Bug? Try/Catch Error - variable not shown when run on webportal


Strange behaviour when using try and catch on webportal vs on analytics platform.

In my workflow I use the error message from catch error to fill out text on a webpage to tell the user more specifically why it failed:

Whereas, when exactly the same error is produced in this workflow on the webportal the message does not get displayed, but when I open the job the variable has still been created:

Is there a reason this message doesn’t get displayed on the webportal compared to AP?


Hi @REIDK_AIRP , I’m just taking a guess here, this could be by design, or the default behaviour by design.

The webportal is Front-End, so the users do not see what is happening in the back end. In most Front-Ends, that’s usually how you want to keep it, and this would mean that you don’t necessarily want the front-end user to see any information in relation to what is happening in the back-end as it could give out some sensitive information.

Some information are good for the back end team to troubleshoot, but that you may not want the front end user to have access to.

Usually the back end team has access to everything in the workflow, while the front-end is meant for regular user.

Also, when a webportal execution fails, you can always go to that session in the backend to see the error message/logs.


that is strange indeed - the content of a flow variable should be displayed in both cases. I don’t think this is by design.

To reproduce this, I just tried to build a component that feeds the output of a String Widget into the Math Formula and catch any errors - but the later node warns, if the expression could not be parsed: How did you catch this warning? Would you mind sharing the relevant part of your workflow? And this was KNIME AP 4.4.1 and Server 4.13.1, right?

Best, Lukas

Hi @LukasS

We are using server 4.13.1 but still on AP 4.3.4 - I assume as it works fine on the AP its not to do with versioning but perhaps there is a compatibility issue?

I’ve created a demo of the problem - run on AP it shows the content of the flow variable, but run on the webportal it does not.

Demo_Error_display.knwf (31.3 KB)

Thanks for your help,

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uh oh, that looks like something is fishy with the flow variables that come out of the try/catch: additional to your issue, I get a NullPointerException when trying to display the StackTrace Flow variable in the text output. Ticket created (internal reference: AP-17473).

I like the trick to catch the Math Formula Warning by wrapping it in a component, very nicely done!

Thanks once again for reporting & example, best


Sorry, I misunderstood the issue. I thought the error message, as in Knime’s error message was not displaying in the front-end, in which case it could be by design, but from what I gathered, the issue here is that the error message was captured as part of the Catch and assigned to a variable (FailingNodeMessge), and you are trying display the content of that variable which is not happening, in which case yes, then it should display what’s from the variable.

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an update to your strangely behaving flow variable: It was hidden all along in the grey box - if you have the window too wide, the Custom CSS makes it such that it get lost there. It reappears if you make your window smaller :smiley: