Is it possible to change the io_context of a socket in boost::asio?

Question

I'm currently writing a multi-threaded server where each thread has an io_context and a list of task objects to execute, with each task object having an associated ip::tcp::socket object.

For load-balancing I'm sometimes migrating tasks from one thread to another, however I'd like to migrate their sockets as well without dropping connection.

I could simply pass ownership of the socket object between threads, however the socket's io_context will remain that of its original thread, which would add significant complexity/slow-down.

Is there any way for me to keep a socket connection whilst moving it to a different io_context? Or is there another recommended approach?

Many thanks

Moyer

I don't think an io_context really owns the socket. I suppose it owns the services, and it definitely owns the pending/active handlers. Now, I /think/ with sockets, it's just the handle that gets actually shared. At least, I've run with this (multiple io_services and passing sockets to and fro) for some time (years) and didn't observe any problems with that. Interesting question though. +1 – Ustkamenogorsk

Answer 1

You can not change the io_context directly, but there's a workaround

just use release and assign

here's an example:

const char* buff = "send";
boost::asio::io_context io;
boost::asio::io_context io2;
boost::asio::ip::tcp::socket socket(io);
socket.open(boost::asio::ip::tcp::v4());

std::thread([&](){
    auto worker1 = boost::asio::make_work_guard(io);
    io.run();
}).detach();
std::thread([&](){
    auto worker2 = boost::asio::make_work_guard(io2);
    io2.run();
}).detach();

socket.connect(boost::asio::ip::tcp::endpoint(boost::asio::ip::address_v4::from_string("127.0.0.1"), 8888));

socket.async_send(boost::asio::buffer(buff, 4),
        [](const boost::system::error_code &ec, std::size_t bytes_transferred)
        {
            std::cout << "send\n";
            fflush(stdout);
        });

// any pending async ops will get boost::asio::error::operation_aborted
auto fd = socket.release();
// create another socket using different io_context
boost::asio::ip::tcp::socket socket2(io2);
// and assign the corresponding fd
socket2.assign(boost::asio::ip::tcp::v4(), fd);
// from now on io2 is the default executor of socket2
socket2.async_send(boost::asio::buffer(buff, 4),
        [](const boost::system::error_code &ec, std::size_t bytes_transferred){
            std::cout << "send via io2\n";
            fflush(stdout);
        });

getchar();

Jethro