const backends = ["origin_0", "origin_1"];

addEventListener("fetch", event => {
  // Get the request from the client.
  const req = event.request;

  // Pick which origin to use as a backend randomly.
  const randomBackend = backends[Math.floor(Math.random() * backends.length)];

  // Send the request to a random backend.
  const backendResponse = fetch(req, {
    backend: randomBackend
  });

  // Send the backend response back to the client.
  event.respondWith(backendResponse);
});