CakeDC Blog

This article is part of the CakeDC Advent Calendar 2025 (December 20th 2025)

As PHP developers, when we hear "real-time," our minds immediately jump to WebSockets. We think of complex setups with Ratchet, long-running server processes, and tricky Nginx proxy configurations. And for many applications (like live chats or collaborative editing) WebSockets are absolutely the right tool.

But, if you don't need all that complexity or if you just want to push data from your server to the client? Think of a new notification, a "users online" counter, or a live dashboard update. For these one-way-street use cases, WebSockets are often overkill.

Enter Server-Sent Events (SSE). It's a simple, elegant, and surprisingly powerful W3C standard that lets your server stream updates to a client over a single, long-lasting HTTP connection.

SSE vs. WebSockets: The Showdown

The most important difference is direction.

• WebSockets (WS): Bidirectional. The client and server can both send messages to each other at any time. It's a two-way conversation.
• Server-Sent Events (SSE): Unidirectional. Only the server can send messages to the client. It's a one-way broadcast.

This single difference has massive implications for simplicity and implementation.

Pros for SSE:

• It's just HTTP. No new protocol, no special ports.
• Automatic reconnection is a life-saver.
• The server-side implementation can be a simple controller action.

Cons for SSE:

• Strictly one-way. The client can't send data back on the same connection.
• Some older proxies or servers might buffer the response, which can be tricky.

Infrastructure Note:

Since SSE keeps a persistent connection open, each active client will occupy one PHP-FPM worker. For high-traffic applications, ensure your server is configured to handle the concurrent load or consider a non-blocking server like RoadRunner. Additionally, using HTTP/2 is strongly recommended to bypass the 6-connection-per-domain limit found in older HTTP/1.1 protocols

The Implementation: A Smart, Reusable SSE System in CakePHP

We're not going to build a naive while(true) loop that hammers our database every 2 seconds. That's inefficient.

Instead, we'll build an event-driven system. The while(true) loop will only check a cache key. This is lightning-fast. A separate "trigger" class will update that cache key's timestamp only when a new notification is actually created.

This design is clean, decoupled, and highly performant.

Note:

This example uses CakePHP, but the principles (a component, a trigger, and a controller) can be adapted to any framework like Laravel or Symfony.

1. The Explicit SseTrigger Class

First, we need a clean, obvious way to "poke" our SSE stream. We'll create a simple class whose only job is to update a cache timestamp. This is far better than a "magic" Cache::write() call hidden in a model.

src/Sse/SseTrigger.php

<?php
namespace App\Sse;

use Cake\Cache\Cache;

/**
 * Provides an explicit, static method to "push" an SSE event.
 * This simply updates a cache key's timestamp, which the
 * SseComponent is watching.
 */
class SseTrigger
{
    /**
     * Pushes an update for a given SSE cache key.
     *
     * @param string $cacheKey The key to "touch".
     * @return bool
     */
    public static function push(string $cacheKey): bool
    {
        // We just write the current time. The content doesn't
        // matter, only the timestamp.
        return Cache::write($cacheKey, microtime(true));
    }
}

CRITICAL PERFORMANCE WARNING: The PHP-FPM Bottleneck

In a standard PHP-FPM environment, each SSE connection is synchronous and blocking. This means one active SSE stream = one locked PHP-FPM worker.

If your max_children setting is 50, and 50 users open your dashboard, your entire website will stop responding because there are no workers left to handle regular requests.

How to mitigate this:

Dedicated Pool: Set up a separate PHP-FPM pool specifically for SSE requests.

Go Asynchronous: Use a non-blocking server like RoadRunner, Swoole or FrankenPHP. These can handle thousands of concurrent SSE connections with minimal memory footprint.

HTTP/2: Always serve SSE over HTTP/2 to bypass the browser's 6-connection limit per domain.

2. The SseComponent (The Engine)

This component encapsulates all the SSE logic. It handles the loop, the cache-checking, the CallbackStream, and even building the final Response object. The controller will be left perfectly clean.

To handle the stream, we utilize CakePHP's CallbackStream. Unlike a standard response that sends all data at once, CallbackStream allows us to emit data in chunks over time. It wraps our while(true) loop into a PSR-7 compliant stream, enabling the server to push updates to the browser as they happen without terminating the request.

src/Controller/Component/SseComponent.php

<?php
namespace App\Controller\Component;

use Cake\Controller\Component;
use Cake\Http\CallbackStream;
use Cake\Cache\Cache;
use Cake\Http\Response;

class SseComponent extends Component
{
    protected $_defaultConfig = [
        'poll' => 2, // How often to check the cache (in seconds)
        'eventName' => 'message', // Default SSE event name
        'heartbeat' => 30, // Keep-alive to prevent proxy timeouts
    ];

    /**
     * Main public method.
     * Builds the stream and returns a fully configured Response.
     */
    public function stream(callable $dataCallback, string $watchCacheKey, array $options = []): Response
    {
        $stream = $this->_buildStream($dataCallback, $watchCacheKey, $options);

        // Get and configure the controller's response
        $response = $this->getController()->getResponse();
        $response = $response
            ->withHeader('Content-Type', 'text/event-stream')
            ->withHeader('Cache-Control', 'no-cache')
            ->withHeader('Connection', 'keep-alive')
            ->withHeader('X-Accel-Buffering', 'no') // For Nginx: disable response buffering
            ->withBody($stream);

        return $response;
    }

    /**
     * Protected method to build the actual CallbackStream.
     */
    protected function _buildStream(callable $dataCallback, string $watchCacheKey, array $options = []): CallbackStream
    {
        $config = $this->getConfig() + $options;

        return new CallbackStream(function () use ($dataCallback, $watchCacheKey, $config) {

            set_time_limit(0); 
            $lastSentTimestamp = null; 
            $lastHeartbeat = time();

            while (true) {
                if (connection_aborted()) {
                    break;
                }

                // 1. THE FAST CHECK: Read the cache.
                $currentTimestamp = Cache::read($watchCacheKey);

                // 2. THE COMPARE: Has it been updated?
                if ($currentTimestamp > $lastSentTimestamp) {

                    // 3. THE SLOW CHECK: Cache is new, so run the data callback.
                    $data = $dataCallback(); 

                    // 4. THE PUSH: Send the data.
                    echo "event: " . $config['eventName'] . "\n";
                    echo "data: " . json_encode($data) . "\n\n";

                    $lastSentTimestamp = $currentTimestamp; 
                    $lastHeartbeat = time();

                } else if (time() - $lastHeartbeat > $config['heartbeat']) {
                    // 5. THE HEARTBEAT: Send a comment to keep connection alive.
                    echo ": \n\n"; 
                    $lastHeartbeat = time();
                }

                if (ob_get_level() > 0) {
                    ob_flush();
                }
                flush();

                // Wait before the next check
                sleep($config['poll']);
            }
        });
    }
}

3. Connecting the Logic (Model & Controller)

First, we use our SseTrigger in the afterSave hook of our NotificationsTable. This makes it clear: "After saving a notification, push an update."

src/Model/Table/NotificationsTable.php (Partial)

use App\Sse\SseTrigger; // Don't forget to import!

public function afterSave(EventInterface $event, Entity $entity, ArrayObject $options)
{
    // Check if the entity has a user_id
    if ($entity->has('user_id') && !empty($entity->user_id)) {

        // Build the user-specific cache key
        $userCacheKey = 'notifications_timestamp_user_' . $entity->user_id;

        // Explicitly trigger the push!
        SseTrigger::push($userCacheKey);
    }
}

Now, our controller action becomes incredibly simple. Its only jobs are to get the current user, define the data callback, and return the component's stream.

src/Controller/NotificationsController.php

<?php
namespace App\Controller;

use App\Controller\AppController;
use Cake\Http\Exception\ForbiddenException;

class NotificationsController extends AppController
{
    public function initialize(): void
    {
        parent::initialize();
        $this->loadComponent('Sse');
        $this->loadComponent('Authentication.Authentication'); 
    }

    public function stream()
    {
        $this->autoRender = false;

        // 1. Get authenticated user
        $identity = $this->Authentication->getIdentity();
        if (!$identity) {
            throw new ForbiddenException('Authentication required');
        }

        // 2. Define user-specific parameters
        $userId = $identity->get('id');
        $userCacheKey = 'notifications_timestamp_user_' . $userId;

        // 3. Define the data callback (what to run when there's an update)
        $dataCallback = function () use ($userId) {
            return $this->Notifications->find()
                ->where(['user_id' => $userId, 'read' => false]) 
                ->order(['created' => 'DESC'])
                ->limit(5)
                ->all();
        };

        // 4. Return the stream. That's it!
        return $this->Sse->stream(
            $dataCallback, 
            $userCacheKey, 
            [
                'eventName' => 'new_notification', // Custom event name for JS
                'poll' => 2
            ]
        );
    }
}

4. The Frontend (The Easy Part)

Thanks to the native EventSource API, the client-side JavaScript is trivial. No libraries. No complex connection management.

<script>
    // 1. Point to your controller action
    const sseUrl = '/notifications/stream';
    const eventSource = new EventSource(sseUrl);

    // 2. Listen for your custom event
    eventSource.addEventListener('new_notification', (event) => {
        console.log('New data received!');
        const notifications = JSON.parse(event.data);

        // Do something with the data...
        // e.g., update a <ul> list or a notification counter
        updateNotificationBell(notifications);
    });

    // 3. (Optional) Handle errors
    eventSource.onerror = (error) => {
        console.error('EventSource failed:', error);
        // The browser will automatically try to reconnect.
    };

    // (Optional) Handle the initial connection
    eventSource.onopen = () => {
        console.log('SSE connection established.');
    };
</script>

Ideas for Your Projects

You can use this exact pattern for so much more than just notifications:

• Live Admin Dashboard: A "Recent Sales" feed or a "Users Online" list that updates automatically.
• Activity Feeds: Show "John recently commented..." in real-time.
• Progress Indicators: For a long-running background process (like video encoding), push status updates ("20% complete", "50% complete", etc.).
• Live Sports Scores: Push new scores as they happen.
• Stock or Crypto Tickers: Stream new price data from your server.

When NOT to Use SSE: Know Your Limits

While SSE is an elegant solution for many problems, it isn't a silver bullet. You should avoid SSE and stick with WebSockets or standard Polling when:

• True Bidirectional Communication is Required: If your app involves heavy "back-and-forth" (like a fast-paced multiplayer game or a collaborative whiteboarding tool), WebSockets are the correct choice.
• Binary Data Streams: SSE is a text-based protocol. If you need to stream raw binary data (like audio or video frames), WebSockets or WebRTC are better suited.
• Legacy Browser Support (IE11): If you must support older browsers that lack EventSource and you don't want to rely on polyfills, SSE will not work.
• Strict Connection Limits: If you are on a restricted shared hosting environment with very few PHP-FPM workers and no support for HTTP/2, the persistent nature of SSE will quickly exhaust your server's resources.

Conclusion

WebSockets are a powerful tool, but they aren't the only tool. For the wide array of use cases that only require one-way, server-to-client communication, Server-Sent Events are a simpler, more robust, and more maintainable solution.

It integrates perfectly with the standard PHP request cycle, requires no extra daemons, and is handled natively by the browser.

So the next time you need real-time updates, ask yourself: "Do I really need a two-way conversation?" If the answer is no, give SSE a try.

This article is part of the CakeDC Advent Calendar 2025 (December 20th 2025)