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Cloudflare “Error 1101: Worker Threw Exception” – How to troubleshoot

by | Jun 29, 2021

Wondering how to fix Cloudflare “Error 1101: Worker Threw Exception”? We can help you.

Generally the “Error 1101: Rendering error. Worker Threw Exception” occurs when Cloudflare Worker is throwing a runtime JavaScript exception.

Here at Bobcares, we often handle requests from our customers using Cloudflare to fix similar errors as a part of our Server Management Services.

Today we will see how our Support Engineers fix this for our customers.

How to troubleshoot Cloudflare “Error 1101: Worker Threw Exception”

Following are the steps with which our Support Engineers troubleshoot this issue:

Identifying error: Workers Metrics

We can find out whether the application is experiencing any downtime or returning any errors by navigating to Workers Metrics in the dashboard.

Debugging exceptions

After identifying the Workers application that is returning exceptions, we can use wrangler tail to inspect, and fix the exceptions.

Exceptions can be seen under the exceptions field in the JSON returned by wrangler tail.

Once we identify the exception that is causing errors, we can redeploy our code with a fix, and continue trailing the logs to confirm that the issue is fixed.

Setting up a logging service

A Worker can make HTTP requests to any HTTP service on the public internet. We can use a service like Sentry to collect error logs from the Worker, by making an HTTP request to the service to report the error.

While logging in using this strategy, we must remember that outstanding asynchronous tasks are canceled as soon as a Worker finishes sending its main response body to the client.

To ensure that a logging subrequest completes, we can pass the request promise to event.waitUntil()
For example:

addEventListener("fetch", event => {  event.respondWith(handleEvent(event))}
async function handleEvent(event) {  // ...
  // Without event.waitUntil(), our fetch() to our logging service may  // or may not complete.  event.waitUntil(postLog(stack))  return fetch(event.request)}
function postLog(data) {  return fetch("", {    method: "POST",    body: data,  })}

Go to Origin on Error

By using event.passThroughOnException, the Workers application will pass requests to our origin if it throws an exception.

As a result, it allows us to add logging, tracking, or other features with Workers, without harming our website’s functionality.

addEventListener("fetch", event => { event.passThroughOnException() event.respondWith(handleRequest(event.request))}) async function handleRequest(request) { // An error here will return the origin response, as if the Worker wasn’t present. // ... return fetch(request)}

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To conclude, we saw the steps that our Support Techs follow to fix this error for our customers.


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