Causal dependencies in deadline requirements

摘要

Causal dependency and time requirements are often confused. Things take a particularly interesting twist if causal dependency is called for in a requirement involving time events. A paradoxical argument emerges, which claims that a deadline requirement is equivalent to postulating that occurrence of the deadline be delayed in case the deadline-conditioned event did not happen. To resolve the paradox, a descriptive and a prescriptive part of a deadline requirement are discerned. Further, distinction is made between contingent and strict deadline requirements. The paradox does not appear in contingent deadline requirements. For these, the essence of their causal relations is worked out. For strict deadline requirements, the paradox is finally resolved by tracing it to an asymmetry in the prescriptive part. The causal structure of strict deadline requirements is quite straightforward. The criticism is anticipated that strict deadline requirements are bound to be incorrect for any real system because they are idealizations. It is argued that idealizations, hence strict deadline requirements, have their place in the designer's toolbox.