Modelling Features as Statecharts Pourya Shaker and Joanne M. Atlee

Feature-Oriented Requirements Modelling Research study will evaluate the readability and writeability of two approaches to modelling features –Model a feature AND its context –Model just the feature (state its context) Time Commitment: 2 hours –50 minute tutorial on modelling language (July 17 th ) –50 minute questionnaire about models (July 24 th ) –$30 renumeration Asked to consent us to use questionnaire answers in thesis or publication –Can withdraw consent at any time –Can withdraw from study at any time

Agenda Remember SE463? – A reference model for requirements engineering – Modelling requirements using UML class and statechart models Feature-oriented software development FORML: A feature-oriented requirements modelling language – Two approaches to evolving FORML models with new features Overview of questionnaire

A Requirements Engineering Framework Requirements – desired goals or behaviour

A Requirements Engineering Framework Requirements – desired goals or behaviour Requirements – desired changes to the world – Expressed in terms of world phenomena World (aka domain, environment) Software System Requirements World Phenomena Software-System Phenomena Interface Phenomena

Modelling Requirements Using UML The world can be modelled as a UML class diagram

Modelling Requirements Using UML The world can be modelled as a UML class diagram

Modelling Requirements Using UML The requirements can be modelled as UML state machines – Events, guards, and actions in terms of world phenomena

Feature-Oriented Software Development (FOSD) Stakeholders (e.g., users, developers) often think of a software system in terms of its features – Feature – a coherent bundle of functionality (e.g., call waiting, call forwarding, voice mail) FOSD – advocates the use of features as first-class entities throughout the lifecycle of a software system – Make features explicit in software artefacts (e.g., code, design models, requirements models)

FORML: A Feature-Oriented Requirements Modelling Language World Model Behaviour Model feature modules events & conditions actions FOSD + UML-based requirements modelling

Example A telephone system with a basic-call service (BCS) feature – Responds to user commands to start, accept, and end calls – Notifies users about the callees status

FORML World Model feature: groups shared phenomena by features software system

FORML: A Feature-Oriented Requirements Modelling Language World Model Behaviour Model feature modules events & conditions actions

World States The inputs and outputs of the behaviour model are over world states – instances of the world model, representing a possible snapshot of the world

BCS Feature Module One statechart instance per TelephoneSystem object Keyword this refers to the TelephoneSystem object of a statechart instance

BCS Feature Module Transition Labels: id: e [c] / id 1 : [c 1 ] a 1, …, id n : [c n ] a n Transition or action name Triggering event: world-change event Guard condition: world-state conditions Action: world-change action macros

FORML World Model

Adding a New Feature to a FORML Model Example: adding the requirements of call-forwarding on busy (CFB) to the model in the previous slides – CFB: when a user of a telephone system receives a call while he/she is busy (i.e., already in a call), forward the call to a designated user of a different telephone system Update the world model (new phenomena) Add a new feature module to the behaviour model (add new behaviours, modify existing behaviours)

Alternative Approaches to Modelling New Features Approach 1 – The requirements of a new feature can always be specified as a complete statechart Approach 2 – But sometimes, it is convenient to specify a new features requirements as differences with the requirements of existing features – i.e., as statechart fragments that extend the feature modules of existing features

Approach 1: CFB as a Complete Statechart Updated World Model CFB Feature Module override action a2 of transition t7 in the BDS feature module updates

BCS Feature Module

Approach 1: Composed Behaviour model in parallel with

Approach 2: CFB as Statechart Fragments Updated World Model CFB Feature Module updates add actions a1, a2, and a3 to transition t7 in the BDS feature module

Approach 2: Composed Behaviour Model element names qualified with the name of the feature module in which they were introduced CFB extensions of BDS feature module

Statechart Fragments F1 feature module F2 feature module Composed model

Statechart Fragments F1 feature module F2 feature module Composed model new region existing state

Statechart Fragments F1 feature module F2 feature module Composed model new transition and destination state existing region and source state

Statechart Fragments F1 feature module F2 feature module Composed model new action existing transition

Statechart Fragments F1 feature module F2 feature module Composed model new strengthening clause existing transition

Statechart Fragments F1 feature module F2 feature module Composed model new weakening clause existing transition

Statechart Fragments F1 feature module F2 feature module Composed model new strengthening clause existing action

Statechart Fragments F1 feature module F2 feature module Composed model new weakening clause existing action

FORML: A Feature-Oriented Requirements Modelling Language World Model Behaviour Model feature modules events & conditions actions How are these expressed?

World-State Expressions Basic Set Expressions: – Calls : the set of Call objects – this : the TelephoneSystem object of a statechart instance – o : an world-change event parameter Selection – Calls[voice = true] : the set of Call objects whose voice attribute has the value true

World-State Expressions Basic Set Expressions: – Calls : the set of Call objects – this : the TelephoneSystem object of a statechart instance – o : an world-change event parameter Selection – Calls[voice = true] : the set of Call objects whose voice attribute has the value true

World-State Expressions Navigation – set-expr.feature-name this.BCS – set-expr.association-name.role-name this.BCS.Processing.Call – set-expr.attribute-name this.BCS.Processing.Call.voice – set-expr.role-name this.BCS.Processing.Call.caller

World-State Expressions Navigation – set-expr.feature-name this.BCS – set-expr.association-name.role-name this.BCS.Processing.Call – set-expr.attribute-name this.BCS.Processing.Call.voice – set-expr.role-name this.BCS.Processing.Call.caller

World-State Expressions Navigation – set-expr.feature-name this.BCS – set-expr.association-name.role-name this.BCS.Processing.Call – set-expr.attribute-name this.BCS.Processing.Call.voice – set-expr.role-name this.BCS.Processing.Call.caller

World-State Expressions Navigation – set-expr.feature-name this.BCS – set-expr.association-name.role-name this.BCS.Processing.Call – set-expr.attribute-name this.BCS.Processing.Call.voice – set-expr.role-name this.BCS.Processing.Call.caller

World-State Expressions Set, boolean, and integer operations: Binary Set OpsSet-Cardinality PredicatesBoolean OpsInteger Ops + (union)no set-expr (set has zero elements) and# set-expr (number of elements in set) - (intersection)one set-expr (set has exactly one element) or=>, =< = (equals)lone (set has zero or one elements) implies>, < in (subset of)some (set has one or more elements) iff== not!=

World-State Expressions World-change events – s ? M(o) : message o sent to a system s this ? StartCall(o) – C+(o) : object o of type C added to world state Call+(o) – C-(o) : object o of type C removed from world state Call-(o) – C.a~(o): attribute a of object o of type C changed value Call.voice~(o)

World-State Expressions World-change actions – o = +C(a 1 = exp 1,..., a n = exp n ) creates a C object (that can be referenced by the variable o) whose attributes a i have values exp i o = +User() – +A(a 1 = exp 1,..., a n = exp n, r 1 = o 1,..., r m = o m ) creates an A link that relates objects o j in roles r j, and whose link attributes a i have values exp i c = +Call(caller = set-expr, callee = set-expr, voice = false)

World-State Expressions World-change actions – s ! M(p 1 = exp 1,..., p n = exp n ) creates an M message object sent by system s whose parameters p i have values exp i this ! Busy() – -o removes the objects o and their dependent links -Calls[voice = true] – o.a := exp changes the value of os attribute a to value exp o.voice := true

Transition and Action Overrides

An action override specifies a new action, a2, that overrides an existing action, F{a1} in a transition t in feature-module F. – F{t} : / a2: override(a1) [c] – Literally, this states that whenever Fs transition t executes, action a2 is performed in place of a1 provided that the guard condition c is true.

Transition and Action Overrides Transition priority: specifies that a new transition, t2, has priority over an existing transition, F{t1} in feature module F, whenever both are simultaneously enabled. – t2 > F{ t1} :... Transition override: specifies that a new transition, t2, that overrides an existing transition, F{t1} in feature-module F. An override differs from a transition priority in that the enabling condition of t2 is implicitly the same as that of t1, but could be strengthened with an additional guard c: – t2 : override(F{t1}) [c] /... – Literally, this states that whenever Fs transition t1 is enabled, t2 executes instead provided that the behaviour model is in t2s source state and guard condition c is true.

Questionnaire You will be asked to perform model-reading and writing tasks with respect to one of the two approaches to adding a new feature to a FORML model of TelephoneSystem with BCS Model-reading task: – Reading-comprehension questions about a given model of a new feature added to TelephoneSystem Model-writing task: – Modelling of a new feature added to TelephoneSystem

Thank you for your participation! Any Questions? Slides will be made available at: Also, please feel free to contact me (Pourya) at or Joanne M. Atlee at with any questions about this presentation or about the study