gistfile1.txt

location ~* \.(json)$ {
    set memcached_key $key;
    memcached_pass $uri;
    echo_location_async /json_available;
    error_page 404 @proxy;
}

location /json_available {
    echo_duplicate 1 "$arg_callback(";
    echo_location_async "/json_memcached";
    echo_duplicate 1 ")";
}

location /json_memcached {
    set memcached_key $key;
    memcached_pass $uri;
    error_page 404 @proxy;
}