At some point you'll build a xap that doesn't work, and you'll need to debug it. To do this, you'll use the Event Log and various indicators from the components in the dataflow. We recommend you familiarize yourself with the dataflow UI in general before reading this article.
The xap log is available via this button in the lower right of the dataflow interface:
The log outputs a range of information on component and xap level events:
You can send data through your components with this arrow icon in the component:
After sending data, when a port further down the dataflow turns blue the data was successfully input to or output from that port. When it turns orange an error occurred, and you should check the log.