Check if data is available to read in ASIO

Question

I am migrating old c++ code to more modern c++ with ASIO and i have a problem which i'm not sure how to solve with ASIO.

The general ideia is:

                    Act as Client and 
                     Server (same socket)       
            +----------BACKEND--------------+                
            |        ---------------+       |
            |       |  +-- MSG-+    |       |  Read/Write same Socket
            |       |  |       |    |     SOCKET         
FRONTEND  --|-----> v  inQueue  | inQueue <-|-----  HOST
         <--|----- outQueue    -> outQueue--|---->  MACHINE
            |-------------------------------|

BACKEND starts by waiting for a HOST to connect and for a Frontend to connect. Requests from the FRONTEND are arrive at the backend via socket and are put in inQueue, processed by the BACKEND and then sent to the HOST MACHINE.

From time to time the HOST MACHINE may send a msg that needs to be responded.

My problem is between BACKEND and HOST MACHINE. I need to use the same socket to also receive messages. Do i need to one thread waiting for msg from the HOST MACHINE ? Is there a way to check for read evens on the socket like the old code (not done by me)?

I can't have something like this because in other thread i may receive a msg from the FRONTEND that needs to be sent to the HOST MACHINE by the BACKEND and the socket is being used:

Thread waiting for data

...
asio::async_read(*sock_.get(),....
...

The old code uses "select" to detect for read events blocking till a read event occurs. when this event occurs, the msg is red from the socket and then responded.

Answer 1

Using the reactor-style example provided by asio i have put put the socket in non-blocking mode waiting for events to be read.

sock->non_blocking(true); 
sock->async_wait(asio::ip::tcp::socket::wait_read,    
std::bind(&HostServerComm::read_handler, this, 
std::placeholders::_1));

And my read_handler

void HostServerComm::read_handler(const asio::error_code& ec)
{
    if (!ec)
    {
        LOG(INFO) << "read_handler msg received";
        strand_.post([&]()
        {
                host_conn->read_message();
        });
    }
}

I'm still working on this. I'm still struggling with the use of strands.