Real-Time Streams

Quick Start

#[derive(Entity, Serialize, Deserialize)]
#[entity(table = "orders", events, streams)]
pub struct Order {
 #[id]
 pub id: Uuid,
 #[field(create, update, response)]
 pub status: String,
 #[field(create, response)]
 pub customer_id: Uuid,
}

[dependencies]
entity-derive = { version = "0.3", features = ["postgres", "streams"] }
serde = { version = "1", features = ["derive"] }

Generated Code

Channel Constant

impl Order {
 /// Postgres NOTIFY channel name.
 pub const CHANNEL: &'static str = "entity_orders";
}

Subscriber Struct

/// Subscriber for real-time Order changes.
pub struct OrderSubscriber {
 listener: PgListener,
}
impl OrderSubscriber {
 /// Connect and subscribe to the channel.
 pub async fn new(pool: &PgPool) -> Result<Self, sqlx::Error>;
 /// Wait for next event (blocking).
 pub async fn recv(&mut self) -> Result<OrderEvent, StreamError<sqlx::Error>>;
 /// Check for event without blocking.
 pub async fn try_recv(&mut self) -> Result<Option<OrderEvent>, StreamError<sqlx::Error>>;
}

Automatic Notifications

// In generated create() method:
async fn create(&self, dto: CreateOrderRequest) -> Result<Order, Self::Error> {
 let order = /* insert */;
 // Auto-generated notification
 let event = OrderEvent::created(order.clone());
 let payload = serde_json::to_string(&event)?;
 sqlx::query("SELECT pg_notify(1,ドル 2ドル)")
 .bind(Order::CHANNEL)
 .bind(&payload)
 .execute(self)
 .await?;
 Ok(order)
}

Usage Examples

Basic Subscription

use entity_derive::StreamError;
async fn watch_orders(pool: &PgPool) -> Result<(), Box<dyn std::error::Error>> {
 let mut subscriber = OrderSubscriber::new(pool).await?;
 loop {
 match subscriber.recv().await {
 Ok(event) => {
 match event {
 OrderEvent::Created(order) => {
 println!("New order: {}", order.id);
 }
 OrderEvent::Updated { old, new } => {
 println!("Order {} updated: {} -> {}", new.id, old.status, new.status);
 }
 OrderEvent::HardDeleted { id } => {
 println!("Order {} deleted", id);
 }
 _ => {}
 }
 }
 Err(StreamError::Database(e)) => {
 eprintln!("Database error: {}", e);
 break;
 }
 Err(StreamError::Deserialize(e)) => {
 eprintln!("Invalid event payload: {}", e);
 }
 }
 }
 Ok(())
}

Real-Time Dashboard (Axum WebSocket)

use axum::{
 extract::{State, WebSocketUpgrade, ws::{Message, WebSocket}},
 response::IntoResponse,
};
async fn ws_handler(
 ws: WebSocketUpgrade,
 State(pool): State<PgPool>,
) -> impl IntoResponse {
 ws.on_upgrade(|socket| handle_socket(socket, pool))
}
async fn handle_socket(mut socket: WebSocket, pool: PgPool) {
 let mut subscriber = match OrderSubscriber::new(&pool).await {
 Ok(s) => s,
 Err(_) => return,
 };
 loop {
 match subscriber.recv().await {
 Ok(event) => {
 let json = serde_json::to_string(&event).unwrap();
 if socket.send(Message::Text(json)).await.is_err() {
 break;
 }
 }
 Err(_) => break,
 }
 }
}

Cache Invalidation

struct CacheInvalidator {
 cache: Redis,
 pool: PgPool,
}
impl CacheInvalidator {
 async fn run(&self) -> Result<(), StreamError<sqlx::Error>> {
 let mut subscriber = OrderSubscriber::new(&self.pool).await
 .map_err(StreamError::Database)?;
 loop {
 let event = subscriber.recv().await?;
 let key = format!("order:{}", event.entity_id());
 match event {
 OrderEvent::Created(_) | OrderEvent::Updated { .. } => {
 self.cache.del(&key).await.ok();
 }
 OrderEvent::HardDeleted { id } | OrderEvent::SoftDeleted { id } => {
 self.cache.del(&format!("order:{}", id)).await.ok();
 }
 _ => {}
 }
 }
 }
}

Background Worker with Graceful Shutdown

use tokio::sync::watch;
async fn notification_worker(
 pool: PgPool,
 mut shutdown: watch::Receiver<bool>,
) {
 let mut subscriber = OrderSubscriber::new(&pool).await.unwrap();
 loop {
 tokio::select! {
 result = subscriber.recv() => {
 match result {
 Ok(event) => process_event(event).await,
 Err(e) => {
 eprintln!("Stream error: {:?}", e);
 tokio::time::sleep(Duration::from_secs(1)).await;
 }
 }
 }
 _ = shutdown.changed() => {
 println!("Shutting down notification worker");
 break;
 }
 }
 }
}

Error Handling

use entity_derive::StreamError;
match subscriber.recv().await {
 Ok(event) => { /* process */ }
 Err(StreamError::Database(sqlx_error)) => {
 // Connection lost, query failed, etc.
 // Subscriber will auto-reconnect on next recv()
 }
 Err(StreamError::Deserialize(message)) => {
 // Invalid JSON payload
 // Log and continue - don't crash the loop
 }
}

Architecture

CRUD Operation (create/update/delete)
 │
 ▼
 pg_notify(channel, event_json)
 │
 ▼
 Postgres NOTIFY
 │
 ┌────┴────┐
 ▼ ▼
Subscriber Subscriber (multiple listeners)
 │ │
 ▼ ▼
 WebSocket Cache
 Dashboard Invalidator

Best Practices

With Soft Delete

#[derive(Entity, Serialize, Deserialize)]
#[entity(table = "documents", events, streams, soft_delete)]
pub struct Document {
 #[id]
 pub id: Uuid,
 #[field(create, response)]
 pub title: String,
 #[field(skip)]
 pub deleted_at: Option<DateTime<Utc>>,
}
// Events include:
// - DocumentEvent::SoftDeleted { id }
// - DocumentEvent::Restored { id }
// - DocumentEvent::HardDeleted { id }

See Also