How to receive messages multithreaded?

[autoantwort]

In the only example how to receive messages this is done like this:

mqtt5/example/receiver.cpp

Lines 91 to 93 in 5375062

	for (;;) {
	// Receive an Appplication Message from the subscribed Topic(s).
	auto&& [ec, topic, payload, publish_props] = co_await client.async_receive(use_nothrow_awaitable);

Which means only a single thread is used to process incoming messages. It is possible to use multiple threads to process subscribed topics?

[autoantwort]

Ok probably it is better to only have one thread getting the messages and then multiple worker threads handling the hard work (writing stuff into a database (not async))

[autoantwort]

Interestingly the example crashes with a read access violation on windows if you include <spdlog/spdlog.h> before <boost/asio/use_awaitable.hpp>

[biljazovic]

mqtt_client is not thread-safe.

Ok probably it is better to only have one thread getting the messages and then multiple worker threads handling the hard work (writing stuff into a database (not async))

This sounds like reasonable approach.

Interestingly the example crashes with a read access violation on windows if you include <spdlog/spdlog.h> before <boost/asio/use_awaitable.hpp>

I couldn't reproduce this with MSVC 19.44.35213, C++20 and spdlog v1.x. It would help a lot if you could share a full example with the versions of your compiler and dependencies. Thanks!

One thing to watch out for is that you need to construct the client with either an io_context running on a single thread, or a boost::asio::strand, and call all member functions of the client within that context/strand.