Weighted traffic splitting

Send a set percentage of visitors to a second origin and keep each visitor on the same origin across requests.

This script hashes a stable key (the client IP) into a 0-99 bucket and routes by percentage. The same key always lands in the same bucket, so a visitor stays on one origin across requests.

import * as BunnySDK from "@bunny.net/edgescript-sdk";

const ORIGINS = {
  a: "https://origin-a.example.com",
  b: "https://origin-b.example.com",
};

const B_PERCENTAGE = 10; // send 10% of visitors to origin B

// Polynomial string hash (Java String.hashCode style), mapped into a 0-99 bucket
function bucket(key: string): "a" | "b" {
  let h = 0;
  for (let i = 0; i < key.length; i++) {
    h = (h * 31 + key.charCodeAt(i)) >>> 0;
  }
  return h % 100 < B_PERCENTAGE ? "b" : "a";
}

BunnySDK.net.http.serve(async (request: Request): Promise<Response> => {
  const ip = request.headers.get("x-forwarded-for") ?? "0.0.0.0";
  const variant = bucket(ip);

  const target = new URL(request.url);
  const origin = new URL(ORIGINS[variant]);
  target.protocol = origin.protocol;
  target.host = origin.host;

  return fetch(new Request(target, request));
});

Adjust B_PERCENTAGE to change the split. To pin on something other than the client IP, such as a cookie or a header, hash that value instead.