Fastly VCL config for A/B tests

vcl_deliver.vcl

sub vcl_deliver {
  # ----- snip -----
  
  # Set a cookie with the experiment group sequence, if the client didn't
  # already have the cookie and provided it in the request, and if the origin
  # hasn't already set it. Use `setcookie.get_value_by_name` to handle the
  # cases when the backend sets multiple cookies.

  # Only do this on the edge nodes, not the origin shield, otherwise the edge
  # nodes will see a response with a `Set-Cookie` header and not cache it.
  if (!req.http.Fastly-FF && req.http.X-Tt-Backend-AB-First ~ "1" && !setcookie.get_value_by_name(resp, "tt_bab")) {
    add resp.http.Set-Cookie = "tt_bab=" req.http.X-Tt-Backend-AB "; Expires=" time.add(now, 3650d) "; Path=/";
  }
  
  # ----- snip -----
}

vcl_recv.vcl

sub vcl_recv {
  # ----- snip -----
  
  # Create and manage the Tictail Backend AB Experiments cookie. This cookie is
  # a random sequence of As and Bs, and is assigned to each client. The backend
  # can use this cookie, and the corresponding X-Tt-Backend-AB header, to place
  # the client into groups for backend A/B tests.

  # For example: an A as the first character could mean that the client is in
  # group A, control, for test X, while a B would mean group B, treatment. 50%
  # of all clients would then be in each group. The second character could be
  # used for another test Y. X and Y would then be independent.

  # The backend can decide whether to have this cookie/experiment group
  # placement as part fo then Fastly cache key by including `X-Tt-Backend-AB`
  # in its `Vary` response header.

  if (!req.http.Fastly-FF) {
    # Only do this at the edge nodes, not the origin shield. There's no need to
    # do it twice.

    if (req.http.Cookie ~ "tt_bab=") {
      set req.http.X-Tt-Backend-AB = req.http.cookie:tt_bab;
    } else {
      set req.http.X-Tt-Backend-AB-First = "1";

      set req.http.X-Tt-Backend-AB = randomstr(2, "AB");

      if (req.http.Cookie) {
        set req.http.Cookie = req.http.Cookie "; tt_bab=" req.http.X-Tt-Backend-AB;
      } else {
        set req.http.Cookie = "tt_bab=" req.http.X-Tt-Backend-AB;
      }
    }
  }
  
  # ----- snip -----
}