How to Prevent Interleaved `async_write` Calls in Boost Asio?

How to Prevent Interleaving of async_write Calls in Boost Asio

In distributed systems, it is common for clients to send messages to servers asynchronously. To handle incoming messages, servers typically implement a queue-based mechanism where messages are processed sequentially in the order they are received. However, there are certain scenarios where messages can become interleaved, leading to unexpected behavior.

Problem Description

Consider a scenario involving a server that receives messages from multiple clients simultaneously. Each client's messages are processed asynchronously using async_write. If the clients send messages at a rapid pace, it is possible for the async_write calls to become interleaved, resulting in messages being processed out of order.

Solution: Queue-Based Approach

To prevent the interleaving of async_write calls, a queue-based approach can be employed. Here's how it works:

1. Each client has a dedicated outgoing message queue.
2. When a client sends a message, it is added to the outgoing queue.
3. The server checks the size of the outgoing queue for each client.
4. If the queue is not empty, the server initiates an async_write operation to send the first message in the queue.
5. Upon completion of the async_write operation, the server checks the queue again.
6. If there are more messages in the queue, another async_write operation is initiated.
7. This process repeats until all messages in the queue have been sent successfully.

Implementation Example

The following code snippet demonstrates how to implement this queue-based approach:

// Include necessary headers
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <deque>
#include <iostream>
#include <string>

class Connection
{
public:
    Connection(
            boost::asio::io_service& io_service
            ) :
        _io_service( io_service ),
        _strand( _io_service ),
        _socket( _io_service ),
        _outbox()
    {

    }

    void write( 
            const std::string& message
            )
    {
        _strand.post(
                boost::bind(
                    &Connection::writeImpl,
                    this,
                    message
                    )
                );
    }

private:
    void writeImpl(
            const std::string& message
            )
    {
        _outbox.push_back( message );
        if ( _outbox.size() > 1 ) {
            // outstanding async_write
            return;
        }

        this->write();
    }

    void write()
    {
        const std::string& message = _outbox[0];
        boost::asio::async_write(
                _socket,
                boost::asio::buffer( message.c_str(), message.size() ),
                _strand.wrap(
                    boost::bind(
                        &Connection::writeHandler,
                        this,
                        boost::asio::placeholders::error,
                        boost::asio::placeholders::bytes_transferred
                        )
                    )
                );
    }

    void writeHandler(
            const boost::system::error_code& error,
            const size_t bytesTransferred
            )
    {
        _outbox.pop_front();

        if ( error ) {
            std::cerr << "could not write: " << boost::system::system_error(error).what() << std::endl;
            return;
        }

        if ( !_outbox.empty() ) {
            // more messages to send
            this->write();
        }
    }


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

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

int
main()
{
    boost::asio::io_service io_service;
    Connection foo( io_service );
}

Conclusion

By implementing a queue-based approach, the interleaving of async_write calls can be effectively prevented, ensuring that messages are processed in the correct order. This is particularly important in scenarios where the order of message processing has a significant impact on the overall functionality of the system.

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