  /**
   * @notice Process stored callback function
   * @param transferId The transferId to process
   */
  function process(bytes32 transferId, bytes calldata _message) public nonReentrant {
    // parse out the return data and callback address from message
    bytes32 messageHash = messageHashes[transferId];
    if (messageHash == bytes32(0)) revert PromiseRouter__process_invalidTransferId();

    bytes29 _msg = _message.ref(0).mustBePromiseCallback();
    if (messageHash != _msg.keccak()) revert PromiseRouter__process_invalidMessage();

    // enforce relayer is whitelisted by calling local connext contract
    if (!connext.approvedRelayers(msg.sender)) revert PromiseRouter__process_notApprovedRelayer();

    address callbackAddress = _msg.callbackAddress();

    if (!AddressUpgradeable.isContract(callbackAddress)) revert PromiseRouter__process_notContractCallback();

    uint256 callbackFee = callbackFees[transferId];

    // remove message
    delete messageHashes[transferId];

    // remove callback fees
    callbackFees[transferId] = 0;

    // execute callback
    ICallback(callbackAddress).callback(transferId, _msg.returnSuccess(), _msg.returnData());

    emit CallbackExecuted(transferId, msg.sender);

    // Should transfer the stored relayer fee to the msg.sender
    if (callbackFee > 0) {
      AddressUpgradeable.sendValue(payable(msg.sender), callbackFee);
    }
  }
