Scenarios, Stories, Use Cases: Through the Systems Development Life-Cycle

CHAPTER 24

WHAT SCENARIOS (STILL) AREN'T GOOD FOR

Ian Alexander¹ and Neil Maiden²

¹Scenario Plus, London, UK

²Centre for HCI Design, City University, London, UK

Scenarios are extremely versatile and useful in systems development—and in other places, but they do not offer help in every situation, and may even be misleading if over-enthusiastically applied. This short chapter suggests some possible limits to scenario use—at least until we gain a better understanding. We hope this may restrain the over-enthusiastic, give readers a better feel for what scenarios really can do well, and perhaps indicate some open areas where researchers and innovative practitioners can explore and develop new techniques.

CONTINUOUS BEHAVIOUR

Scenario modelling assumes that behaviour can be divided up into meaningful episodes or chunks, such as transactions: you go and get some cash from the teller machine. But some systems provide continuous behaviour that just doesn't fall into that pattern. As Michael Jackson writes, possibly damning with faint praise,

The use case view can work quite well when it makes sense to think of the machine as a facility offering discrete services that are used in clearly delimited episodes. (Jackson 2001)

Consider a couple of examples:

• System A is a message router for a large organisation. A huge stream of traffic of all kinds arrives from all over the world, is split into its components, and sent on to the addressees of the individual messages. Another stream travels ...