Union sporadically breaks when an incoming stream doesn't have data??

[Alexandra_Mannerings]

I have a bunch of analytic apps that rely on unions to manage variable numbers of incoming data streams. This has always worked in the past for me. Suddenly and seemingly randomly, it has stopped working for some of these workflows (but not all?). 

The problem looks like this:

The data just stop and don't flow through a union when stream #2 has no data . But if input #1 doesn't have data but #2 does, the union works fine and passes data through. So I feel like it can't be the union configuration? But I don't know what else is wrong. A week ago this worked fine. I was on 2023.2, and hadn't updated or changed versions, or even added a patch when it stopped working. Since then, I've tried upgrading to 2024 but that didn't fix it. If I copy the section of workflow that isn't working and paste it into a new workbook, it still doesn't work. If I replace the incoming data streams with yxdb copies of the data at the point they go into the union, the union works fine regardless of which streams are 'active' with incoming data and which streams don't have data. So it's not the contents of the data that make it not work. 

I tried the configuration settings recommended in this post (Union when no data is available - Alteryx Community) but that also did not help.

I'm totally stumped. I've opened a support ticket a week ago, but haven't gotten any answers yet. 

[dgaines1972]

@Alexandra_Mannerings Thank you!!!  The tip about why the Union tool kept throwing errors... has saved me hours of frustration!

[Alexandra_Mannerings]

So glad I could help :) This was driving me NUTS!

[sd4w95sf8e]

Same here.  After upgrading to 2024.2 I started getting this error on Union when no records were fed into it:
Error: Union (55): No incoming metadata from one or more upstream tools. Check the input data and run the workflow again

Removing AMP fixed the issue, thanks for the suggestion! It worked fine on 2023.2 with AMP.

Union has the following settings:

Auto Config by Name

Ignore - Continue Processing Records

Output All Fields

Output Order is not set