RSI - A Structured Approach Use Cases and HCI Design

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The RSI Approach To HCI Design / Use Case Analysis Page 32 of 42 firstName surname address1 address2 address3 ::Customer * customers 1 ::Account * items ::AccountItem value description date 1 account 1 reservedby * reservations ::Reservation start-date end-date checked-in W5a - RSI LONG PAPER [42 PAGES].doc( Rev: 5) - 03/09/00 1 1 ::Room 1 reserved * rooms * reservations *1 1 ::Hotel smoking-rule reservations Invariant: The is no overlap in Reservation dates for all the Reservations associated with any particular Customer values: smoking,non-smoking invarant: start-date < end-date Invariant: Room reservations do not overlap. Invariant: All Reservations must have an associated Customer in Hotel.customers. All Reservations must have an associated Room in Hotel.rooms. The model shown contains a number of invariants (business rules) which must be adhered to at all times (e.g. "Room reservations do not overlap"). Using invariants reduces the complexity and removes redundancy in pre- and post-condition descriptions (which are, conceptually at least, implicitly 'and'-ed with them at the beginning and end of each use case). Note that the core specification model must contain at least one 'anchor point' (Hotel in this example). Pre- and post-conditions are always described in a manner that is relative to some known point within the model (e.g. a post-condition stating that 'aCustomer.reservations is empty' is relative to aCustomer). Without a well known anchor point, it would not be possible to find a way into the model! Two example service use case specifications are shown below. A full description of the service use case model for this system can be found in the companion document to this paper on www.ratio.co.uk/rsi-info.htm. in:- [what information is passed from the actor to the system] start-date, end-date, aRoom, aCustomer; out:- [what information does the system pass back to the actor] status [shorthand for ok/fail] pre:-conditions:- [what must be true before this service can be invoked] start-date
The RSI Approach To HCI Design / Use Case Analysis Page 33 of 42 • aReservation.start-date = start-date • aReservation.end-date = end-date • aReservation.reservedby = aCustomer • aReservation.reserved = aRoom W5a - RSI LONG PAPER [42 PAGES].doc( Rev: 5) - 03/09/00 update service use case 'make reservation' The descriptions above use a number of semi-formal conventions (based loosely on OCL), which experience shows assist in removing ambiguity: • Names that correspond to types in the specification model are capitalised; • Use of an instance of a type in the specification model is indicated by pre-fixing by 'a' or 'an' onto the type name. Additional qualification (aFirstCustomer, etc.) may be required if more than one instance needs to be identified. • Use of attributes is shown using a '.' between the type name and the attribute name of the model. So 'aReservation.start-date' indicates the start-date attribute of aReservation. • Navigation via associations is shown the same convention: so 'aReservation.reservedby' is the Customer who has reserved the Room identified by Reservation.reserved. • All associations on the model are, at this stage, assumed to be bi-directional, so if aReservation.reservedby = aCustomer, aCustomer.reserves = aReservation. Note that 'Make reservation' adds a new instance of a Reservation to the model. When an instance is added (or deleted or updated) it must be added to something (or deleted from or updated in). In this case, a Reservation instance is added to the set of reservations associated with (the one and only) Hotel. Hotel provides the context within which the Reservation instance is added - hence its inclusion in the post-condition description. Fans of OCL, or those on projects which demand a high degree of formality, may choose to document the above example in a more formal manner as follows [7]: Hotel::MakeReservation(start:Date, end:Date, aRoom:Room, aCustomer:Customer) pre: ( start forAll( (end < start-date) or (start > end-date) ) ) and ( aCustomer.reservations->forAll( (end < start-date) or (start > end-date) ) ) post: ( self.reservations->includes(aReservation) ) and ( aReservation.start-date = start ) and ( aReservation.end-date = end ) and ( aReservation.reservedby = aCustomer ) and
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RSI - A Structured Approach Use Cases and HCI Design

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