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I have a software application which communicates with a Graphic User Interface using MQTT protocol.

The first software (let's call it app) publish messages and the second software (let's call it gui) subscribes to topics.

App software

The app software is a C++ application which embeds a MQTT client based on mosquittopp.

It sends messages on different topics in order to notify gui software.

Gui software

The gui software is a Qt application which embeds a MQTT client also based on mosquittopp.

When a message is received on a topic, a signal messageReceived is emitted.

The signal messageReceived is connected to a slot Message_received_from_topic.

All messages from all topics are processed into Message_received_from_topic slot.

Presenting the issue

A gui screen is composed of several fields. A field is updated when a message is received on the associated topic.

When gui stays on the same screen, receiving messages through MQTT allow to update fields without reloading the full screen.

However when changing to a screen which contains many fields, there are some glitches that appear because the screen is refreshed before all fields are generated.

In order to avoid those glitches, I want to lock screen refreshing before updating all fields at least once and then unlock refresh.

That's why not receiving MQTT messages in the right order is problematic.

Issue analysis

The issue I'm facing is that messages are not received in order by gui.

Here is a table that present how messages are sent and received.

+----------------------------------------------+----------------------------------------------+----------------------------------------------+
|               Sent by **app**                |       On broker (using mosquitto_sub)        |             Received by **gui**              |
+---------------------------+------------------+---------------------------+------------------+---------------------------+------------------+
| Topic                     | Message          | Topic                     | Message          | Topic                     | Message          |
+---------------------------+------------------+---------------------------+------------------+---------------------------+------------------+
| widget/refresh            | 0                | widget/refresh            | 0                | widget/refresh            | 0                |
| widget/input/value/9      | {"label":"2020"} | widget/input/value/9      | {"label":"2020"} | widget/refresh            | 1                |
| widget/input/value/10     | {"label":"02"}   | widget/input/value/10     | {"label":"02"}   | widget/input/value/9      | {"label":"2020"} |
| widget/input/value/11     | {"label":"20"}   | widget/input/value/11     | {"label":"20"}   | widget/input/position/9   | 1                |
| widget/input/value/12     | {"label":"15"}   | widget/input/value/12     | {"label":"15"}   | widget/input/selection/9  | true             |
| widget/input/value/13     | {"label":"06"}   | widget/input/value/13     | {"label":"06"}   | widget/input/value/10     | {"label":"02"}   |
| widget/input/position/12  | 0                | widget/input/position/12  | 0                | widget/input/position/10  | 0                |
| widget/input/selection/12 | false            | widget/input/selection/12 | false            | widget/input/selection/10 | true             |
| widget/input/position/13  | 0                | widget/input/position/13  | 0                | widget/input/value/11     | {"label":"20"}   |
| widget/input/selection/13 | false            | widget/input/selection/13 | false            | widget/input/position/11  | 0                |
| widget/input/position/9   | 0                | widget/input/position/9   | 0                | widget/input/selection/11 | true             |
| widget/input/selection/9  | true             | widget/input/selection/9  | true             | widget/input/value/12     | {"label":"15"}   |
| widget/input/position/10  | 0                | widget/input/position/10  | 0                | widget/input/position/12  | 0                |
| widget/input/selection/10 | true             | widget/input/selection/10 | true             | widget/input/selection/12 | false            |
| widget/input/position/11  | 0                | widget/input/position/11  | 0                | widget/input/value/13     | {"label":"06"}   |
| widget/input/selection/11 | true             | widget/input/selection/11 | true             | widget/input/position/13  | 0                |
| widget/input/position/9   | 1                | widget/input/position/9   | 1                | widget/input/selection/13 | false            |
| widget/refresh            | 1                | widget/refresh            | 1                |                           |                  |
+---------------------------+------------------+---------------------------+------------------+---------------------------+------------------+

Here messages are received in the wrong order, I'm also missing one message on topic "widget/input/position/9"

All publications and subscriptions are using QoS 2.

Questions

Does anybody knows why messages are not received in the right order by gui when they are correctly received using mosquitto_sub ?

Is there any configuration to apply to mosquitto broker in order to keep message order ?

eyllanesc
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Arkaik
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    Is it guaranteed that the message sequence is kept OVER topics? I doubt that. Also a hint to different QoS: https://stackoverflow.com/questions/30955110/is-message-order-preserved-in-mqtt-messages – Mario The Spoon Feb 20 '20 at 16:17
  • It seems there is something wrong in your qt mqtt client. It is possible, that the client is receiving messages in order, but signal/sllot distribution is somehow wrong. You can try to connect all slots using `Qt::BlockingQueuedConnection` and see what happens. – j123b567 Feb 21 '20 at 09:48

0 Answers0