CODEWITHSUNDEEP
c++boost-asioboost-beast
Julien
Julien

Reputation: 51

Boost::Beast : server with websocket pipelining

I am writing a c++ websocket server with boost beast 1.70 and mysql 8 C connector. The server will have several clients simultaneously connected. The particularity is that each client will perform like 100 websocket requests in a row to the server. Each request is "cpu light" for my server but the server perform a "time heavy" sql request for each request.

I have started my server with the websocket_server_coro.cpp example. The server steps are :

1) a websocket read

2) a sql request

3) a websocket write

The problem is that for a given user, the server is "locked" at the step 2 and cannot read until this step and the step 3 are finished. Thus, the 100 requests are solved sequentially. This is too slow for my use case.

I have read that non blocking read/write are not possible with boost beast. However, what I am trying to do now is to execute async_read and async_write in a coroutine.

void ServerCoro::accept(websocket::stream<beast::tcp_stream> &ws) {
    beast::error_code ec;

    ws.set_option(websocket::stream_base::timeout::suggested(beast::role_type::server));

    ws.set_option(websocket::stream_base::decorator([](websocket::response_type &res) {
                res.set(http::field::server, std::string(BOOST_BEAST_VERSION_STRING) + " websocket-Server-coro");
            }));

    ws.async_accept(yield[ec]);
    if (ec) return fail(ec, "accept");

    while (!_bStop) {
        beast::flat_buffer buffer;
        ws.async_read(buffer, yield[ec]);

        if (ec == websocket::error::closed) {
            std::cout << "=> get closed" << std::endl;
            return;
        }

        if (ec) return fail(ec, "read");

        auto buffer_str = new std::string(boost::beast::buffers_to_string(buffer.cdata()));
        net::post([&, buffer_str] {

            // sql async request such as :
            // while (status == (mysql_real_query_nonblocking(this->con, sqlRequest.c_str(), sqlRequest.size()))) {
            //    ioc.poll_one(ec);
            // }
            // more sql ...

            ws.async_write(net::buffer(worker->getResponse()), yield[ec]); // this line is throwing void boost::coroutines::detail::pull_coroutine_impl<void>::pull(): Assertion `! is_running()' failed.
            if (ec) return fail(ec, "write");

        });
    }
}

The problem is that the line with async_write throw an error :

void boost::coroutines::detail::pull_coroutine_impl::pull(): Assertion `! is_running()' failed.

If a replace this line with a sync_write, it works but the server remains sequential for a given user. I have tried to execute this code on a single threaded server. I have also tried to use the same strand for async_read and async_write. Still have the assertion error.

Is such server impossible with boost beast for websocket ? Thank you.

Upvotes: 3

Views: 1692

Answers (1)

Julien
Julien

Reputation: 51

By following the suggestion of Vinnie Falco, I rewrite the code by using "websocket chat" and "async server" as exemple. Here is the final working result of the code :

void Session::on_read(beast::error_code ec, std::size_t bytes_transferred)
{
    boost::ignore_unused(bytes_transferred);

    if(ec == websocket::error::closed) return;  // This indicates that the Session was closed
    if(ec) return fail(ec, "read");

    net::post([&, that = shared_from_this(), ss = std::make_shared<std::string const>(std::move(boost::beast::buffers_to_string(_buffer.cdata())))] {
        /* Sql things that call ioc.poll_one(ec) HERE, for me the sql response go inside worker.getResponse() used below */

        net::dispatch(_wsStrand, [&, that = shared_from_this(), sss = std::make_shared < std::string const>(worker.getResponse())] {
            async_write(sss);
        });
    });
    _buffer.consume(_buffer.size()); // we remove from the buffer what we just read
    do_read(); // go for another read
}

void Session::async_write(const std::shared_ptr<std::string const> &message) {
    _writeMessages.push_back(message);

    if (_writeMessages.size() > 1) {
        BOOST_LOG_TRIVIAL(warning) << "WRITE IS LOCKED";
    } else {
        _ws.text(_ws.got_text());
            _ws.async_write(net::buffer(*_writeMessages.front()), boost::asio::bind_executor(_wsStrand, beast::bind_front_handler(
                    &Session::on_write, this)));
    }
}

void Session::on_write(beast::error_code ec, std::size_t)
{
    // Handle the error, if any
    if(ec) return fail(ec, "write");

    // Remove the string from the queue
    _writeMessages.erase(_writeMessages.begin());

    // Send the next message if any
    if(!_writeMessages.empty())
        _ws.async_write(net::buffer(*_writeMessages.front()), boost::asio::bind_executor(_wsStrand, beast::bind_front_handler(
                        &Session::on_write, this)));
}

Thank you.

Upvotes: 2

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