c++ - boost ASIO : Send message to all connected clients

Question

我正在从事一个涉及 boost::beast websocket/http 混合服务器的项目，该服务器在 boost::asio 之上运行。我的项目很大程度上基于 advanced_server.cpp示例来源。

它工作正常，但现在我正在尝试添加一项功能，该功能需要向所有 连接的客户端发送消息。

我对 boost::asio 不是很熟悉，但现在我看不出有什么方法可以产生类似“广播”事件的东西(如果这是正确的术语)。

我天真的方法是看看我是否可以让 websocket_session() 的构造附加事件监听器之类的东西，然后析构函数分离监听器。那时，我可以触发事件，并让所有当前有效的 websocket session (websocket_session() 的生命周期限定在其中)执行回调。

有https://stackoverflow.com/a/17029022/268006 ，它或多或少地通过 (ab) 使用 boost::asio::steady_timer 来完成我想要的，但这似乎是一种可怕的黑客来完成本应非常简单的事情。

基本上，给定一个有状态的 boost::asio 服务器，我如何对多个连接执行操作？

Answer 1

首先:您可以广播 UDP，但那不是连接的客户端。那只是……UDP。

其次，该链接显示了如何在 Asio 中拥有类似条件变量(事件)的界面。那只是你问题的一小部分。您忘记了大局:您需要以某种方式了解一组打开的连接:

1. 例如为每个连接保留 session 指针 (weak_ptr) 的容器
2. 每个连接订阅一个信号槽(例如 Boost Signals )。

选项 1. 非常适合性能，选项 2. 更适合灵 active (将事件源与订阅者解耦，使异构订阅者成为可能，例如，不来自连接)。

因为我认为选项 1. 对于线程来说更简单，更好的是 w.r.t.效率(例如，您可以在不复制的情况下从一个缓冲区为所有客户端提供服务)并且您可能不需要双重解耦信号/插槽，让我引用一个答案，其中我已经展示了尽可能多的纯 Asio(没有 Beast):

• How to design proper release of a boost::asio socket or wrapper thereof

它展示了“连接池”的概念 - 它本质上是 weak_ptr<connection> 的线程安全容器具有一些垃圾收集逻辑的对象。

演示:介绍 Echo Server

在 chatting about things 之后我想花时间实际演示这两种方法，所以我在说什么完全清楚。

首先让我们展示一个简单的、普通的异步 TCP 服务器

• 具有多个并发连接
• 每个连接的 session 逐行从客户端读取，并将相同的内容回显给客户端
• 3秒后停止接受，并在最后一个客户端断开连接后退出

master branch on github

#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>

namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;

static bool s_verbose = false;

struct connection : std::enable_shared_from_this<connection> {
    connection(ba::io_context& ioc) : _s(ioc) {}

    void start() { read_loop(); }
    void send(std::string msg, bool at_front = false) {
        post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
            if (enqueue(std::move(msg), at_front))
                write_loop();
        });
    }

  private:
    void do_echo() {
        std::string line;
        if (getline(std::istream(&_rx), line)) {
            send(std::move(line) + '\n');
        }
    }

    bool enqueue(std::string msg, bool at_front)
    { // returns true if need to start write loop
        at_front &= !_tx.empty(); // no difference
        if (at_front)
            _tx.insert(std::next(begin(_tx)), std::move(msg));
        else
            _tx.push_back(std::move(msg));

        return (_tx.size() == 1);
    }
    bool dequeue()
    { // returns true if more messages pending after dequeue
        assert(!_tx.empty());
        _tx.pop_front();
        return !_tx.empty();
    }

    void write_loop() {
        ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                if (!ec && dequeue()) write_loop();
            });
    }

    void read_loop() {
        ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                do_echo();
                if (!ec)
                    read_loop();
            });
    }

    friend struct server;
    ba::streambuf          _rx;
    std::list<std::string> _tx;
    tcp::socket            _s;
};

struct server {
    server(ba::io_context& ioc) : _ioc(ioc) {
        _acc.bind({{}, 6767});
        _acc.set_option(tcp::acceptor::reuse_address());
        _acc.listen();
        accept_loop();
    }

    void stop() {
        _ioc.post([=] {
                _acc.cancel();
                _acc.close();
            });
    }

  private:
    void accept_loop() {
        auto session = std::make_shared<connection>(_acc.get_io_context());
        _acc.async_accept(session->_s, [this,session](error_code ec) {
             auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
             std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;

             session->start();
             if (!ec)
                 accept_loop();
        });
    }

    ba::io_context& _ioc;
    tcp::acceptor _acc{_ioc, tcp::v4()};
};

int main(int argc, char** argv) {
    s_verbose = argc>1 && argv[1] == "-v"s;

    ba::io_context ioc;

    server s(ioc);

    std::thread th([&ioc] { ioc.run(); }); // todo exception handling

    std::this_thread::sleep_for(3s);
    s.stop(); // active connections will continue

    th.join();
}

方法一、添加广播消息

因此，让我们添加同时发送到所有事件连接的“广播消息”。我们添加两个:

• 在每个新连接时一个(说“玩家 ## 已进入游戏”)

• 模拟全局“服务器事件”的一个，就像您在问题中描述的那样)。它从 main 中触发:

  std::this_thread::sleep_for(1s);
  
  auto n = s.broadcast("random global event broadcast\n");
  std::cout << "Global event broadcast reached " << n << " active connections\n";
  

请注意我们是如何通过向每个已接受的连接注册一个弱指针并对每个连接进行操作来做到这一点的:

    _acc.async_accept(session->_s, [this,session](error_code ec) {
         auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
         std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;

         if (!ec) {
             auto n = reg_connection(session);

             session->start();
             accept_loop();

             broadcast("player #" + std::to_string(n) + " has entered the game\n");
         }

    });

broadcast也直接从main中使用并且很简单:

size_t broadcast(std::string const& msg) {
    return for_each_active([msg](connection& c) { c.send(msg, true); });
}

using-asio-post branch on github

#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>

namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;

static bool s_verbose = false;

struct connection : std::enable_shared_from_this<connection> {
    connection(ba::io_context& ioc) : _s(ioc) {}

    void start() { read_loop(); }
    void send(std::string msg, bool at_front = false) {
        post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
            if (enqueue(std::move(msg), at_front))
                write_loop();
        });
    }

  private:
    void do_echo() {
        std::string line;
        if (getline(std::istream(&_rx), line)) {
            send(std::move(line) + '\n');
        }
    }

    bool enqueue(std::string msg, bool at_front)
    { // returns true if need to start write loop
        at_front &= !_tx.empty(); // no difference
        if (at_front)
            _tx.insert(std::next(begin(_tx)), std::move(msg));
        else
            _tx.push_back(std::move(msg));

        return (_tx.size() == 1);
    }
    bool dequeue()
    { // returns true if more messages pending after dequeue
        assert(!_tx.empty());
        _tx.pop_front();
        return !_tx.empty();
    }

    void write_loop() {
        ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                if (!ec && dequeue()) write_loop();
            });
    }

    void read_loop() {
        ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                do_echo();
                if (!ec)
                    read_loop();
            });
    }

    friend struct server;
    ba::streambuf          _rx;
    std::list<std::string> _tx;
    tcp::socket            _s;
};

struct server {
    server(ba::io_context& ioc) : _ioc(ioc) {
        _acc.bind({{}, 6767});
        _acc.set_option(tcp::acceptor::reuse_address());
        _acc.listen();
        accept_loop();
    }

    void stop() {
        _ioc.post([=] {
                _acc.cancel();
                _acc.close();
            });
    }

    size_t broadcast(std::string const& msg) {
        return for_each_active([msg](connection& c) { c.send(msg, true); });
    }

  private:
    using connptr = std::shared_ptr<connection>;
    using weakptr = std::weak_ptr<connection>;

    std::mutex _mx;
    std::vector<weakptr> _registered;

    size_t reg_connection(weakptr wp) {
        std::lock_guard<std::mutex> lk(_mx);
        _registered.push_back(wp);
        return _registered.size();
    }

    template <typename F>
    size_t for_each_active(F f) {
        std::vector<connptr> active;
        {
            std::lock_guard<std::mutex> lk(_mx);
            for (auto& w : _registered)
                if (auto c = w.lock())
                    active.push_back(c);
        }

        for (auto& c : active) {
            std::cout << "(running action for " << c->_s.remote_endpoint() << ")" << std::endl;
            f(*c);
        }

        return active.size();
    }

    void accept_loop() {
        auto session = std::make_shared<connection>(_acc.get_io_context());
        _acc.async_accept(session->_s, [this,session](error_code ec) {
             auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
             std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;

             if (!ec) {
                 auto n = reg_connection(session);

                 session->start();
                 accept_loop();

                 broadcast("player #" + std::to_string(n) + " has entered the game\n");
             }

        });
    }

    ba::io_context& _ioc;
    tcp::acceptor _acc{_ioc, tcp::v4()};
};

int main(int argc, char** argv) {
    s_verbose = argc>1 && argv[1] == "-v"s;

    ba::io_context ioc;

    server s(ioc);

    std::thread th([&ioc] { ioc.run(); }); // todo exception handling

    std::this_thread::sleep_for(1s);

    auto n = s.broadcast("random global event broadcast\n");
    std::cout << "Global event broadcast reached " << n << " active connections\n";

    std::this_thread::sleep_for(2s);
    s.stop(); // active connections will continue

    th.join();
}

方法 2:广播但带有 boost 信号2

信号方法是 Dependency Inversion 的一个很好的例子.

最重要的注意事项:

• 信号槽在调用它的线程上被调用(“引发事件”)
• scoped_connection当 connection 时，订阅会被*自动 删除吗？被破坏
• 有subtle difference in the wording of the console message从“达到 # 个活跃连接”到“达到 # 个活跃订阅者”。

The difference is key to understanding the added flexibility: the signal owner/invoker does not know anything about the subscribers. That's the decoupling/dependency inversion we're talking about

using-signals2 branch on github

#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>
#include <boost/signals2.hpp>

namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;

static bool s_verbose = false;

struct connection : std::enable_shared_from_this<connection> {
    connection(ba::io_context& ioc) : _s(ioc) {}

    void start() { read_loop(); }
    void send(std::string msg, bool at_front = false) {
        post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
            if (enqueue(std::move(msg), at_front))
                write_loop();
        });
    }

  private:
    void do_echo() {
        std::string line;
        if (getline(std::istream(&_rx), line)) {
            send(std::move(line) + '\n');
        }
    }

    bool enqueue(std::string msg, bool at_front)
    { // returns true if need to start write loop
        at_front &= !_tx.empty(); // no difference
        if (at_front)
            _tx.insert(std::next(begin(_tx)), std::move(msg));
        else
            _tx.push_back(std::move(msg));

        return (_tx.size() == 1);
    }
    bool dequeue()
    { // returns true if more messages pending after dequeue
        assert(!_tx.empty());
        _tx.pop_front();
        return !_tx.empty();
    }

    void write_loop() {
        ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                if (!ec && dequeue()) write_loop();
            });
    }

    void read_loop() {
        ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
                if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
                do_echo();
                if (!ec)
                    read_loop();
            });
    }

    friend struct server;
    ba::streambuf          _rx;
    std::list<std::string> _tx;
    tcp::socket            _s;

    boost::signals2::scoped_connection _subscription;
};

struct server {
    server(ba::io_context& ioc) : _ioc(ioc) {
        _acc.bind({{}, 6767});
        _acc.set_option(tcp::acceptor::reuse_address());
        _acc.listen();
        accept_loop();
    }

    void stop() {
        _ioc.post([=] {
                _acc.cancel();
                _acc.close();
            });
    }

    size_t broadcast(std::string const& msg) {
        _broadcast_event(msg);
        return _broadcast_event.num_slots();
    }

  private:
    boost::signals2::signal<void(std::string const& msg)> _broadcast_event;

    size_t reg_connection(connection& c) {
        c._subscription = _broadcast_event.connect(
                [&c](std::string msg){ c.send(msg, true); }
            );

        return _broadcast_event.num_slots();
    }

    void accept_loop() {
        auto session = std::make_shared<connection>(_acc.get_io_context());
        _acc.async_accept(session->_s, [this,session](error_code ec) {
             auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
             std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;

             if (!ec) {
                 auto n = reg_connection(*session);

                 session->start();
                 accept_loop();

                 broadcast("player #" + std::to_string(n) + " has entered the game\n");
             }

        });
    }

    ba::io_context& _ioc;
    tcp::acceptor _acc{_ioc, tcp::v4()};
};

int main(int argc, char** argv) {
    s_verbose = argc>1 && argv[1] == "-v"s;

    ba::io_context ioc;

    server s(ioc);

    std::thread th([&ioc] { ioc.run(); }); // todo exception handling

    std::this_thread::sleep_for(1s);

    auto n = s.broadcast("random global event broadcast\n");
    std::cout << "Global event broadcast reached " << n << " active subscribers\n";

    std::this_thread::sleep_for(2s);
    s.stop(); // active connections will continue

    th.join();
}

See the diff between Approach 1. and 2.: Compare View on github

针对 3 个并发客户端运行时的输出示例:

(for a in {1..3}; do netcat localhost 6767 < /etc/dictionaries-common/words > echoed.$a& sleep .1; done; time wait)