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A/B testing with same-URL direct access

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.
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) {
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 js import Response, Headers, fetch
NAME = "myExampleWorkersABTest"
async def on_fetch(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=/'
headers = Headers.new(headers.items())
return Response.new(res.body, headers=headers)
return fetch(urlunparse(url))