Is async_mqtt5::async_publish thread-safe ?

Hi,

Asio-compliant objects (like our mqtt_client) and their member functions (like async_publish) are not thread-safe. This is intentional to avoid the overhead associated with mutexes and other synchronisation mechanisms in single-threaded applications.

Regardless, you can use more than one thread to run io_context. In that case, you should make sure that the mqtt_client's functions are executed within the strand (an executor that guarantees that tasks assigned to it are executed in a serialised manner, preventing concurrent execution).

You can achieve that by:

constructing the mqtt_client with a strand object as its executor
dispatching all mqtt_client's function calls to that strand using post or dispatch
ensuring that the async_xxx's completion handler's associated executor is that same strand

This is how you would call async_publish in a multi-threaded environment:

  boost::asio::strand strand = boost::asio::make_strand(ioc.get_executor());
  async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket> client(strand);
  client.brokers("<your-mqtt-broker>", 1883);
  boost::asio::post(
      strand,
      [&client, &strand] {
        // Considering that the default associated executor of all completion handlers is the strand,
        // it is not necessary to explicitly bind it to async_run or other async_xxx's handlers.
          client.async_run(boost::asio::detached);
      }
  );
  boost::asio::post(
      strand,
      [&client, &strand] {
          client.async_publish<async_mqtt5::qos_e::at_least_once>(
              "<your-mqtt-topic>", "Hello world!", async_mqtt5::retain_e::no,
              async_mqtt5::publish_props {},
              [&client, &strand](
                  async_mqtt5::error_code ec, async_mqtt5::reason_code rc,
                  async_mqtt5::puback_props props
              ) {
              }
          );
      }
  );

Now, you can freely use as many threads as you'd like for your io_context and not worry about any concurrency issues.

Additionally, we have thoroughly covered this topic in our documentation. I'd recommend giving it a glance.

Hope this helps!

mireo / async-mqtt5

Is async_mqtt5::async_publish thread-safe ? #10