Server-side A/B testing with NGINX & Varnish (WP Engine)

Before In Location

add_header Set-Cookie "my_test_group=$test_group;Domain=.mydomain.com;Path=/;Max-Age=604800";

Before Location

#############################
# Define our A/B Test group #
#############################
set $test_group a;
 
if ( $time_iso8601 ~ '[13579]\+00:00$' ) {
    set $test_group b;
}
 
if ( $cookie_wpe_test_group != '' ) {
    set $test_group $cookie_my_test_group;
}
 
####################
# Redirect Group B #
####################
set $variant '';
 
# Redirect Group B
if ( $test_group = b ) {
    set $variant 'new';
}
 
# Don't redirect if we're hitting a WP resource file.
if ( $request_uri ~* ^/(wp-admin|wp-content|wp-includes)/? ) {
    set $variant '';
}

# Don't redirect if we're already in the experiment page.
if ( $request_uri ~* ^/new/? ) {
    set $variant '';
}
 
# Redirect to language path if we have one
if ( $variant != '' ) {
    rewrite .* "/$variant/$request_uri" permanent;
}

This setup in WP Engine's NGINX rules will send 1/2 of all visitors from mydomain.com/path/to/post/ to mydomain.com/new/path/to/post/.