Server-Sent Events (SSE) 是一種允許服務(wù)器通過單個(gè)持久化的HTTP 連接向客戶端推送更新的技術(shù)。 1. 它採用單向通信，僅支持服務(wù)器向客戶端發(fā)送數(shù)據(jù)；2. 基於標(biāo)準(zhǔn)HTTP 協(xié)議，便於集成到現(xiàn)有架構(gòu)；3. 支持自動(dòng)重連，提升連接穩(wěn)定性；4. 使用text/event-stream 格式傳輸事件流?？蛻舳送ㄟ^EventSource API 接收消息，適用於實(shí)時(shí)通知、股票行情、監(jiān)控面板等場景。對(duì)於需要頻繁反向通信的應(yīng)用（如多人遊戲），應(yīng)選擇WebSocket。 SSE 在現(xiàn)代瀏覽器中廣泛支持，但在IE 和舊版移動(dòng)瀏覽器中需使用回退方案或polyfill 處理。

Server-Sent Events (SSE) is a technology that allows a server to push updates to a client over a single, long-held HTTP connection. Unlike traditional request-response models where the client has to poll the server for updates, SSE enables real-time communication from the server to the client.

What makes SSE different from other real-time technologies?

SSE is often compared to WebSocket and HTTP/2 Server Push, but it's simpler and more lightweight. Here are a few key differences:

• Unidirectional Communication : SSE only allows servers to send data to clients, whereas WebSockets support bidirectional communication.
• Built on HTTP : It uses standard HTTP protocols, making it easier to implement with existing web infrastructure.
• Automatic Reconnection : If the connection drops, the browser automatically tries to reconnect.
• Event Stream Format : The server sends data using a special MIME type ( text/event-stream ), which the browser parses into events.

How does SSE work in practice?

When a client wants to receive updates, it opens an HTTP connection to a server endpoint that supports SSE. The server then keeps this connection open and sends data as new events occur.

Here's a basic example of how you might use SSE in JavaScript:

 const eventSource = new EventSource("example.php");

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

On the server side (in PHP here), you'd output something like:

 data: This is a message\n\n

This sends "This is a message" to the client whenever the condition is met.

When should you use Server-Sent Events?

SSE works best when your application needs to receive real-time updates from the server, but doesn't need to send frequent messages back. Common use cases include:

• Live news feeds or stock tickers
• Real-time notifications
• Monitoring dashboards
• Chat applications where the client mostly listens

If you're building something where the client needs to talk back to the server frequently (like a multiplayer game), you might want to consider WebSockets instead.

Browser support and fallbacks

Most modern browsers support SSE, including Chrome, Firefox, Safari, and Edge. However, Internet Explorer and some older mobile browsers do not support it natively.

If you need to support older browsers, you may have to fall back to polling or use a polyfill library that mimics SSE behavior using techniques like long polling.

Also, because SSE relies on text-based streaming, binary data isn't supported directly — though you can encode binary data as base64 if needed.

That's basically what Server-Sent Events are and how they work — not complicated, but powerful when used in the right context.

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