How Server-Sent Events (SSE) Work Internally

In the previous article, we understood why Server-Sent Events (SSE) exist. Now let’s go deeper: 👉 How does SSE actually work under the hood? ⚙️ Step 1 — It Starts with a Normal HTTP Request From the browser’s perspective, SSE begins like any other request: GET /events But the server responds differently. 📡 Step 2 — Special Response Header The server sends: Content-Type: text/event-stream This header tells the browser: “This response is not going to end. Keep listening.” 🔁 Step 3 — The Connection Stays Open Unlike normal APIs: Client → request → response → close SSE does this: Client → request Server → response (never ends) 🧠 Mental Model Think of it like opening a tap: Client → open connection Server → stream data continuously Instead of sending a bucket of water, the server sends a flow . 📦 Step 4 — Event Format SSE sends data in a very specific format. Example: data: Order received data: Cooking started data: Out for delivery Important Rules: Each message starts with data: Each event