asio :how does server actively send information to client while listening to client at the same time?

The server accept new connections :

void do_accept()
{
    acceptor_.async_accept(socket_,
        [this](boost::system::error_code ec)
    {
        if (!ec)
        {
            connection_count++;
            all_session.push_back(std::make_shared<session>(std::move(socket_), io_service_));
            all_session.back()->start();
        }

        do_accept();
    });
}

and the session->start works like below:

void start()
{
    std::cout << "connection from " << socket_.remote_endpoint().address() << std::endl;
    do_read_header();
}

the do_read_header tackle the messages then recursively call itself. When tackling the message, it would call write to send some information to client. The write function is below:

void write(const chat_message& msg)
{
    write_msgs_ = msg;
    do_write(msg.length());
}
void do_write(std::uint32_t length)
{
    auto self(shared_from_this());
    boost::asio::async_write(socket_, boost::asio::buffer(write_msgs_.data(), length),
        [this, self](boost::system::error_code ec, std::size_t /*length*/)
    {
        if (!ec)
        {
            do_read_header();
        }
    });
}

Then here is the question: how does the server actively send information to client outside the do_read_header() recursion? I need the server dispatch some work which is decided by future user input to all the clients. The boost asio c++11 examples don't metion how to do it. And I can't find some relevant resources on it.