RSI - A Structured Approach Use Cases and HCI Design

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