How Can Per-Client Outbound Queues Solve Async_write Interleaving in Boost Asio?

Async Writes and Client Interactions in boost asio

In asynchronous programming using boost asio, managing concurrent client interactions can pose challenges. Specifically, the interleaving of async_write calls can occur when multiple clients send messages rapidly.

The Problem: Interleaving Async Writes

Consider a scenario where Client A sends messages to Client B. The server processes these messages asynchronously, reading the specified data amount and waiting for further data from Client A. Once information is processed, the server sends a response to Client B via async_write.

However, if Client A sends messages at a rapid pace, the async_write operations for these messages may interleave before the completion handler for the previous message is invoked.

A Solution: Per-Client Outbound Queues

To address this issue, consider implementing an outgoing queue for each client. By checking the queue size in the async_write completion handler, the server can determine if additional messages are available for sending. If so, it triggers another async_write operation.

Code Example

The following code snippet illustrates a possible implementation using an Outbox queue:

class Connection {
public:
    ...
private:
    void writeImpl(const std::string& message) {
        _outbox.push_back(message);
        if (_outbox.size() > 1) {
            // Outstanding async_write
            return;
        }
        this->write();
    }

    void write() {
        ... // Asynchronous write operation
    }

    void writeHandler(const boost::system::error_code& error, const size_t bytesTransferred) {
        _outbox.pop_front();
        if (error) {
            // Handle error
            ...
        }
        if (!_outbox.empty()) {
            // More messages to send
            this->write();
        }
    }

private:
    typedef std::deque<:string> Outbox;

private:
    boost::asio::io_service& _io_service;
    boost::asio::io_service::strand _strand;
    boost::asio::ip::tcp::socket _socket;
    Outbox _outbox;
};</:string>

Key Points

• The boost::asio::io_service::strand protects concurrent access to the per-client Outbox queue.
• The write() method is invoked from the writeImpl() handler because it is public.

By implementing per-client outgoing queues, you can effectively prevent the interleaving of async_write calls, ensuring that messages are processed in the intended order.

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