A/B testing with same-URL direct access (2)
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A/B testing with same-URL direct access (2)

Set up an A/B test by controlling what response is served based on cookies. This version supports passing the request through to test and control on the origin, bypassing random assignment.

Last updated Apr 23, 2026

const NAME = "myExampleWorkersABTest";

export default {
    async fetch(req) {
        const url = new URL(req.url);

        // Enable Passthrough to allow direct access to control and test routes.
        if (url.pathname.startsWith("/control") || url.pathname.startsWith("/test"))
            return fetch(req);

        // Determine which group this requester is in.
        const cookie = req.headers.get("cookie");

        if (cookie && cookie.includes(`${NAME}=control`)) {
            url.pathname = "/control" + url.pathname;
        } else if (cookie && cookie.includes(`${NAME}=test`)) {
            url.pathname = "/test" + url.pathname;
        } else {
            // If there is no cookie, this is a new client. Choose a group and set the cookie.
            const group = Math.random() < 0.5 ? "test" : "control"; // 50/50 split
            if (group === "control") {
                url.pathname = "/control" + url.pathname;
            } else {
                url.pathname = "/test" + url.pathname;
            }
            // Reconstruct response to avoid immutability
            let res = await fetch(url);
            res = new Response(res.body, res);
            // Set cookie to enable persistent A/B sessions.
            res.headers.append("Set-Cookie", `${NAME}=${group}; path=/`);
            return res;
        }
        return fetch(url);
    },
};

const NAME = "myExampleWorkersABTest";

export default {
    async fetch(req): Promise<Response> {
        const url = new URL(req.url);
        // Enable Passthrough to allow direct access to control and test routes.
        if (url.pathname.startsWith("/control") || url.pathname.startsWith("/test"))
            return fetch(req);
        // Determine which group this requester is in.
        const cookie = req.headers.get("cookie");
        if (cookie && cookie.includes(`${NAME}=control`)) {
            url.pathname = "/control" + url.pathname;
        } else if (cookie && cookie.includes(`${NAME}=test`)) {
            url.pathname = "/test" + url.pathname;
        } else {
            // If there is no cookie, this is a new client. Choose a group and set the cookie.
            const group = Math.random() < 0.5 ? "test" : "control"; // 50/50 split
            if (group === "control") {
                url.pathname = "/control" + url.pathname;
            } else {
                url.pathname = "/test" + url.pathname;
            }
            // Reconstruct response to avoid immutability
            let res = await fetch(url);
            res = new Response(res.body, res);
            // Set cookie to enable persistent A/B sessions.
            res.headers.append("Set-Cookie", `${NAME}=${group}; path=/`);
            return res;
        }
        return fetch(url);
    },
} satisfies ExportedHandler;

import random
from urllib.parse import urlparse, urlunparse
from workers import Response, fetch, WorkerEntrypoint

NAME = "myExampleWorkersABTest"

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        url = urlparse(request.url)
        # Uncomment below when testing locally
        # url = url._replace(netloc="example.com") if "localhost" in url.netloc else url

        # Enable Passthrough to allow direct access to control and test routes.
        if url.path.startswith("/control") or url.path.startswith("/test"):
            return fetch(urlunparse(url))

        # Determine which group this requester is in.
        cookie = request.headers.get("cookie")

        if cookie and f'{NAME}=control' in cookie:
            url = url._replace(path="/control" + url.path)
        elif cookie and f'{NAME}=test' in cookie:
            url = url._replace(path="/test" + url.path)
        else:
            # If there is no cookie, this is a new client. Choose a group and set the cookie.
            group = "test" if random.random() < 0.5 else "control"
            if group == "control":
                url = url._replace(path="/control" + url.path)
            else:
                url = url._replace(path="/test" + url.path)

            # Reconstruct response to avoid immutability
            res = await fetch(urlunparse(url))
            headers = dict(res.headers)
            headers["Set-Cookie"] = f'{NAME}={group}; path=/'
            return Response(res.body, headers=headers)

        return fetch(urlunparse(url))

import { Hono } from "hono";
import { getCookie, setCookie } from "hono/cookie";

const app = new Hono();

const NAME = "myExampleWorkersABTest";

// Enable passthrough to allow direct access to control and test routes
app.all("/control/*", (c) => fetch(c.req.raw));
app.all("/test/*", (c) => fetch(c.req.raw));

// Middleware to handle A/B testing logic
app.use("*", async (c) => {
    const url = new URL(c.req.url);

    // Determine which group this requester is in
    const abTestCookie = getCookie(c, NAME);

    if (abTestCookie === "control") {
        // User is in control group
        url.pathname = "/control" + c.req.path;
    } else if (abTestCookie === "test") {
        // User is in test group
        url.pathname = "/test" + c.req.path;
    } else {
        // If there is no cookie, this is a new client
        // Choose a group and set the cookie (50/50 split)
        const group = Math.random() < 0.5 ? "test" : "control";

        // Update URL path based on assigned group
        if (group === "control") {
            url.pathname = "/control" + c.req.path;
        } else {
            url.pathname = "/test" + c.req.path;
        }

        // Set cookie to enable persistent A/B sessions
        setCookie(c, NAME, group, {
            path: "/",
        });
    }

    const res = await fetch(url);

    return c.body(res.body, res);
});

export default app;

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description: Access custom Cloudflare properties and control how Cloudflare features are applied to every request. title: Accessing the Cloudflare Object image: https://developers.cloudflare.com/workers/examples/accessing-the-cloudflare-object/og.png?v=dac3928a0cc946ff

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A/B testing with same-URL direct access

A/B testing with same-URL direct access (2)

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