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::threadfor thread abstraction.boost::thread_groupto manage lifetime of threads.boost::asio::io_serviceto provide a threadpool. Will likely want to useboost::asio::io_service::workto 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::multisetorstd::multimapcan be used. However, it is unspecified in C++03 as to the order of request with the same key (priority).- If
Requestdo not need an guard method, thenstd::priority_queuecould be used.
Request could be an unspecified type:
boost::functionandboost::bindcould be used to provide a type-erasure, while binding to callable types without introducing aRequesthierarchy.
Futures could use Boost.Thread’s Futures support.
future.valid()will return true ifRequesthas been added toActivationList.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
Requestwill be passed to theFuture.
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