Information Systems Development Problem Frames Problems and Subproblems

Information Systems Development Problem Frames: Problems and Subproblems October 2002 J. B. Wordsworth: J 2 ISDPPS 1

A problem diagram (1) Lights controller a Lights unit b Lights regime (from Jackson's Problem Frames) a: LC! {RPulse[i], GPulse[i]} b: LU! {Stop[i], Go[i]} October 2002 J. B. Wordsworth: J 2 ISDPPS 2

A problem diagram (2) Description of shared phenomena are amplified by showing the controlling domain. Context diagram extended by adding the requirement. The requirement might refer to the phenomena of one or more domains. The requirement constrains the behaviour of one or more domains. October 2002 J. B. Wordsworth: J 2 ISDPPS 3

Problem analysis Requirement: an optative description of what the customer would like to be true in the problem domain. Domain properties: an indicative description of the properties of the domains. Machine specification: an optative description of the machine's desired behaviour at its interfaces. October 2002 J. B. Wordsworth: J 2 ISDPPS 4

A subproblem: editing periods and ranges (1) n PREdit machine m Periods & ranges Medical staff o m Data entry rules (from Jackson's Problem Frames) October 2002 J. B. Wordsworth: J 2 ISDPPS 5

A subproblem: editing periods and ranges (2) m: MS!{Enter. Period, Enter. Range, . . . } n: PM! {Edit. Opns} o: PR! {Data. Values} The requirement constrains the values of the periods and ranges according to the edit operations initiated by the medical staff. October 2002 J. B. Wordsworth: J 2 ISDPPS 6

Projections or partitions? A partition divides a problem into separate parts, each part having no domains or phenomena in common with any other. A projection divides a problem into parts that might overlap, some parts sharing domains or phenomena with others. October 2002 J. B. Wordsworth: J 2 ISDPPS 7

Summary A problem diagram shows the domains, their interfaces, and the requirement. Problem analysis produces a specification of the machine domain, descriptions of the other domains, and a requirements statement. Descriptions can be optative (desired behaviour) or indicative (given behaviour). A problem can be decomposed by projection into simpler subproblems. October 2002 J. B. Wordsworth: J 2 ISDPPS 8