Exception Cases

Exceptions are unwanted but acceptable deviations from the normal case. They are unwanted in the sense that the owner of the work would rather that they did not ever happen. However, we know that they will happen on occasion, so we have to cater for them. For example, an end user could enter the wrong data, the passenger might not know the locator number, or the online customer might forget his password.

Exceptions are unwanted but acceptable deviations from the normal case.

The goal of the exception scenario is to show how the work safely handles the exception. In other words, which steps have to be taken to rejoin the path of the normal case? You can write a separate scenario, but in most cases it is more convenient to add the exception steps to your normal scenario.

Consider step 5 as an example:

5. Find a seat.

E5.1 The passenger's choice of seat is not available.

E5.1 Record a request for a seat change by the gate agent.

You find the exceptions by examining each step of the normal case and asking questions such as these:

• What happens if this step cannot be completed, or does not complete, or comes up with the wrong or unacceptable result?

• What can go wrong at this step?

• What could happen to prevent the work from reaching this step?

• Could any external entity disrupt or prevent this step, or this business use case?

• Could any technology used to implement this step fail, or be unavailable?

• Could the end user fail to understand what is required of him, or misunderstand the information presented by the product?

• Could the end user take the wrong actionintentionally or unintentionallyor fail to respond?

There are many questions, each of which seeks to find a different potential error. We suggest you make a checklist of the questions pertinent to your particular situation, adding to it each time a new project discovers new questions. We have also seen some successful attempts to use automation to help with this task. Our colleague Neil Maiden at City University in London is having success using the university's ART-SCENE scenario presenter. This tool works with the normal case scenario to automatically generate a list of potential exceptions. It also uses rich-media scenarios where additional information in the form of photographs, movies, sound, and so on is integrated into the scenario.

The exception questions are intended to prompt the interested stakeholders to discover all the exceptions. Question each step carefully. We know from experience that the exceptions have the potential to necessitate a great amount of remedial rework if they are not found at requirements time. Look for anything that could go wrong. You can ignore the scenario in which the work is struck by a meteor, but almost anything else is possible.

Details of ART-SCENE can be found at hcid.soi.city.ac.uk/research/Artsceneindex.html