In-Order Delivery

Here is an example that sends and receives cross-chain messages with guaranteed in-order delivery.

Source code at GitHub.

// a simple example to enforce in-order message delivery
contract MsgExampleInOrder is MessageApp {
    event MessageReceived(
        address srcContract,
        uint64 srcChainId,
        address sender,
        uint64 seq,
        bytes message
    );
    
    // map at source chain. (dstChainId, dstContract) -> seq
    mapping(uint64 => mapping(address => uint64)) public sendSeq;

    // map at destination chain (srcChainId, srcContract) -> seq
    mapping(uint64 => mapping(address => uint64)) public recvSeq;

    constructor(address _messageBus) MessageApp(_messageBus) {}

    // called by user on source chain to send cross-chain message
    function sendMessage(
        address _dstContract,
        uint64 _dstChainId,
        bytes calldata _message
    ) external payable {
        uint64 seq = sendSeq[_dstChainId][_dstContract];
        bytes memory message = abi.encode(msg.sender, seq, _message);
        sendMessage(_dstContract, _dstChainId, message, msg.value);
        sendSeq[_dstChainId][_dstContract] += 1;
    }

    // called by MessageBus on destination chain to receive message
    function executeMessage(
        address _srcContract,
        uint64 _srcChainId,
        bytes calldata _message,
        address // executor
    ) external payable override onlyMessageBus returns (ExecutionStatus) {
        (address sender, uint64 seq, bytes memory message) = abi.decode(
            (_message),
            (address, uint64, bytes)
        );
        uint64 expectedSeq = recvSeq[_srcChainId][_srcContract];
        if (seq != expectedSeq) {
            // sequence number not expected, let executor retry.
            // Note: cannot revert here, because once a message execute tx is 
            // reverted, it cannot be retried later.
            return ExecutionStatus.Retry;
        }
        emit MessageReceived(_srcContract, _srcChainId, sender, seq, message);
        recvSeq[_srcChainId][_srcContract] += 1;
        return ExecutionStatus.Success;
    }
}