Provide an example of how to receive only one retained message

[dmak]

I wonder how to implement the following functionality:

mosquitto_sub -C 1 -t ...

i.e. read one (retained) message from the topic.

The following code does not wait for 5 seconds and also does not receive any message from the topic (prints Tick: 1 to the console):

my $mqtt = Net::MQTT::Simple->new(MQTT_HOST);
$mqtt->login($mqtt_user, $mqtt_pass);
$mqtt->subscribe(MQTT_TOPIC, sub {
	shift;
	print "Received the message: " . shift . "\n";
});
print "Tick: " . $mqtt->tick(5) . "\n";
$mqtt->disconnect();

Thanks in advance!

[Juerd]

Dmitry Katsubo skribis 2022-02-17 14:52 (-0800):

I wonder how to implement the following functionality: ``` mosquitto_sub -C 1 -t ... ``` i.e. read one (retained) message from the topic.

The accompanied command line utility `mqtt-simple` supports this feature. You could look at that for inspiration.

[dmak]

Thanks. The utility calls run() function (which I cannot afford as it enters the endless loop) and calls exit() to abort the program from message handler.
I need exactly the logic I've described: receive the message from the topic and then the program flow continues. If there is no message send by the server, then wait some time and continue:

my $message = undef;
$mqtt->subscribe(MQTT_TOPIC, sub {
	shift;
	$message = shift;
});
# ... some long-running initialization ...
if (!$message) {
	$mqtt->tick(5); # give one more chance to receive the message
	if (!$message) {
		$mqtt->unsubscribe(MQTT_TOPIC);
		$message = { 'default' => 'value' };
	}
}
# ... the program continues either with received message or with default values ...

The only way I see this can be implemented with utility approach is when:

• The function run() is added a timeout argument which specifies now many seconds the loop may cycle.
• The message handler may return a special return code to signal that loop should exit.

Then it can be implemented the suggested way:

$mqtt->subscribe(MQTT_TOPIC, sub {
	$message = $_[1];
	return BREAK_LOOP;
});
$mqtt->run(5);

[christianTF]

See this code used in one of our libraries:
https://github.com/mschlenstedt/Loxberry/blob/f517648bdcf8d17a3290165f2c83a7369ba4ba29/libs/perllib/LoxBerry/IO.pm#L730

[dmak]

Thanks for the example, that is actually what I was thinking about: call subscribe() then sleep() + tick() (however that is not efficient). But if this is a "standard" usecase, why it cannot library help here?

Currently tick() returns also when PINGRESP (type 13) is received while generally one would be interested only in messages for subscribed topics. The meta-code for the better helper is:

my $time_ms = time_ms();
while ($timeout_ms > 0) {
  if (select($timeout_ms / 1000)) {
    sysread();
    _parse();
    my $message_dispatched_to_topic = _dispatch_message();
    if ($message_dispatched_to_topic) {
      last;
    }
    // message is a service message
    my $current_time_ms = time_ms();
    $timeout_ms -= ($current_time_ms - $time_ms);
    $time_ms = $current_time_ms;
  } else {
    // no event for the given timeout
    last;
  }
}