MQTT Maximum Message count Maximum Topic count

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Hey guys,

I have a question about mqtt. What is the maximum number of messages I can send to the broker? Is there a limit? My MQTT.fx clients numbers the messages with a ID. I don't know if it is part of mqtt (which can overflow) or if this is just a solution of mqtt.fx.

Is there a maximum count of the currently active topics?

Jul 10, 2020 in IoT (Internet of Things) by Dingsbums
• 120 points

1 answer to this question.

0 votes

Hello @ Dingsbums ,

The number of topics is going to be set by the number of topics that clients subscribe to as this list is what is searched when a new message is published, so assuming this list is kept I some kind of tree structure (which is very likely due to the hierarchical nature of topics) then the limit is likely to be the amount of memory on the server (assuming 64bit host).

The length of the actual topic string is at most 65536 bytes. This is a limit imposed by the mqtt spec, you can't change it. It is also worth noting that the topic is encoded with utf-8, so you may have less than 65536 characters available.

The payload of the message is limited to 268,435,456 bytes. Again, this is defined by the spec.

If you are routinely approaching either of these limits you should be thinking about whether what you are doing is sensible.

Hope it is helpfull!!
Thank you!

answered Jul 27, 2020 by Niroj
• 82,880 points

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