c++ - 如何在更多线程上运行 boost asio 解析器服务？

Question

我在 SNMPV2 实现中使用 boost::asio::ip::udp::resolver 来确定主机是否可达。

using Resolver = boost::asio::ip::udp::resolver;
Resolver resolver(ioService);
Resolver::query query(connectOptions.getHost(),
        connectOptions.getPort());
Resolver::iterator endpointIterator;
BOOST_LOG_SEV(logger, Severity::debug) << "Waiting for async resolve";
endpointIterator = resolver.async_resolve(query, yield);
BOOST_LOG_SEV(logger, Severity::debug) << "Async resolve done";
if (endpointIterator == Resolver::iterator{}) { // unreachable host
    using namespace boost::system;
    throw system_error{error_code{SnmpWrapperError::BadHostname}};
}

我有一个测试用例，我测试了当一个不存在的主机名和一个存在的主机名被并行查询时发生了什么:

2013-09-16 10:45:28.687001: [DEBUG   ] 0x88baf8 SnmpConnection: connect                                                                                              
2013-09-16 10:45:28.687396: [DEBUG   ] 0x88baf8 SnmpConnection: host: non_existent_host_name_                                                                        
2013-09-16 10:45:28.687434: [DEBUG   ] 0x88baf8 SnmpConnection: port: 1611                                                                                           
2013-09-16 10:45:28.687456: [DEBUG   ] 0x88baf8 SnmpConnection: Waiting for async resolve                                                                            
2013-09-16 10:45:28.687675: [DEBUG   ] 0x88c608 SnmpConnection: connect                                                                                              
2013-09-16 10:45:28.687853: [DEBUG   ] 0x88c608 SnmpConnection: host: 127.0.0.1                                                                                      
2013-09-16 10:45:28.687883: [DEBUG   ] 0x88c608 SnmpConnection: port: 1611                                                                                           
2013-09-16 10:45:28.687904: [DEBUG   ] 0x88c608 SnmpConnection: Waiting for async resolve                                                                            
2013-09-16 10:45:31.113527: [ERROR   ] 0x88baf8 SnmpConnection: Host not found (authoritative)                                                                       
2013-09-16 10:45:31.113708: [DEBUG   ] 0x88c608 SnmpConnection: Async resolve done                                                                                   
2013-09-16 10:45:31.113738: [DEBUG   ] 0x88c608 SnmpConnection: Connecting to 127.0.0.1:1611
...

从日志中可以看出，具有可达地址的对象被阻塞，直到另一个对象的解析完成并出现错误(3 秒)。我的假设是 Asio 解析器服务使用一个线程，因此对一个无法访问的主机的一次查询可能会阻止即将到来的解析请求的处理。

解决方法是在更多线程上运行解析器服务，这可能吗？或者是否有可能像 udp 服务那样在套接字上工作的解析器服务(而不是使用::getaddrinfo)？

Answer 1

如 documentation 中所述, Boost.Asio 将为每个 io_service 创建一个额外的线程，以在第一次调用 resolver::async_resolve() 时模拟异步主机解析。

创建多个 io_service 对象将仅在对与不同 io_service 关联的 resolver 启动异步解析操作时允许并发主机解析。例如，以下代码不会执行并发主机解析，因为两个解析器使用相同的服务:

boost::asio::io_service service1;
boost::asio::ip::udp::resolver resolver1(service1); // using service1
boost::asio::ip::udp::resolver resolver2(service1); // using service1
resolver1.async_resolve(...); 
resolver2.async_resolve(...);

另一方面，以下将执行并发主机解析，因为每个解析器都使用不同的服务:

boost::asio::io_service service1;
boost::asio::io_service service2;
boost::asio::ip::udp::resolver resolver1(service1); // using service1
boost::asio::ip::udp::resolver resolver2(service2); // using service2
resolver1.async_resolve(...); 
resolver2.async_resolve(...);

假设每个 io_service 有一个 resolver，为了获得并发性，应用程序有责任将解析操作分派(dispatch)给不同的解析器。一个简单的工作分配策略，例如循环法，可能就足够了。

另一方面，可以将此职责委托(delegate)给 io_service，允许它分配工作，以类似于 Boost.Asio 内部执行的方式模拟异步主机解析。同步resolver::resolve()成员函数在调用线程中执行工作。因此，应用程序可以创建一个由线程池提供服务的 io_service。当需要进行异步主机解析时，会将一个作业发布到 io_service 中，这将创建一个 resolver 并执行同步解析，用结果调用用户处理程序。下面是一个完整的基本示例，其中 resolver 类使用线程池模拟异步主机解析:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/optional.hpp>
#include <boost/thread.hpp>

/// @brief Type used to emulate asynchronous host resolution with a 
///        dedicated thread pool.
class resolver
{
public:
  resolver(const std::size_t pool_size)
    : work_(boost::ref(io_service_))
  {
    // Create pool.
    for (std::size_t i = 0; i < pool_size; ++i)
      threads_.create_thread(
        boost::bind(&boost::asio::io_service::run, &io_service_));
  }

  ~resolver()
  {
    work_ = boost::none;
    threads_.join_all();
  }

  template <typename QueryOrEndpoint, typename Handler>
  void async_resolve(QueryOrEndpoint query, Handler handler)
  {
    io_service_.post(boost::bind(
        &resolver::do_async_resolve<QueryOrEndpoint, Handler>, this,
        query, handler));
  }

private:
  /// @brief Resolve address and invoke continuation handler.
  template <typename QueryOrEndpoint, typename Handler>
  void do_async_resolve(const QueryOrEndpoint& query, Handler handler)
  {
    typedef typename QueryOrEndpoint::protocol_type protocol_type;
    typedef typename protocol_type::resolver        resolver_type;

    // Resolve synchronously, as synchronous resolution will perform work
    // in the calling thread.  Thus, it will not use Boost.Asio's internal
    // thread that is used for asynchronous resolution.
    boost::system::error_code error;
    resolver_type resolver(io_service_);
    typename resolver_type::iterator result = resolver.resolve(query, error);

    // Invoke user handler.
    handler(error, result);
  }

private:
  boost::asio::io_service io_service_;
  boost::optional<boost::asio::io_service::work> work_;
  boost::thread_group threads_;
};

template <typename ProtocolType>
void handle_resolve(
    const boost::system::error_code& error,
    typename ProtocolType::resolver::iterator iterator)
{
  std::stringstream stream;
  stream << "handle_resolve:\n"
            "  " << error.message() << "\n";
  if (!error)
    stream << "  " << iterator->endpoint() << "\n";

  std::cout << stream.str();
  std::cout.flush();
}

int main()
{
  // Resolver will emulate asynchronous host resolution with a pool of 5
  // threads.
  resolver resolver(5);

  namespace ip = boost::asio::ip;
  resolver.async_resolve( 
      ip::udp::resolver::query("localhost", "12345"),
      &handle_resolve<ip::udp>);
  resolver.async_resolve(
      ip::tcp::resolver::query("www.google.com", "80"),
      &handle_resolve<ip::tcp>);
  resolver.async_resolve(
      ip::udp::resolver::query("www.stackoverflow.com", "80"),
      &handle_resolve<ip::udp>);
  resolver.async_resolve(
      ip::icmp::resolver::query("some.other.address", "54321"),
      &handle_resolve<ip::icmp>);
}

和带注释的输出:

handle_resolve:
  Success
  127.0.0.1:12345   // localhost
handle_resolve:
  Service not found // bogus
handle_resolve:
  Success
  173.194.77.147:80 // google
handle_resolve:
  Success
  198.252.206.16:80 // stackoverflow