PHP & MQTT - Publish and subscribe in same function - php

Question

Is there a way to make this work..
I'm trying to publish a 'PING' request to a device, and then subscribe to another channel and wait for the 'PONG' response. Here's the code:
public function ping(Request $request)
{
$device = Device::find($request->id);
if ($device) {
//Listen on laravel_device_DEVICEID
$listen_topic = 'laravel_device_' . $device->device_id;
$result = [];
$mqtt = MQTT::connection();
$mqtt->subscribe($listen_topic, function (string $topic, string $message) use ($mqtt, &$result) {
$result['topic'] = $topic;
$result['message'] = $message;
$mqtt->interrupt();
}, 0);
$mqtt->loop(true, true);
//Submit PING message
$topic = $device->device_id;
$message = json_encode([
'type' => 'ping',
'device_id' => $device->device_id
]);
$mqtt = MQTT::connection();
$mqtt->publish($topic, $message);
$device->last_ping_response = 2;
$device->save();
}
}

Answer 1

I'm so free to copy my response from the original question in the php-mqtt/laravel-client repository.
Yes, it is possible and a fairly common thing to do as well. The pattern is called RPC (Remote Procedure Call).
Your code already looks quite good, the only issue is the order of subscribe(), publish() and loop(). Subscribing first is cruicial, since you do not want to miss a response before your subscription goes through. This part is correct already. But you may start the loop only after publishing the request, otherwise the publish() is never reached (because the loop does not exit by itself, only when it receives a message).
Here is the updated piece of code of yours, with some best practices added in. I assumed the response message is what you want to save as $device->last_ping_response:
use PhpMqtt\Client\MqttClient;
use PhpMqtt\Client\Exceptions\MqttClientException;
public function ping(Request $request)
{
$device = Device::find($request->id);
// Exit early if we have no device to ping. Doing it this way round,
// we save one level of identation for the rest of the code.
if ($device === null) {
return;
}
// Prepare all data before connecting to the broker. This way, we do not waste
// time preparing data while already connected.
$subscribeTopic = "laravel_device_{$device->device_id}";
$publishTopic = $device->device_id;
$request = json_encode([
'type' => 'ping',
'device_id' => $device->device_id,
]);
try {
$mqtt = MQTT::connection();
// Ensure we are ready to receive a response to the request
// we are going to send.
$mqtt->subscribe(
$subscribeTopic,
function (string $topic, string $message) use ($mqtt, $device) {
// Update the device based on the response.
$device->last_ping_response = $message;
$device->save();
$mqtt->interrupt();
},
MqttClient::QOS_AT_LEAST_ONCE
);
// Here we register a timeout using a loop event handler.
// The event handler is passed the elapsed time
// the loop runs already (in seconds).
// We do this because in case the receiver of our request is offline,
// we would be stuck in a loop forever.
$mqtt->registerLoopEventHandler(
function function (MqttClient $client, float $elapsedTime) {
// After 10 seconds, we quit the loop.
if ($elapsedTime > 10) {
$client->interrupt();
}
}
);
// Send the request. We use QoS 1 to have guaranteed delivery.
$mqtt->publish($publishTopic, $request, MqttClient::QOS_AT_LEAST_ONCE);
// Wait for a response. This will either return when we receive
// a response on the subscribed topic, or after 10 seconds
// due to our registered loop event handler.
$mqtt->loop();
// Always disconnect gracefully.
$mqtt->disconnect();
} catch (MqttClientException $e) {
\Log::error("Pinging device failed.", [
'device' => $device->id,
'exception' => $e->getMessage(),
]);
}
}