Delving into Server-Sent Events (SSE): Unidirectional Data Streaming Made Simple

In the expansive cosmos of real-time technologies, Server-Sent Events (SSE) often remain unnoticed. However, SSE is an ideal solution for numerous use cases due to its lightweight nature, reliability, native support for HTTP, and suitability for continuous data streaming from server to client.

While WebSockets provide bidirectional communication, SSE is a unidirectional push mechanism over HTTP. The server streams events to the client — including notifications, logs, dashboard updates, live counters, or telemetry data — through a persistently open connection and a straightforward event protocol.

SSE merges the best of both realms: the immediacy of real-time updates with the simplicity and universality of HTTP.

Unraveling SSE

SSE is natively built into browsers through the EventSource API, a feature of HTML5. This means no need for polyfills, custom protocols, or specialized client SDKs, which simplifies its implementation.

const source = new EventSource("/api/stream");

source.onmessage = (event) => {
  console.log("New message:", event.data);
};

The server maintains the connection open and streams text-based events formatted as follows:

data: {"message": "Hello, world!"}

Each event can include:

• event: for denoting a custom event name,
• id: for identifying a reconnection,
• retry: for setting the reconnection delay in milliseconds (ms).

Why Utilize SSE?

SSE proves beneficial when:

• Real-time delivery from server to browser is necessary,
• Only the server sends data (such as alerts or counter updates),
• Automatic reconnection and handling are desirable,
• Compatibility with HTTP/1.1 is required (avoiding custom upgrade).

Compared to WebSockets, SSE is:

• Simpler to set up,
• Compatible with HTTP/2,
• More cooperative with proxies, firewalls, and load balancers.

Illustration: A Basic SSE Server in Node.js

app.get("/api/stream", (req, res) => {
  res.setHeader("Content-Type", "text/event-stream");
  res.setHeader("Cache-Control", "no-cache");
  res.setHeader("Connection", "keep-alive");

  setInterval(() => {
    res.write(`data: ${JSON.stringify({ timestamp: Date.now() })}\n\n`);
  }, 1000);
});

The connection remains open — and the browser continuously receives onmessage events every second.

Ideal Use Cases for SSE

Notifications

SSE can push real-time notifications to the UI, eliminating the need for polling. This ensures messages are delivered promptly as they are emitted.

Live Counters/Dashboards

SSE is ideal for metrics, analytics, and dashboards that require frequent updates. It provides continuous streams without introducing overhead.

CI/CD Logs

Streaming logs from a build pipeline or deployment job is straightforward with SSE. Events are received as soon as they are flushed server-side, ensuring real-time updates.

Comparing SSE and WebSockets

| Feature | SSE | WebSocket | |-----------------------|-------------------|---------------------| | Direction | Server → Client | Bi-directional | | Browser support | Native (EventSource) | Native + JS API | | Transport | HTTP | Custom protocol | | Reconnection | Auto | Manual | | Ease of use | ✅ Simple | ⚠️ More complex | | Proxies/CDN friendly | ✅ Yes | ⚠️ Can be problematic | | Typical use case | Notifications, logs | Chats, games |

Limitations of SSE

• One-way only: Data cannot be sent from the client to the server over the same channel,
• Limited browser support: Not supported in Internet Explorer or some older browsers,
• Not suitable for binary data: Text-based only,
• Some CDNs and proxies might buffer output unless X-Accel-Buffering or flush() is used.

Reconnection and Reliability with SSE

SSE embeds recovery logic:

• Clients automatically reconnect upon disconnection,
• You can use id: to mark the last message,
• The server receives a Last-Event-ID header to resume the stream.

id: 172456
data: {"message": "Hello again"}

Use these features for:

• Recovering from disconnection,
• Resuming missed messages,
• Implementing "at-least-once" delivery guarantees.

Real-World Applications of SSE

Netlify/Vercel

These platforms use SSE to stream logs and deployment events to developer dashboards.

GitHub Actions

GitHub Actions employs SSE (or WebSockets depending on the browser) to stream CI logs live to the job viewer.

Firebase Emulator

The Firebase Emulator uses SSE for local data streaming in the developer console, making mock data updates visible instantly.

Server-Side Considerations for SSE

• Set Content-Type: text/event-stream,
• Flush headers immediately,
• Disable buffering on reverse proxies (e.g., X-Accel-Buffering: no),
• Stream heartbeats every 20–30s to avoid idle timeout.

Example in Express:

res.write(":

"); // heartbeat

Anti-Patterns with SSE

• Using SSE for high-throughput chat applications (WebSockets are more suitable),
• Forgetting to manually reconnect in legacy fallback,
• Not closing connections upon client inactivity,
• Streaming large payloads (SSE is better suited for small messages).

Conclusion: Streamlined Streaming with SSE

SSE is the most straightforward, frictionless method to push real-time data from server to client — no need for upgrades, custom headers, just plain HTTP.

While it won't replace WebSockets for every scenario, for notifications, metrics, logs, and unidirectional updates, it's a remarkably efficient tool.

Don't overcomplicate real-time communication. Sometimes, a stream of text is all you need.