How to Prevent Async Write Call Interleaving in Boost Asio?

Asynchronous Write Calls Interleaving in Boost Asio

In boost asio, async_write calls can interleave if multiple messages are sent in rapid succession. This can lead to unexpected behavior and data corruption. To avoid this issue, a simple solution is to maintain an outgoing queue for each client.

Using an Outgoing Queue

By using an outgoing queue, you can ensure that messages are sent in the order they were received. When a new message arrives, it is added to the queue. The async_write handler checks the queue size and starts a new async_write operation if there are more messages to send.

Example Implementation

Here is a sample implementation of an async write handler that uses an outgoing queue:

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();
    }
}

In this example, the writeHandler starts a new async_write operation if there are more messages in the _outbox.

Protecting Access to the Outbox

To protect access to the _outbox, you should use a strand or synchronization primitive. A strand ensures that only one thread can execute code that accesses the _outbox at a time, preventing race conditions.

Key Points

• Maintain an outgoing queue for each client.
• Use an async_write handler that checks the queue size and starts a new async_write operation if there are more messages to send.
• Protect access to the outgoing queue using a strand or synchronization primitive.

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