Elements of scenarios

A scenario is a sequence of: triggers, system reactions, waiting delays, guard realizations and assertions. The following is an example of scenario describing a normal login to the PMSystem.

Figure 7.1: Scenario describing a normal login to the PMSystem.

Scenario: Normal login

1.

Trigger: USER insert card

2.

Guard: USER Card status is NOT irregular AND PMSystem security is NOT high

3.

Reaction: PMSystem ask Pin

4.

Wait: 30 sec

5.

Trigger: USER type PIN

6.

Reaction: PMSystem validate USER identification

7.

Guard: USER identification is valid

8.

Reaction: PMSystem display welcome message

9.

Assertion: User is logged

• A trigger is an operation of an actor of the system (a concept in the domain model).

  Line 1 of scenario Normal login is a trigger corresponding to operation ``insert card'' of concept ``User''.

• A system reaction is an operation of the system under consideration (a system concept in the domain model).

  Line 3 of scenario Normal login is a system reaction.

• A waiting delay specifies a point in a scenario where a certain amount of time passes without any trigger or system reaction.

  Line 4 of scenario Normal login is a waiting delay of 30 seconds.

  Timeouts may be enabled by waiting delays during scenario execution.

• A guard realization is a condition set to hold at a certain point in a scenario.

  Line 7 of scenario Normal login is a guard realization.

  Guard realizations are used set conditions necessary for choosing among several execution paths.

• A assertion is a condition that needs to be true at a certain point in a scenario.

  Line 9 of scenario Normal login is an assertion.

  Assertions serve to check that certain conditions are realized at specific points in an interaction sequence.

A scenario may be ``positive'' (by default) or ``negative''. A positive scenario describes interactions that need to be supported while a negative scenario describes interactions that need to be avoided.