Solving Server-Sent Events (SSE) Cache Problems: Realtime Response Held Back by Nginx Caching

Introduction

SSE allows a web server to send updates to the client browser in real-time, without the client having to request each update. This is particularly useful for applications that require real-time data updates, such as live news feeds, chat applications, or real-time analytics. For example, AI assistant applications such as ChatGPT and Google Bard leverage SSE to deliver real-time, interactive responses.

However, a common challenge in implementing SSE is addressing caching issues that can impede the real-time nature of the data stream. Often caused by Nginx's default caching behavior, these issues can lead to significant delays or even complete interruptions in the data stream. A notable symptom of this problem is that real-time responses may only be sent in one batch at the end when cached, instead of continuously as events occur.

In this blog post, we'll explore how to resolve these caching problems by using specific HTTP headers in your Node.js application to override Nginx's default settings

Solving Caching with HTTP Headers

app.get('/events', function(req, res) {
    // Make sure to set Content-Type to text/event-stream
    res.setHeader('Content-Type', 'text/event-stream');
    res.setHeader('Cache-Control', 'no-cache');
    res.setHeader('Connection', 'keep-alive');
    // Override nginx buffering for real-time events
    res.setHeader('X-Accel-Buffering', 'no');

    // ... SSE logic to send data to client ...
});

1. Content-Type: text/event-stream: This header is crucial for SSE. It tells the client that the server is sending a stream of events.
2. Cache-Control: no-cache: This directive ensures that the response is not cached by the client or any proxies. It's a key header for maintaining the real-time nature of SSE.
3. Connection: keep-alive: This header indicates that the server should keep the connection open. It's essential for SSE, as the server needs to send events over the same connection without closing it after each response.
4. X-Accel-Buffering: no: This is a non-standard header used by Nginx to prevent buffering of responses. It ensures that data is sent to the client in real-time.

Conclusion

SSE is a powerful tool for building real-time applications. However, it's important to understand how to deal with caching issues that can affect the real-time nature of the data stream. By using the HTTP headers described above, you can ensure that your SSE application works as intended.