Hi I want to catch an exception value in a variable. for example if

You can do it by capturing the Fault object. This is available in the Outputs tab in the Call Web Service node.

You can add a new custom output to the Call Web Service smart service, use this expression:

xpathsnippet(ac!Fault,"//faultstring/text()")

and save it in a process variable.

Make sure to enhance this expression to handle cases where the ac!Fault is null

thanks eduardo, suppose I have been thrown a exception in a query rule how will I handle that or how can I take the value of that in my process variable

When an activity fails an alert is generated to notify the relevant parties for it to be corrected. There's no automatic exception handling for this type of activities (in contrast to the Call Web Service Node which has a retry logic and a Fault object). For more information on Exception Handling review the following link forum.appian.com/.../Exception_Handling

You can still add logic to your model to take actions when there's a failure but the implementation of such mechanisms depends on the design of the model. For instance, let's say you know Script Task A runs a query rule; you could add a custom output to store a "success" flag in a process variable and monitor this variable after X hours (through a timer), let's say that based on your model if this flag is still null or false after those X hours then you can consider something happened with the Script Task and you can configure your flow to notify you or take other actions.

Other people may have other ideas on how to handle failures on nodes but they will all depend on the design of your model.