为什么 onmessage 侦听器不处理初始 SSE 事件？

Question

在此示例中，客户端通过创建新的 EventSource 连接到 Node Express 服务器。服务器在初始连接时发送 SSE 事件，然后每隔 30 秒发送一次。问题在于客户端的 onmessage 处理程序不是由连接时发送的初始 SSE 事件触发的，而是由所有后续事件触发的。如果我在建立连接后立即停止服务器，这实际上会在客户端触发一个事件，该事件表明数据实际上是在没有被处理的情况下被接收到的（或者初始 res.write 仅由服务器正在关闭）：

服务器：

let data = {mydata: 123};

function eventsHandler(req, res, next) {

  const headers = {
    "Content-Type": "text/event-stream",
    Connection: "keep-alive",
    "Cache-Control": "no-cache",
  };

  res.writeHead(200, headers);
  // After client opens connection we send data string, but client doesn't handle this correctly
  // Event is only triggered on client if I stop the server after sending this message
  res.write(`data: ${JSON.stringify(data)}\n\n`);
  console.log("sent SSE");

  const clientId = Date.now();
  console.log(clientId, " connected");
  const newClient = {
    id: clientId,
    res,
  };
  clients.push(newClient);

  req.on("close", () => {
    console.log(`${clientId} Connection closed`);
    clients = clients.filter((c) => c.id !== clientId);
  });
}

// these events are received and handled correctly on client
function sendEventsToAll(datadict) {
  clients.forEach((c) => c.res.write(`data: ${JSON.stringify(datadict)}\n\n`));
}

// Route to open SSE connection
app.get("/events", eventsHandler);

// Loop to send SSE every 30 seconds - these are all received and handled correctly
async function wait() {
  sendEventsToAll(data);
  setTimeout(wait, 30000);
}

wait();

浏览器：

      const events = new EventSource(url)

      // This handles every SSE event except the one that is written immediately upon connection
      // If I stop the server after connection, this will trigger and reveal the data was received but not handled
      events.onmessage = event => {
        const newData = JSON.parse(event.data)
        console.log("newData", newData)
      }

编辑：我还应该补充一点，我正在使用 nginx 作为反向代理 - 这是否可以缓冲第一个 res.write 而不是出于某种原因立即发送它？

Answer 1

这是由 nginx 中的 proxy_buffering 引起的，它默认处于“打开”状态。

在 nginx 配置中关闭它解决了问题：

server {
# server config here...

   location / {
   # proxy config here...
   proxy_buffering off; # resolves issue
   }

}

为什么 onmessage 侦听器不处理初始 SSE 事件？

Why is the initial SSE event not handled by the onmessage listener?

javascript

nginx

node.js

express

server-sent-events