Messages directed only to the first target when a format function is defined

[gsaviane]

The following configuration

[telegraf/raspberrypi/weather]
format = openweathermap_format()
targets = smtp:user, file:mqttwarnlog, log:info
title = "Weather forecast"

Routes messages only to the first target (smtp), although the documentation states that all the targets should receive messages. If I change the order of the targets, like

[telegraf/raspberrypi/weather]
format = openweathermap_format()
targets = file:mqttwarnlog, smtp:user, log:info
title = "Weather forecast"

Then the file target only would receive the messages.

[amotl]

Dear @gsaviane,

omg, thank you for reporting that flaw. Why don't we have a corresponding test case, and why didn't anyone else report this problem before? We will look into it on the next development iteration.

With kind regards,
Andreas.

[amotl]

Does the problem just happen if, how you are describing it, »when a format function is defined«?

[gsaviane]

@amotl I think I figured it out. My format function keeps a state to prevent redundant notifications.
Question: does the format function get called for each target, or only once upon the message arrives?
If it's the former, I can close the issue

[gsaviane]

Confirmed, the format function gets called for each target. I changed the configuration to have one single target calling the function that republish the message into another topic. That solves the problem.
I would suggest to improve the documentation and suggest not to use shared resources like files in functions to prevent race conditions.

[amotl]

Dear @gsaviane,

thanks for your swift reply. Do you have a recommendation how and where to improve the documentation?

With kind regards,
Andreas.

[gsaviane]

In https://mqttwarn.readthedocs.io/en/latest/configure/transformation.html#id5
maybe a note informing:

Functions get invoked once per target. If idempotency of a custom function is not guaranteed (function(msg) returns always the same result), the message might not get dispatched to every target the same way

[amotl]

Thank you. We've staged a relevant patch.

• Documentation: Educate about idempotency of user-defined functions #714