Apa iku LISTEN lan NOTIFY?

Question

IT Interview Practice · Accepted Answer

**`LISTEN`** lan **`NOTIFY`** nyediyakaké sistem messagingan **publish/subscribe** sing dibangun-bangun ing PostgreSQL — sawijining sesi bisa ngirim notifikasi ing saluran, lan sesi-sesi liyane sing ngrungokake saluran kasebut bakal nampa kanthi langsung. Iku cara ringan kanggo push event real-time saka database tanpa polling.

## Mekanisme dasar

```sql
-- a session SUBSCRIBES to a channel
LISTEN order_created;

-- another session/transaction PUBLISHES a notification (with an optional payload)
NOTIFY order_created, 'order id 123';
-- → all sessions listening on 'order_created' receive the message immediately
```

LISTEN langganan saluran; NOTIFY menerbitkan (karo payload teks opsional). Pamrungokake bakal kaandhani kanthi asinkron — polling ora perlu.

## Triggers NOTIFY otomatis (pola sing umum)

```sql
-- a trigger that notifies whenever a row is inserted
CREATE FUNCTION notify_new_order() RETURNS trigger AS $$
BEGIN
  PERFORM pg_notify('order_created', NEW.id::text);   -- notify with the new id
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER order_notify AFTER INSERT ON orders
  FOR EACH ROW EXECUTE FUNCTION notify_new_order();
-- now inserting an order automatically pushes a notification to listeners
```

Nggabung NOTIFY karo **trigger** biyen database bisa push event kanthi otomatis nalika data berubah (pesanan anyar, update status) — server aplikasi sing ngrungokake saluran nganggep kanthi real time.

## Cara aplikasi nggunakake

```text
Application code (Node, Python, etc.) opens a connection and LISTENs on a channel.
When a NOTIFY fires (e.g. from a trigger on data change), the app receives the event
and reacts — pushing to websockets, invalidating a cache, triggering a job, etc.
→ Real-time reactions to database changes WITHOUT polling the database.
```

## Keterbatasan sing kudu dimangerteni

```text
✗ Notifications are NOT persisted — if no one is listening, the message is LOST
  (no durability/queue guarantees like a real message broker)
✗ Payload is small (limited size); delivered only after the transaction COMMITS
→ Good for lightweight real-time signaling; for durable/reliable messaging use a
  proper broker (RabbitMQ, Kafka) or a queue table.
```

## Mulane penting

`LISTEN`/`NOTIFY` minangka fitur PostgreSQL sing berguna, kewan ora banyak sing kenal, sing nyediyakaké **pub/sub messagingan sing dibangun-bangun**, berharga kanggo nambah **real-time reactivity** ing aplikasi tanpa polling.

Nilai inti minangka nggawaké database bisa **push event** menyang sesi aplikasi sing ngrungokake kanthi langsung nalika ana kedadean (pesanan anyar, perubahan status) — biasane digabung karo **trigger** supaya perubahan data kanthi otomatis ngandhani pamrungokake, lan server aplikasi nganggep (push menyang websocket, ngrusak cache, nggerakake pekerjaan background).

Iku berharga amarga nyediyakaké tingkah laku event-driven real-time tanpa inefisiensi **polling** database berkali-kali kanggo mriksa owah-owahan (pola umum, sia-sia sing LISTEN/NOTIFY gawa ganti kanthi apik).

Mangertèni mekanisme (LISTEN kanggo subscribe, NOTIFY kanggo publish, karo trigger kanggo notifikasi otomatis ing perubahan data) lan cara aplikasi nggunakake (ngrungokake event kanggo pager real-time reaksi) berguna kanggo bangun aplikasi sing responsif, event-driven nggunakake mung-mungan PostgreSQL.

Sama pentinge iku mangertèni **keterbatasan**: notifikasi **ora disetèkaké** (ilang yen ora ana sing ngrungokake — ora ada jaminan durability), duwé payload cilik, lan mung mairun sawise commit — mula cocok kanggo signaling real-time ringan, *ora* gawa sulih broker pesan tahan lama (RabbitMQ, Kafka) nalika reliability lan persistence penting.

Kambi real-time reactivity minangka kebutuhan umum lan LISTEN/NOTIFY nyediyakaké cara dibangun-bangun, polling-free kanggo kasusan ringan, mangertèni — mekanisme pub/sub, pola trigger, lan kang paling penting keterbatasan (nalika cukup vs nalika broker nyata diperlu) — iku berharga, praksis-relevan pangertèn PostgreSQL kanggo bangun aplikasi event-driven, fitur dibangun-bangun berguna sing asring ora kapeduliake.

Mekanisme dasar

Triggers NOTIFY otomatis (pola sing umum)

Cara aplikasi nggunakake

Keterbatasan sing kudu dimangerteni

Mulane penting

Pitakon sing gegandhengan