | Direct method call (signal 불필요) |
| Heavy work (email, ML inference) | Signal → enqueue Celery/RQ task |
| Cross-process / cross-service | Kafka/RabbitMQ — 매 signal 은 in-process 만 |
| Need ordering / replay | Outbox pattern + message broker |

**기본값**: signal 은 light decouple 만, heavy work 는 즉시 task queue 의 enqueue.

## 🔗 Graph
- 부모: [[Observer-Pattern]]
- 변형: [[Django-Signals]] ·  (blinker)

## 🤖 LLM 활용
**언제**: in-process decoupling 이 필요할 때, ORM lifecycle hook (post_save 등) 이 자연스러울 때, 매 third-party app 의 own model 의 alter 못할 때.
**언제 X**: cross-service eventing — 매 Kafka/Outbox 의 use; complex workflow orchestration — 매 Celery chain / Temporal 의 use; testability 가 critical 한 critical path — 매 explicit service call 의 prefer.

## ❌ 안티패턴
- **Heavy work in receiver**: 매 sync send 면 request latency 의 block — Celery enqueue.
- **Signals for in-app flow**: 매 traceability 의 lose — 매 explicit method call 의 use.
- **No transaction.on_commit**: post_save 시점 의 transaction 미commit — race condition 발생.
- **Forgetting weak=False**: lambda receiver 가 GC 의 collected — 매 module-level def 또는 weak=False.
- **Test pollution**: signal 의 test 사이 의 leak — fixture 의 disconnect.

## 🧪 검증 / 중복
- Verified (docs.djangoproject.com/en/5.1/topics/signals/, Django source dispatch/dispatcher.py).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Django 5.1 signal patterns + async receiver + transaction.on_commit |