Debugging logs (2)
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Debugging logs (2)

Send debugging information in an errored response to a logging service.

Last updated Apr 23, 2026 If you want to get started quickly, click on the button below.

This creates a repository in your GitHub account and deploys the application to Cloudflare Workers.

export default {
    async fetch(request, env, ctx) {
        // Service configured to receive logs
        const LOG_URL = "https://log-service.example.com/";

        async function postLog(data) {
            return await fetch(LOG_URL, {
                method: "POST",
                body: data,
            });
        }

        let response;

        try {
            response = await fetch(request);
            if (!response.ok && !response.redirected) {
                const body = await response.text();
                throw new Error(
                    "Bad response at origin. Status: " +
                        response.status +
                        " Body: " +
                        // Ensure the string is small enough to be a header
                        body.trim().substring(0, 10),
                );
            }
        } catch (err) {
            // Without ctx.waitUntil(), your fetch() to Cloudflare's
            // logging service may or may not complete
            ctx.waitUntil(postLog(err.toString()));
            const stack = JSON.stringify(err.stack) || err;
            // Copy the response and initialize body to the stack trace
            response = new Response(stack, response);
            // Add the error stack into a header to find out what happened
            response.headers.set("X-Debug-stack", stack);
            response.headers.set("X-Debug-err", err);
        }
        return response;
    },
};

interface Env {}
export default {
    async fetch(request, env, ctx): Promise<Response> {
        // Service configured to receive logs
        const LOG_URL = "https://log-service.example.com/";

        async function postLog(data) {
            return await fetch(LOG_URL, {
                method: "POST",
                body: data,
            });
        }

        let response;

        try {
            response = await fetch(request);
            if (!response.ok && !response.redirected) {
                const body = await response.text();
                throw new Error(
                    "Bad response at origin. Status: " +
                        response.status +
                        " Body: " +
                        // Ensure the string is small enough to be a header
                        body.trim().substring(0, 10),
                );
            }
        } catch (err) {
            // Without ctx.waitUntil(), your fetch() to Cloudflare's
            // logging service may or may not complete
            ctx.waitUntil(postLog(err.toString()));
            const stack = JSON.stringify(err.stack) || err;
            // Copy the response and initialize body to the stack trace
            response = new Response(stack, response);
            // Add the error stack into a header to find out what happened
            response.headers.set("X-Debug-stack", stack);
            response.headers.set("X-Debug-err", err);
        }
        return response;
    },
} satisfies ExportedHandler<Env>;

from workers import WorkerEntrypoint
from pyodide.ffi import create_proxy
from js import Response, fetch

async def post_log(data):
    log_url = "https://log-service.example.com/"
    await fetch(log_url, method="POST", body=data)

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        # Service configured to receive logs
        response = await fetch(request)

        try:
            if not response.ok and not response.redirected:
                body = await response.text()
            # Simulating an error. Ensure the string is small enough to be a header
            raise Exception(f'Bad response at origin. Status:{response.status} Body:{body.strip()[:10]}')
        except Exception as e:
            # Without ctx.waitUntil(), your fetch() to Cloudflare's
            # logging service may or may not complete
            self.ctx.waitUntil(create_proxy(post_log(str(e))))
            # Copy the response and add to header
            response = Response.new(stack, response)
            response.headers["X-Debug-err"] = str(e)

        return response

import { Hono } from 'hono';

// Define the environment with appropriate types
interface Env {}

const app = new Hono<{ Bindings: Env }>();

// Service configured to receive logs
const LOG_URL = "https://log-service.example.com/";

// Function to post logs to an external service
async function postLog(data: string) {
  return await fetch(LOG_URL, {
    method: "POST",
    body: data,
  });
}

// Middleware to handle error logging
app.use('*', async (c, next) => {
  try {
    // Process the request with the next handler
    await next();

    // After processing, check if the response indicates an error
    if (c.res && (!c.res.ok && !c.res.redirected)) {
      const body = await c.res.clone().text();
      throw new Error(
        "Bad response at origin. Status: " +
        c.res.status +
        " Body: " +
        // Ensure the string is small enough to be a header
        body.trim().substring(0, 10)
      );
    }

  } catch (err) {
    // Without waitUntil, the fetch to the logging service may not complete
    c.executionCtx.waitUntil(
      postLog(err.toString())
    );

    // Get the error stack or error itself
    const stack = JSON.stringify(err.stack) || err.toString();

    // Create a new response with the error information
    const response = c.res ?
      new Response(stack, {
        status: c.res.status,
        headers: c.res.headers
      }) :
      new Response(stack, { status: 500 });

    // Add debug headers
    response.headers.set("X-Debug-stack", stack);
    response.headers.set("X-Debug-err", err.toString());

    // Set the modified response
    c.res = response;
  }
});

// Default route handler that passes requests through
app.all('*', async (c) => {
  return fetch(c.req.raw);
});

export default app;

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Debugging logs (2)

Debugging logs (2)

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