[Solved] event.waitUntil() doesn't do anything

Developers Workers
#1

Consider this basic worker that needs to respond to the caller immedialty, then hand off the request to another API.

async function saveToDatabase(notification) {
  return fetch('https://example.com/api', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body: JSON.stringify(notification, null, 0)
  });
}

async function saveRequest(request) {
  let body;

  const contentType = request.headers.get('content-type')
  if (contentType.includes('application/json')) {
    body = await request.json();
  }
  
  if (body) {
    return saveToDatabase(body);
  }
}

async function handleRequest(event) {
  if (event.request.method === 'POST') {
    event.waitUntil(saveRequest(event.request));
    // try {
    //   await saveRequest(event.request);
    // } catch (error) {
    //   return new Response(error.toString(), { status: 500 })
    // }
   
    return new Response(undefined, { status: 200 })
  }
  return new Response(undefined, { status: 405 })
}

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event));
})

The function in waitUntil is never executed. If however, I await the function as shown in the commented out code - it works. So the handling + posting obviously works, but I don’t want to force the caller to wait.

What’s the issue?

#2

a quick read might be missing a await on the fetch.

#3

I don’t await it because it can be returned direct form the async function as it, and the caller of saveToDatabase gets the fetch promise.

#4

One of the quirks of the Service Worker specification is that It’s not possible to read event.request's body after having generated a response, so I expect that the call to request.json() is resulting in an exception.

A quick workaround would be to read the request body up front, then call event.waitUntil() and return the Response object.

Edit: I had thought this behavior was mandated by the Service Worker spec, but after searching for a reference, I’m not so sure. At any rate, what I described is accurate for our implementation – still researching why.

2 Likes
#5

That solved it for me thank you @harris. I await loading the body before beginning processing the response or the waitUntil. I tested this with an setTimeout to exaggerate the delay and the request responded instantly, while the saveToDatabase function would now be called after. :white_check_mark:

async function handleRequest(event) {
  if (event.request.method === 'POST') {
    const body = await getBody(event.request);
    if (body) event.waitUntil(saveToDatabase(body)); // this might actually take a few seconds
    return new Response(undefined, { status: 200 }) // caller sees instant response regardless
  }
  return new Response(undefined, { status: 405 })
}