How to Prevent Interleaving in Asynchronous Write Operations in a Client-Server Architecture?

Avoiding Interleaving in async_write Calls

In a client-server architecture, maintaining the order of async_write calls is crucial to ensure data integrity. The issue arises when multiple clients send messages rapidly, causing subsequent async_write operations to interweave.

Solution: Queuing for Each Client

To resolve this, it is recommended to employ an outgoing queue for each client. The queue serves as a buffer for messages, ensuring that they are processed and sent in the correct sequence.

How It Works

1. Upon a client's message reception, the server enqueues it into the client's outgoing queue.
2. When the async_write operation for the current message completes, the server inspects the queue size.
3. If the queue is non-empty, a new async_write operation is initiated to send the next message in the queue.
4. If the queue is empty, the server waits for the next message to arrive.

Code Sample

The following code demonstrates a server implementation with an outgoing queue:

class Connection {
public:
    // ...
    void write(const std::string& message) {
        _strand.post([this, message] { writeImpl(message); });
    }

private:
    void writeImpl(const std::string& message) {
        _outbox.push(message);
        if (_outbox.size() > 1) return;
        write();
    }

    void write() {
        const std::string& message = _outbox.front();
        async_write(_socket, buffer(message), _strand.wrap(
            [this, err, bytesTransferred] { writeHandler(err, bytesTransferred); }));
    }

    void writeHandler(const std::error_code& error, size_t bytesTransferred) {
        _outbox.pop_front();
        handle error or send next message if the queue is not empty.
    }

private:
    boost::asio::strand _strand;
    std::deque<std::string> _outbox;
};

Key Points

• Each client has its own outgoing queue, protected by the boost::asio::io_service::strand.
• The write() method is responsible for enqueuing messages and triggering asynchronous send operations.
• The writeHandler() method processes asynchronous async_write completion and manages the queue accordingly.

By employing these measures, the server effectively avoids interleaving of async_write calls, ensuring the proper ordering of messages for each client.

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