### Timer.5 - Synchronising completion handlers in multithreaded programs

This tutorial demonstrates the use of the strand class template to synchronise completion handlers in a multithreaded program.

The previous four tutorials avoided the issue of handler synchronisation by calling the asio::io_context::run() function from one thread only. As you already know, the asio library provides a guarantee that completion handlers will only be called from threads that are currently calling asio::io_context::run(). Consequently, calling asio::io_context::run() from only one thread ensures that completion handlers cannot run concurrently.

The single threaded approach is usually the best place to start when developing applications using asio. The downside is the limitations it places on programs, particularly servers, including:

- Poor responsiveness when handlers can take a long time to complete.
- An inability to scale on multiprocessor systems.

If you find yourself running into these limitations, an alternative approach is to have a pool of threads calling asio::io_context::run(). However, as this allows handlers to execute concurrently, we need a method of synchronisation when handlers might be accessing a shared, thread-unsafe resource.

    #include <functional>
    #include <iostream>
    #include <thread>
    #include <asio.hpp>

We start by defining a class called `printer`, similar to the class in the previous tutorial. This class will extend the previous tutorial by running two timers in parallel.

    class printer
    {
    public:

In addition to initialising a pair of asio::steady_timer members, the constructor initialises the `strand_` member, an object of type asio::strand<asio::io_context::executor_type>.

The strand class template is an executor adapter that guarantees that, for those handlers that are dispatched through it, an executing handler will be allowed to complete before the next one is started. This is guaranteed irrespective of the number of threads that are calling asio::io_context::run(). Of course, the handlers may still execute concurrently with other handlers that were not dispatched through an strand, or were dispatched through a different strand object.

      printer(asio::io_context& io)
        : strand_(asio::make_strand(io)),
          timer1_(io, asio::chrono::seconds(1)),
          timer2_(io, asio::chrono::seconds(1)),
          count_(0)
      {

When initiating the asynchronous operations, each completion handler is "bound" to an asio::strand<asio::io_context::executor_type> object. The asio::bind_executor() function returns a new handler that automatically dispatches its contained handler through the strand object. By binding the handlers to the same strand, we are ensuring that they cannot execute concurrently.

        timer1_.async_wait(asio::bind_executor(strand_,
              std::bind(&printer::print1, this)));

        timer2_.async_wait(asio::bind_executor(strand_,
              std::bind(&printer::print2, this)));
      }

      ~printer()
      {
        std::cout << "Final count is " << count_ << std::endl;
      }

In a multithreaded program, the handlers for asynchronous operations should be synchronised if they access shared resources. In this tutorial, the shared resources used by the handlers (`print1` and `print2`) are `std::cout` and the `count_` data member.

      void print1()
      {
        if (count_ < 10)
        {
          std::cout << "Timer 1: " << count_ << std::endl;
          ++count_;

          timer1_.expires_at(timer1_.expiry() + asio::chrono::seconds(1));

          timer1_.async_wait(asio::bind_executor(strand_,
                std::bind(&printer::print1, this)));
        }
      }

      void print2()
      {
        if (count_ < 10)
        {
          std::cout << "Timer 2: " << count_ << std::endl;
          ++count_;

          timer2_.expires_at(timer2_.expiry() + asio::chrono::seconds(1));

          timer2_.async_wait(asio::bind_executor(strand_,
                std::bind(&printer::print2, this)));
        }
      }

    private:
      asio::strand<asio::io_context::executor_type> strand_;
      asio::steady_timer timer1_;
      asio::steady_timer timer2_;
      int count_;
    };

The `main` function now causes asio::io_context::run() to be called from two threads: the main thread and one additional thread. This is accomplished using an thread object.

Just as it would with a call from a single thread, concurrent calls to asio::io_context::run() will continue to execute while there is "work" left to do. The background thread will not exit until all asynchronous operations have completed.

    int main()
    {
      asio::io_context io;
      printer p(io);
      std::thread t([&]{ io.run(); });

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