Building Real-Time Features with Server-Sent Events

Server-Sent Events (SSE) are the most underused real-time technology on the web. If your use case is server-to-client only — dashboards, notifications, live feeds, progress updates — SSE is simpler, more reliable, and better supported than WebSockets.

Why SSE Over WebSockets

SSE uses plain HTTP, which means it works through every proxy, load balancer, and CDN without special configuration. It automatically reconnects when the connection drops, with a configurable retry interval. It supports event types, so you can multiplex different data streams on a single connection. And it works with HTTP/2 multiplexing, allowing many SSE connections over a single TCP connection.

Implementation in Next.js

In a Next.js Route Handler, return a ReadableStream with the appropriate headers: Content-Type text/event-stream, Cache-Control no-cache, and Connection keep-alive. Each event is formatted as data: followed by your JSON payload and two newlines. You can add event type prefixes for different message categories.

Client-Side Handling

The browser EventSource API handles connection, reconnection, and parsing automatically. Create a new EventSource pointing to your API route, add event listeners for message types, and handle errors with an onerror callback. For authentication, pass tokens via query parameters since EventSource does not support custom headers natively. Alternatively, use the fetch-based approach with ReadableStream for full header control.

Scaling Considerations

SSE connections are long-lived HTTP connections. On Node.js servers, each connection consumes a file descriptor but minimal memory. For horizontal scaling, use the same Redis Pub/Sub pattern as WebSockets — each server instance subscribes to a channel and forwards events to its connected clients.

When to Use WebSockets Instead

Choose WebSockets when you need client-to-server streaming (collaborative editing, multiplayer games), binary data transfer, or custom protocols. For everything else — activity feeds, price tickers, build logs, notification streams — SSE is the simpler and more robust choice.