boost::asio and Active Object

Boost.Asio can be used to encompass the intention of Active Object: decouple method execution from method invocation. Additional requirements will need to be handled at a higher-level, but it is not overly complex when using Boost.Asio in conjunction with other Boost libraries.

Scheduler could use:

• boost::thread for thread abstraction.
• boost::thread_group to manage lifetime of threads.
• boost::asio::io_service to provide a threadpool. Will likely want to use boost::asio::io_service::work to keep threads alive when no work is pending.

ActivationList could be implemented as:

• A Boost.MultiIndex for obtaining highest priority method request. With a hinted-position insert(), the insertion order is preserved for request with the same priority.
• std::multiset or std::multimap can be used. However, it is unspecified in C++03 as to the order of request with the same key (priority).
• If Request do not need an guard method, then std::priority_queue could be used.

Request could be an unspecified type:

• boost::function and boost::bind could be used to provide a type-erasure, while binding to callable types without introducing a Request hierarchy.

Futures could use Boost.Thread's Futures support.

• future.valid() will return true if Request has been added to ActivationList.
• future.wait() will block waiting for a result to become available.
• future.get() will block waiting for the result.
• If caller does nothing with the future, then caller will not be blocked.
• Another benefit to using Boost.Thread's Futures is that exceptions originating from within a Request will be passed to the Future.

Here is a complete example leveraging various Boost libraries and should meet the requirements:

// Standard includes
#include <algorithm> // std::find_if
#include <iostream>
#include <string>

// 3rd party includes
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/make_shared.hpp>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/thread.hpp>
#include <boost/utility/result_of.hpp>

/// @brief scheduler that provides limits with prioritized jobs.
template <typename Priority,
          typename Compare = std::less<Priority> >
class scheduler
{
public:
  typedef Priority priority_type;
private:

  /// @brief method_request is used to couple the guard and call
  ///        functions for a given method.
  struct method_request
  {
    typedef boost::function<bool()> ready_func_type;
    typedef boost::function<void()> run_func_type;

    template <typename ReadyFunctor,
              typename RunFunctor>
    method_request(ReadyFunctor ready,
                   RunFunctor run)
      : ready(ready),
        run(run)
    {}

    ready_func_type ready;
    run_func_type run;
  };

  /// @brief Pair type used to associate a request with its priority.
  typedef std::pair<priority_type,
                    boost::shared_ptr<method_request> > pair_type;

  static bool is_method_ready(const pair_type& pair)
  {
    return pair.second->ready();
  }

public:

  /// @brief Construct scheduler.
  ///
  /// @param max_threads Maximum amount of concurrent task.
  /// @param max_request Maximum amount of request.  
  scheduler(std::size_t max_threads,
            std::size_t max_request)
    : work_(io_service_),
      max_request_(max_request),
      request_count_(0)
  {
    // Spawn threads, dedicating them to the io_service.
    for (std::size_t i = 0; i < max_threads; ++i)
      threads_.create_thread(
        boost::bind(&boost::asio::io_service::run, &io_service_));
  }

  /// @brief Destructor.
  ~scheduler()
  {
    // Release threads from the io_service.
    io_service_.stop();
    // Cleanup.
    threads_.join_all();
  }

  /// @brief Insert a method request into the scheduler.
  ///
  /// @param priority Priority of job.
  /// @param ready_func Invoked to check if method is ready to run.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename ReadyFunctor,
            typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(priority_type priority, 
         const ReadyFunctor& ready_func,
         const RunFunctor& run_func)
  {
    typedef typename boost::result_of<RunFunctor()>::type result_type;
    typedef boost::unique_future<result_type> future_type;

    boost::unique_lock<mutex_type> lock(mutex_);

    // If max request has been reached, then return an invalid future.
    if (max_request_ &&
        (request_count_ == max_request_))
      return future_type();

    ++request_count_;

    // Use a packaged task to handle populating promise and future.
    typedef boost::packaged_task<result_type> task_type;

    // Bind does not work with rvalue, and packaged_task is only moveable,
    // so allocate a shared pointer.
    boost::shared_ptr<task_type> task = 
      boost::make_shared<task_type>(run_func);

    // Create method request.
    boost::shared_ptr<method_request> request =
      boost::make_shared<method_request>(
        ready_func,
        boost::bind(&task_type::operator(), task));

    // Insert into priority.  Hint to inserting as close to the end as
    // possible to preserve insertion order for request with same priority.
    activation_list_.insert(activation_list_.end(),
                            pair_type(priority, request));

    // There is now an outstanding request, so post to dispatch.
    io_service_.post(boost::bind(&scheduler::dispatch, this));

    return task->get_future();
  }

  /// @brief Insert a method request into the scheduler.
  ///
  /// @param ready_func Invoked to check if method is ready to run.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename ReadyFunctor,
            typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(const ReadyFunctor& ready_func,
         const RunFunctor& run_func)
  {
    return insert(priority_type(), ready_func, run_func);
  }

  /// @brief Insert a method request into the scheduler.
  ///
  /// @param priority Priority of job.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(priority_type priority, 
         const RunFunctor& run_func)
  {
    return insert(priority, &always_ready, run_func);
  }

  /// @brief Insert a method request with default priority into the
  ///        scheduler.
  ///
  /// @param run_func Invoked when ready to run.
  ///
  /// @param functor Job to run.
  ///
  /// @return future associated with the job.
  template <typename RunFunc>
  boost::unique_future<typename boost::result_of<RunFunc()>::type>
  insert(const RunFunc& run_func)
  {
    return insert(&always_ready, run_func);
  }

  /// @brief Cancel all outstanding request.
  void cancel()
  {
    boost::unique_lock<mutex_type> lock(mutex_);
    activation_list_.clear();
    request_count_ = 0;
  } 

private:

  /// @brief Dispatch a request.
  void dispatch()
  {
    // Get the current highest priority request ready to run from the queue.
    boost::unique_lock<mutex_type> lock(mutex_);
    if (activation_list_.empty()) return;

    // Find the highest priority method ready to run.
    typedef typename activation_list_type::iterator iterator;
    iterator end = activation_list_.end();
    iterator result = std::find_if(
      activation_list_.begin(), end, &is_method_ready);

    // If no methods are ready, then post into dispatch, as the
    // method may have become ready.
    if (end == result)
    {
      io_service_.post(boost::bind(&scheduler::dispatch, this));
      return;
    }

    // Take ownership of request.
    boost::shared_ptr<method_request> method = result->second;
    activation_list_.erase(result);

    // Run method without mutex.
    lock.unlock();
    method->run();    
    lock.lock();

    // Perform bookkeeping.
    --request_count_;
  }

  static bool always_ready() { return true; }

private:

  /// @brief List of outstanding request.
  typedef boost::multi_index_container<
    pair_type,
    boost::multi_index::indexed_by<
      boost::multi_index::ordered_non_unique<
        boost::multi_index::member<pair_type,
                                   typename pair_type::first_type,
                                   &pair_type::first>,
        Compare
      >
    >
  > activation_list_type;
  activation_list_type activation_list_;

  /// @brief Thread group managing threads servicing pool.
  boost::thread_group threads_;

  /// @brief io_service used to function as a thread pool.
  boost::asio::io_service io_service_;

  /// @brief Work is used to keep threads servicing io_service.
  boost::asio::io_service::work work_;

  /// @brief Maximum amount of request.
  const std::size_t max_request_;

  /// @brief Count of outstanding request.
  std::size_t request_count_;

  /// @brief Synchronize access to the activation list.
  typedef boost::mutex mutex_type;
  mutex_type mutex_;
};

typedef scheduler<unsigned int, 
                  std::greater<unsigned int> > high_priority_scheduler;

/// @brief adder is a simple proxy that will delegate work to
///        the scheduler.
class adder
{
public:
  adder(high_priority_scheduler& scheduler)
    : scheduler_(scheduler)
  {}

  /// @brief Add a and b with a priority.
  ///
  /// @return Return future result.
  template <typename T>
  boost::unique_future<T> add(
    high_priority_scheduler::priority_type priority,
    const T& a, const T& b)
  {
    // Insert method request
    return scheduler_.insert(
      priority,
      boost::bind(&adder::do_add<T>, a, b));
  }

  /// @brief Add a and b.
  ///
  /// @return Return future result.
  template <typename T>
  boost::unique_future<T> add(const T& a, const T& b)
  {
    return add(high_priority_scheduler::priority_type(), a, b);
  }

private:

  /// @brief Actual add a and b.
  template <typename T>
  static T do_add(const T& a, const T& b)
  {
    std::cout << "Starting addition of '" << a 
              << "' and '" << b << "'" << std::endl;
    // Mimic busy work.
    boost::this_thread::sleep_for(boost::chrono::seconds(2));
    std::cout << "Finished addition" << std::endl;
    return a + b;
  }

private:
  high_priority_scheduler& scheduler_;
};

bool get(bool& value) { return value; }
void guarded_call()
{
  std::cout << "guarded_call" << std::endl; 
}

int main()
{
  const unsigned int max_threads = 1;
  const unsigned int max_request = 4;

  // Sscheduler
  high_priority_scheduler scheduler(max_threads, max_request);

  // Proxy
  adder adder(scheduler);

  // Client

  // Add guarded method to scheduler.
  bool ready = false;
  std::cout << "Add guarded method." << std::endl;
  boost::unique_future<void> future1 = scheduler.insert(
    boost::bind(&get, boost::ref(ready)),
    &guarded_call);

  // Add 1 + 100 with default priority.
  boost::unique_future<int> future2 = adder.add(1, 100);

  // Force sleep to try to get scheduler to run request 2 first.
  boost::this_thread::sleep_for(boost::chrono::seconds(1));

  // Add:
  //   2 + 200 with low priority (5)
  //   "test" + "this" with high priority (99)
  boost::unique_future<int> future3 = adder.add(5, 2, 200);
  boost::unique_future<std::string> future4 = adder.add(99,
    std::string("test"), std::string("this"));

  // Max request should have been reached, so add another.
  boost::unique_future<int> future5 = adder.add(3, 300);

  // Check if request was added.
  std::cout << "future1 is valid: " << future1.valid()
          << "\nfuture2 is valid: " << future2.valid()
          << "\nfuture3 is valid: " << future3.valid()
          << "\nfuture4 is valid: " << future4.valid()
          << "\nfuture5 is valid: " << future5.valid()
          << std::endl;

  // Get results for future2 and future3.  Do nothing with future4's results.
  std::cout << "future2 result: " << future2.get()
          << "\nfuture3 result: " << future3.get()
          << std::endl;

  std::cout << "Unguarding method." << std::endl;
  ready = true;
  future1.wait();
}

The execution uses thread pool of 1 with a max of 4 request.

• request1 is guarded until the end of program, and should be last to run.
• request2 (1 + 100) is inserted with default priority, and should be first to run.
• request3 (2 + 200) is inserted low priority, and should run after request4.
• request4 ('test' + 'this') is inserted with high priority, and should run before request3.
• request5 should fail to insert due to max request, and should not be valid.

The output is as follows:

Add guarded method.
Starting addition of '1' and '100'
future1 is valid: 1
future2 is valid: 1
future3 is valid: 1
future4 is valid: 1
future5 is valid: 0
Finished addition
Starting addition of 'test' and 'this'
Finished addition
Starting addition of '2' and '200'
Finished addition
future2 result: 101
future3 result: 202
Unguarding method.
guarded_call