paho.mqtt.cpp/examples/multithr_pub_sub.cpp

// async_subscribe.cpp
//
// This is a Paho MQTT C++ client, sample application.
//
// This application is an MQTT publisher/subscriber using the C++
// asynchronous client interface, demonstrating how you can share a client
// between multiple threads.
//
// The app will count the number of "data" messages arriving at the broker
// and then emit "events" with updated counts. A data message is any on a
// "data/#" topic, and counts are emitted on the "events/count" topic. It
// emits an event count around once every ten data messages.
//
// Note that this is a fairly contrived example, and it could be done much
// more easily in a single thread. It is meant to demonstrate how you can
// share a client amongst threads if and when that's a proper thing to do.
//
// At this time, there is a single callback or consumer queue for all
// incoming messages, so you would typically only have one thead receiving
// messages, although it _could_ send messages to multiple threads for
// processing, perhaps based on the topics. It could be common, however, to
// want to have multiple threads for publishing.
//
// The sample demonstrates:
//  - Creating a client and accessing it from a shared_ptr<>
//  - Using one thread to receive incoming messages from the broker and
//    another thread to publish messages to it.
//  - Connecting to an MQTT server/broker.
//  - Subscribing to a topic
//  - Using the asynchronous consumer
//  - Publishing messages.
//

/*******************************************************************************
 * Copyright (c) 2020-2023 Frank Pagliughi <fpagliughi@mindspring.com>
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v2.0
 * and Eclipse Distribution License v1.0 which accompany this distribution.
 *
 * The Eclipse Public License is available at
 *    http://www.eclipse.org/legal/epl-v20.html
 * and the Eclipse Distribution License is available at
 *   http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *    Frank Pagliughi - initial implementation and documentation
 *******************************************************************************/

#include <iostream>
#include <cstdlib>
#include <string>
#include <cstring>
#include <cctype>
#include <thread>
#include <chrono>
#include <memory>
#include "mqtt/async_client.h"

using namespace std;
using namespace std::chrono;

const std::string DFLT_SERVER_ADDRESS("mqtt://localhost:1883");
const std::string CLIENT_ID("multithr_pub_sub_cpp");

/////////////////////////////////////////////////////////////////////////////

/**
 * A thread-safe counter that can be used to occasionally signal a waiter on
 * every 10th increment.
 */
class multithr_counter
{
	using guard = std::unique_lock<std::mutex>;

	size_t count_;
	bool closed_;
	mutable bool ready_;
	mutable std::condition_variable cond_;
	mutable std::mutex lock_;

public:
	// Declare a pointer type for sharing a counter between threads
	using ptr_t = std::shared_ptr<multithr_counter>;

	// Create a new thread-safe counter with an initial count of zero.
	multithr_counter() : count_(0), closed_(false), ready_(false) {}

	// Determines if the counter has been closed.
	bool closed() const {
		guard g(lock_);
		return closed_;
	}

	// Close the counter and signal all waiters.
	void close() {
		guard g(lock_);
		closed_ = ready_ = true;
		cond_.notify_all();
	}

	// Increments the count, and then signals once every 10 messages.
	void incr() {
		guard g(lock_);
		if (closed_)
			throw string("Counter is closed");
		if (++count_ % 10 == 0) {
			ready_ = true;
			g.unlock();
			cond_.notify_all();
		}
	}

	// This will block the caller until at least 10 new messages received.
	size_t get_count() const {
		guard g(lock_);
		cond_.wait(g, [this]{ return ready_; });
		ready_ = false;
		return count_;
	}
};

/////////////////////////////////////////////////////////////////////////////

// The MQTT publisher function will run in its own thread.
// It runs until the receiver thread closes the counter object.
void publisher_func(mqtt::async_client_ptr cli, multithr_counter::ptr_t counter)
{
	while (true) {
		size_t n = counter->get_count();
		if (counter->closed()) break;

		string payload = std::to_string(n);
		cli->publish("events/count", payload)->wait();
	}
}

/////////////////////////////////////////////////////////////////////////////

int main(int argc, char* argv[])
{
	 string address = (argc > 1) ? string(argv[1]) : DFLT_SERVER_ADDRESS;

	// Create an MQTT client using a smart pointer to be shared among threads.
	auto cli = std::make_shared<mqtt::async_client>(address, CLIENT_ID);

	// Make a counter object also with a shared pointer.
	auto counter = std::make_shared <multithr_counter>();

	// Connect options for a persistent session and automatic reconnects.
	auto connOpts = mqtt::connect_options_builder()
		.clean_session(false)
		.automatic_reconnect(seconds(2), seconds(30))
		.finalize();

	auto TOPICS = mqtt::string_collection::create({ "data/#", "command" });
	const vector<int> QOS { 0, 1 };

	try {
		// Start consuming _before_ connecting, because we could get a flood
		// of stored messages as soon as the connection completes since
		// we're using a persistent (non-clean) session with the broker.
		cli->start_consuming();

		cout << "Connecting to the MQTT server at " << address << "..." << flush;
		auto rsp = cli->connect(connOpts)->get_connect_response();
		cout << "OK\n" << endl;

		// Subscribe if this is a new session with the server
		if (!rsp.is_session_present())
			cli->subscribe(TOPICS, QOS);

		// Start the publisher thread

		std::thread publisher(publisher_func, cli, counter);

		// Consume messages in this thread

		while (true) {
			auto msg = cli->consume_message();

			if (!msg)
				continue;

			if (msg->get_topic() == "command" &&
					msg->to_string() == "exit") {
				cout << "Exit command received" << endl;
				break;
			}

			cout << msg->get_topic() << ": " << msg->to_string() << endl;
			counter->incr();
		}

		// Close the counter and wait for the publisher thread to complete
		cout << "\nShutting down..." << flush;
		counter->close();
		publisher.join();

		// Disconnect

		cout << "OK\nDisconnecting..." << flush;
		cli->disconnect();
		cout << "OK" << endl;
	}
	catch (const mqtt::exception& exc) {
		cerr << exc.what() << endl;
		return 1;
	}

 	return 0;
}