Vercel Functions: Why Logs Get Mixed

Understanding how asynchronous tasks in serverless environments can cause logs from different requests to appear under the wrong request ID.

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Step 1: Initial State

Function environment is ready to handle requests

Request A
Not Started

Function Execution
Waiting...
Async Task
Not started

Function Environment
Ready

⚪
Waiting for requests

Request B
Not Started

Function Execution
Waiting...
Runtime Logs

Notice how the orphaned task's log appears under Request B's ID

No logs yet...
✅ Solution 1: Await Critical Tasks
// CORRECT: Await tasks needed for response
const [user, products] = await Promise.all([
  db.users.find(request.user.id),
  db.products.getForUser(request.user.id),
]);

return Response.json({ user, products });
✅ Solution 2: Use waitUntil() for Background Tasks
import { waitUntil } from '@vercel/functions';

// Background task won't block response
waitUntil(
  fetch('https://analytics.com/api/log', {
    method: 'POST',
    body: JSON.stringify({ event: 'viewed' }),
  })
);

return new Response('Success!');