class producer final : public so_5::agent_t
{
	// This signal allows to send next request for consumer.
	struct send_next final : public so_5::signal_t {};

public :
	producer( context_t ctx,
		std::string name, 
		so_5::mbox_t logger_mbox,
		so_5::mbox_t consumer_mbox,
		unsigned int requests )
		:	so_5::agent_t{ ctx }
		,	m_name( std::move(name) )
		,	m_logger_mbox{ std::move(logger_mbox) }
		,	m_consumer_mbox{ std::move(consumer_mbox) }
		,	m_requests_left{ requests }
	{
		so_subscribe_self()
			.event( &producer::evt_reply )
			.event( &producer::evt_send_next );
	}

	void so_evt_start() override
	{
		// Initiate request sending loop.
		so_5::send< send_next >( *this );
	}

private :
	const std::string m_name;
	const so_5::mbox_t m_logger_mbox;
	const so_5::mbox_t m_consumer_mbox;

	unsigned int m_requests_left;

	// An event for next attempt to send another requests.
	void evt_send_next(mhood_t< send_next >)
	{
		if( m_requests_left )
		{
			// Send can wait on full mchain. Mark the start time to
			// calculate send call duration later.
			const auto started_at = steady_clock::now();
			try
			{
				// Send another request.
				// Note: this call can wait on full mchain.
				so_5::send< request >( m_consumer_mbox,
						so_direct_mbox(),
						m_name + "_request_" + std::to_string( m_requests_left ) );

				// How much time the send take?
				const auto ms = std::chrono::duration_cast<
						std::chrono::milliseconds >( steady_clock::now()
								- started_at ).count();

				m_logger_mbox <<= msg_maker() << m_name << ": request sent in "
						<< ms << "ms";
			}
			catch( const so_5::exception_t & ex )
			{
				// Log the reason of send request failure.
				m_logger_mbox <<= msg_maker() << m_name << ": request NOT SENT, "
						<< ex.what();

				// Initiate next send attempt.
				so_5::send< send_next >( *this );
			}
		}
		else
			// No more requests to send. Shutdowner must known about it.
			shutdowner::producer_finished( *this );
	}

	void evt_reply( const reply & msg )
	{
		m_logger_mbox <<= msg_maker() << m_name << ": reply received, "
				<< msg.m_payload;

		--m_requests_left;

		so_5::send< send_next >( *this );
	}
};

class consumer final : public so_5::agent_t
{
	// This signal will be sent by not_empty_notificator when
	// the first message is stored to the empty mchain.
	struct chain_has_requests final : public so_5::signal_t {};

public :
	consumer( context_t ctx, so_5::mbox_t logger_mbox )
		:	so_5::agent_t{ ctx + limit_then_drop< chain_has_requests >(1) }
		,	m_logger_mbox{ std::move(logger_mbox) }
	{
		// Appropriate mchain must be created.
		m_chain = so_environment().create_mchain(
			so_5::make_limited_with_waiting_mchain_params(
				// No more than 10 requests in the chain.
				10,
				// Preallocated storage for the chain.
				so_5::mchain_props::memory_usage_t::preallocated,
				// Throw an exception on overload.
				so_5::mchain_props::overflow_reaction_t::throw_exception,
				// Wait no more than 0.5s on overflow.
				std::chrono::milliseconds(150) )
			// A custom notificator must be used for chain.
			.not_empty_notificator( [this] {
					so_5::send< chain_has_requests >( *this );
				} ) );

		so_subscribe_self().event(
				&consumer::process_requests );
	}

	// A mchain will look like an ordinary mbox from outside of consumer.
	so_5::mbox_t consumer_mbox() const
	{
		return m_chain->as_mbox();
	}

private :
	const so_5::mbox_t m_logger_mbox;
	so_5::mchain_t m_chain;

	void process_requests(mhood_t< chain_has_requests >)
	{
		auto r = receive(
				// Handle no more than 5 requests at once.
				// No wait if queue is empty.
				from( m_chain ).handle_n( 5 ).no_wait_on_empty(),
				[]( const request & req ) {
					// Imitation of some hardwork before sending a reply back.
					std::this_thread::sleep_for( random_pause() );
					so_5::send< reply >( req.m_who, req.m_payload + "#handled" );
				} );
		m_logger_mbox <<= msg_maker()
				<< "=== " << r.handled() << " request(s) handled";

		if( !m_chain->empty() )
			// Not all messages from chain have been processed.
			// Initiate new processing by sending the signal to itself.
			so_5::send< chain_has_requests >( *this );
	}

	static std::chrono::milliseconds
	random_pause()
	{
		std::random_device rd;
		std::mt19937 gen{ rd() };
		return std::chrono::milliseconds(
				std::uniform_int_distribution< unsigned int >{2u, 25u}(gen) );
	}
};

m_chain = so_environment().create_mchain(
    so_5::make_limited_with_waiting_mchain_params(
        // No more than 10 requests in the chain.
        10,
        // Preallocated storage for the chain.
        so_5::mchain_props::memory_usage_t::preallocated,
        // Throw an exception on overload.
        so_5::mchain_props::overflow_reaction_t::throw_exception,
        // Wait no more than 0.5s on overflow.
        std::chrono::milliseconds(150) )
    // A custom notificator must be used for chain.
    .not_empty_notificator( [this] {
            so_5::send< chain_has_requests >( *this );
        } ) );

auto r = receive(
        // Handle no more than 5 requests at once.
        // No wait if queue is empty.
        from( m_chain ).handle_n( 5 ).no_wait_on_empty(),
        []( const request & req ) {
            // Imitation of some hardwork before sending a reply back.
            std::this_thread::sleep_for( random_pause() );
            so_5::send< reply >( req.m_who, req.m_payload + "#handled" );
        } );