SSE issues with vercel

[GoldenGamerLP]

Environment

Using NodeJs 21.6.2 and deployed on Vercel. Using chrome Version 124.0.6367.61 (Offizieller Build)

Reproduction

 * import { createEventStream, sendEventStream } from "h3";
 *
 * eventHandler((event) => {
 *   const eventStream = createEventStream(event);
 *
 *   // Send a message every second
 *   const interval = setInterval(async () => {
 *     await eventStream.push("Hello world");
 *   }, 1000);
 *
 *   // cleanup the interval and close the stream when the connection is terminated
 *   eventStream.onClosed(async () => {
 *     console.log("closing SSE...");
 *     clearInterval(interval);
 *     await eventStream.close();
 *   });
 *
 *   return eventStream.send();
 * });
 * ```

• This is the default example

Describe the bug

In a nutshell: The time it takes the client to connect with the SSE is between 10 and up to 30 seconds. I dont know whats going on, but its not usable right now. I hope someone knows whats keeping the time to connect so high.

Regards

Alex

Additional context

The source code on which Iam working on, but has the same issues:

import { addSyncClient, removeSyncClient } from "~/server/utils/sync";

export default eventHandler(async (event) => {
  const { boardId } = getRouterParams(event);
  const { uniqueFingerprint } = getQuery(event);

  if (!boardId || !uniqueFingerprint || typeof uniqueFingerprint !== "string") {
    throw createError({
      status: 400,
      statusText: "Bad Request",
    });
  }

  const eventStream = createEventStream(event);

  // Add the client to the sync clients
  addSyncClient(uniqueFingerprint, boardId.toString(), event.context.user ?? undefined, eventStream);

  // Implement keep-alive mechanism
  const keepAliveInterval = setInterval(() => {
    eventStream.push("\n\nkeep-alive\n\n");
  }, 10000); // Send a keep-alive comment every 10 seconds

  eventStream.onClosed(async () => {
    clearInterval(keepAliveInterval); // Clear the keep-alive interval
    removeSyncClient(boardId.toString(), uniqueFingerprint);
    await eventStream.close();
  });

  return eventStream.send();
});

Logs

No response

[GoldenGamerLP]

New clue: I've figured that the "return #send()" at the end of the event handler is the problem. On vercel, this methods times out.

[GoldenGamerLP]

Found the problem: https://stackoverflow.com/questions/76204143/sse-works-locally-but-not-when-deployed-to-vercel

My question now is, is there a way to implement the vercel/streaming api into h3?