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Re: [paho-dev] MQTT client throttling

Hi Ian,

thanks for bringing the topic to attention. Since I’m involved in this blog series I’m very glad for the feedback and this must indeed be clarified since your point of view on throttling is also valid.

The idea for adding throttling was inspired by the fusesource mqtt-client. I’ll copy the relevant part of their documentation below:

  • setMaxReadRate : Sets the maximum bytes per second that this transport will receive data at. This setting throttles reads so that the rate is not exceeded. Defaults to 0 which disables throttling.

  • setMaxWriteRate : Sets the maximum bytes per second that this transport will send data at. This setting throttles writes so that the rate is not exceeded. Defaults to 0 which disables throttling.


So the idea is to limit the maximum bytes for reading and writing operations on the client side. With that options it’s possible to guard the client from receiving more messages than it can handle (you can try it out with high-throughput public brokers and subscribe to ‘#’). This of course adds backpressure on the broker and this will probably result in load shedding for the subscriber if the broker supports that. This option is also very useful if you are on constrained devices with very limited memory.

While an in-flight window is also good for throttling (outgoing) QoS 1 and 2 messages, my understanding is, that this in-flight feature is useful to guarantee the Ordered Topic requirements of the MQTT 3.1.1 specification. I’m not too familiar with the implementation details of Paho, but I guess the in-flight window is only implemented for the publishing API?

I’d like to hear your opinion on that and if you think the throttling (as in limiting bandwidth) would be useful for Paho.

Nevertheless, we will add a short explanation to the HiveMQ blog post series for throttling soon so readers don’t get confused.

Thanks,
Dominik



On 17 Sep 2015 at 16:45:55, Ian Craggs (icraggs@xxxxxxxxxxxxxxxxxxxxxxx) wrote:

Hello all,

one of the features of MQTT client libraries listed on the HiveMQ blogs
is "throttling". In most, if not all of the client libraries we can
"throttle" outgoing QoS 1 and 2 flows by waiting for acks before sending
new messages, but this does not work for QoS 0. I thought that our
current approach might qualify as throttling - but apparently not :-)

Is there a definition of throttling for MQTT clients that we can all
agree on, and works for any MQTT broker implementation? I'd be
interested in implementing that in various libraries, if it seemed to be
useful.

--
Ian Craggs
icraggs@xxxxxxxxxx IBM United Kingdom
Paho Project Lead; Committer on Mosquitto

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