location / {
        error_page 598 = @retry;
        error_page 599 = @no_retry;
        if ($request_method = POST) {
            return 599;
        }
        return 598;
    }

    location @retry {
        proxy_pass http://backend;
    }

    location @no_retry {
        proxy_pass http://backend;
        proxy_next_upstream off;
    }
}