这些方法并不明确相互排斥。我经常看到第一个和第二个的组合：
- 一个或多个线程正在一个
io_service 中处理网络 I/O。
- 长时间运行或阻塞的任务被发布到不同的
io_service。这个io_service 用作线程池，不会干扰处理网络I/O 的线程。或者，每次需要长时间运行或阻塞的任务时，都可以生成一个分离的线程；但是，线程创建/销毁的开销可能会产生显着影响。
这个answer 提供了一个线程池实现。此外，这里有一个基本示例，试图强调两个io_services 之间的交互。
#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/chrono.hpp>
#include <boost/optional.hpp>
#include <boost/thread.hpp>
/// @brief Background service will function as a thread-pool where
/// long-standing blocking operations may occur without affecting
/// the network event loop.
boost::asio::io_service background_service;
/// @brief The main io_service will handle network operations.
boost::asio::io_service io_service;
boost::optional<boost::asio::io_service::work> work;
/// @brief ODBC blocking operation.
///
/// @brief data Data to use for query.
/// @brief handler Handler to invoke upon completion of operation.
template <typename Handler>
void query_odbc(unsigned int data,
Handler handler)
{
std::cout << "in background service, start querying odbc\n";
std::cout.flush();
// Mimic busy work.
boost::this_thread::sleep_for(boost::chrono::seconds(5));
std::cout << "in background service, posting odbc result to main service\n";
std::cout.flush();
io_service.post(boost::bind(handler, data * 2));
}
/// @brief Functions as a continuation for handle_read, that will be
/// invoked with results from ODBC.
void handle_read_odbc(unsigned int result)
{
std::stringstream stream;
stream << "in main service, got " << result << " from odbc.\n";
std::cout << stream.str();
std::cout.flush();
// Allow io_service to stop in this example.
work = boost::none;
}
/// @brief Mocked up read handler that will post work into a background
/// service.
void handle_read(const boost::system::error_code& error,
std::size_t bytes_transferred)
{
std::cout << "in main service, need to query odbc" << std::endl;
typedef void (*handler_type)(unsigned int);
background_service.post(boost::bind(&query_odbc<handler_type>,
21, // data
&handle_read_odbc) // handler
);
// Keep io_service event loop running in this example.
work = boost::in_place(boost::ref(io_service));
}
/// @brief Loop to show concurrency.
void print_loop(unsigned int iteration)
{
if (!iteration) return;
std::cout << " in main service, doing work.\n";
std::cout.flush();
boost::this_thread::sleep_for(boost::chrono::seconds(1));
io_service.post(boost::bind(&print_loop, --iteration));
}
int main()
{
boost::optional<boost::asio::io_service::work> background_work(
boost::in_place(boost::ref(background_service)));
// Dedicate 3 threads to performing long-standing blocking operations.
boost::thread_group background_threads;
for (std::size_t i = 0; i < 3; ++i)
background_threads.create_thread(
boost::bind(&boost::asio::io_service::run, &background_service));
// Post a mocked up 'handle read' handler into the main io_service.
io_service.post(boost::bind(&handle_read,
make_error_code(boost::system::errc::success), 0));
// Post a mockup loop into the io_service to show concurrency.
io_service.post(boost::bind(&print_loop, 5));
// Run the main io_service.
io_service.run();
// Cleanup background.
background_work = boost::none;
background_threads.join_all();
}
还有输出：
在主服务中，需要查询odbc
在主要服务，做工作。
在后台服务中，开始查询 odbc
在主要服务，做工作。
在主要服务，做工作。
在主要服务，做工作。
在主要服务，做工作。
在后台服务中，将 odbc 结果发布到主服务
在主要服务中，从 odbc 获得 42。
请注意，处理主要io_service 帖子的单线程进入background_service，然后在background_service 阻塞时继续处理其事件循环。一旦background_service 得到结果，它就会将处理程序发布到主io_service。