Data exchange with the IoT 2020 (Node-Red and Arduino) via MQTT and the internal Mosquitto

I am new and need a recommendation on how to exchange data between IoT 2020 (Node-Red) and the Arduino side of IoT 2020 via MQTT and the internal Mosquitto server. I want to activate functions I programmed in an Arduino sketch via Node-Red. In addition, once the function has been executed, I also want to send feedback via MQTT back to Node-Red.

I use the mqtt_basic example from the PubSubClient Arduino library for testing. As server IP I use the 127.0.0.1 for the Arduino example as well as for Node-Red.

After that, the network interface is no longer accessible and I have no access to the IoT 2020.

Does anyone maybe have an example for me or has it ever done that before?

Would it somehow be possible to point it to Mosquitto using the loopback address 127.0.0.1 Is there someway to have the Arduino side not use Eth0 but a software defined port or something of that nature and then use the loopback address thus freeing up Eth0 for use by Node-Red. The opposite may also be useful, have the Arduino use Eth0 and Node-Red use a virtual connection. 

Personally I think the first one is more useful than the second but anything would be a start.

Kas

P.s I do find it odd that Siemens has build in these two sides (Arduino and Linux) without an easy way for them to share data. While MQTT is an easy and obvious choice, the in ability to do so because of insufficient resources (only one ethernet connection) or lack of connectivity (no internal or loopback connection) I find odd and a lack of foresight. I would love to see this reqtified in a future firmware revision.

The issue here is knowing how much data is new data, not missing anything, not duplicating anything and keeping everything neatly in chronological order. 

If there was a python demo on MQTT with the IoT2020 I would look at that. The file option added complexity that is solved with MQTT. 

There is no node that will only report what has changed in a file. It will report that a file has changed and then you need to try figure out what has changed. If you delete the file the node will no longer watch it. If you erase its contents then the node will think there is new data that needs to be processed. If you "publish" multiple data points quickly to the file the node will report multiple changes to the file but when you go to read the file it will only be one long file yet you will process the same file multiple times. All this assumes that you know how many data points you are looking for in the file in the first place. Has only one data point been published or 5. 

In short, you could use a file but you could also use a piece of paper and analog sensor and write down your readings. In reality though they are not the same and are designed to function in two different ways.

K

Hello, 

I have implemented your suggestion with some success. I can still only work one way reliably (Arduino -> Node Red) as this does not require polling my Node Red. The other way (Node Red -> Arduino I can only seem to do this with polling and this not a very reliable solution. 

The main issue I have now is the buffer size. At what appears to be only 64 bytes my messages (JSONs) are being truncated. I have looked around and can't seem to find where the define is for fprintf() buffer size. I have increased the buffer size for both Serial and USB but this has not solved my issue. If you could please point me to where this is defined so I may change it to a larger number I would appreciate it.

Thankfully 

Kas