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    <blockquote> <p>The standard async approach fails here (the send callbacks may block other callbacks while waiting for a new object to send)</p> </blockquote> <p>The async model should work fine, and if used properly is definitely going to scale better&mdash;it only falls apart when you introduce blocking. Instead of ditching async, here's what I would suggest:</p> <p>Remove the condition variable. You need two queues: OUTPUT and WAITING.</p> <p>Then when processing a client:</p> <ol> <li>If there's data in the OUTPUT queue, send it.</li> <li>If not, push it on the WAITING queue.</li> </ol> <p>There is no requirement to perform the next I/O from within the handler of a previous one. Here instead of blocking on a condition variable, we just offhand it onto the WAITING queue for later processing.</p> <p>And in the OUTPUT-pushing code:</p> <ol> <li>If there are clients in the WAITING queue, send the data directly.</li> <li>If not, push onto the OUTPUT queue.</li> </ol> <p>Here's some pseudo-code:</p> <pre><code>queue&lt;packet&gt; output; queue&lt;client&gt; waiting; void try_send(client c) { if(!output.empty()) { // there is output waiting to be sent, send it. packet p = output.pop(); c.async_send(p, on_send_finished); } else { // nothing available, go back to waiting. waiting.push(c); } } void on_send_finished(client c) { // send finished, try again if any more output has accumulated: try_send(c); } void push_output(packet p) { output.push(p); if(!waiting.empty()) { // there is a client waiting to send, give it a try. client c = waiting.pop(); try_send(c); } } </code></pre> <p>This can all be done in a scalable way using a single thread, but multiple threads are pretty easy with asio. If you're going to use multiple threads, you'll want to introduce a lock in the logic that checks the queues.</p>
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    1. POHow can I use boost asio sockets for full duplex streaming efficiently?
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    1. USCory Nelson
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    1. COVery good idea ! Thank you very much...
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