def start_response_callback(start_response, transaction_id):
        """Intercept upstream start_response and adds our headers."""
        def callback(status, headers, exc_info=None):
            """Add our headers to response using a closure."""
            headers.append(('X-Transaction-Id', transaction_id))
            # Call upstream start_response
            start_response(status, headers, exc_info)
        return callback