1. A computer-implemented method comprising: receiving an executable block diagram having a plurality of interconnected message-based blocks, where the message-based blocks send and receive messages that include fixed payloads persisting for a determined time interval; adding an observer block to the executable block diagram, the observer block configured to observe one or more messages generated by the message-based blocks; and displaying information from the one or more messages observed by the observer block during execution of the executable block diagram.