Long Polling & Server-Sent Events (SSE)

Also Known As

TL;DR

Explanation

Long polling: the client makes a request, the server holds it open (sleeping/waiting) until new data is available or a timeout occurs, then responds and the client immediately reconnects. Simple to implement in PHP but holds a PHP-FPM worker for the duration — inefficient under load. Server-Sent Events (SSE): the server sends a text/event-stream response and pushes data: payloads down over a persistent connection using ob_flush() and flush(). SSE is one-directional (server to client) and reconnects automatically. Both approaches consume a PHP worker per open connection — for high concurrency, a Node.js/Go push service with Redis Pub/Sub as the backend is more scalable.

Diagram

sequenceDiagram
    participant C as Client
    participant S as Server
    Note over C,S: Long Polling
    C->>S: GET /events (holds connection)
    Note over S: Waits for event...
    S-->>C: Response when event ready
    C->>S: Immediately reconnect
    Note over C,S: Server-Sent Events (SSE)
    C->>S: GET /stream (persistent)
    S-->>C: data: event1
    S-->>C: data: event2
    Note over C,S: WebSocket - full duplex
    C->>S: WS upgrade
    S-->>C: push anytime
    C->>S: send anytime

Common Misconception

Why It Matters

Common Mistakes

• Not setting a server-side timeout — a connection held indefinitely exhausts worker threads.
• Not handling the client reconnect loop — when the server responds, the client must immediately re-poll.
• Using long polling when WebSockets or SSE are available and more efficient for the use case.
• Not tracking which clients are waiting and what data they have already seen — sending duplicate events.

Code Examples

// Long poll without timeout — holds worker indefinitely:
set_time_limit(0); // Dangerous — worker never freed
while (true) {
    $events = getNewEvents($lastId);
    if (!empty($events)) { echo json_encode($events); exit; }
    sleep(1);
}
// Fix: set_time_limit(30); respond with empty after timeout so client reconnects

// SSE in PHP
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
while (true) {
    echo 'data: ' . json_encode(getUpdate()) . "\n\n";
    ob_flush(); flush();
    sleep(1);
}

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events

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