How many times per minute does the MQTT client poll the server? Is it a big data traffic or not? I know the size of the packet can be small, but how many times the client ping the broker to make itself "online" in the broker.
If I was not clear please comment this question and I'll try explain better my doubt.
My broker is Mosquitto and the clients are small device (sensors and etc.)
Assuming no data flow (which is of course application dependent), the client will periodically send a PINGREQ message to the broker. This is a 2 byte message and the broker replies with a PINGRESP, also 2 bytes.
The rate at which PINGREQ is sent depends on the keepalive parameter set when you connect. This tells the broker the interval at which it should expect at least one message from the client. In the absence of any other message, the client sends a PINGREQ.
60 seconds is often used as a default value (whether or not this is appropriate for you depends on how quickly you want the client/broker to respond to a hung connection). In the absence of any other messages flowing, maintaining the keepalive guarantee would mean 4 bytes total transferred every minute. This is only the application level data of course, the length of the data on the wire will be bigger.
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