2.6.3. Structuring SDsNormally a use case scenario is too long andcomplex to fit on a single (A4?) SD.

We need to hierarchically structure SDs and decompose them into

sub-SDs called by

SD references.1

A reference is a pointer to another SD.

References may not be circular or recursive.

2

Let us give a typical example. A simple dataexchange between two systems can bedivided into 3 phases:

• set-up communication/initialize• exchange data• close-down communication

We can specify the use case “successful dataexchange” as follows.

3

BA

Initialize

Exchange

Shutdown

SD Successful_Data_Exchange

ref

ref

ref

4

The symbol

denotes a sub-SD called name. Thus everySD has a symbolic name also. The keywordref stand for reference.

Then for the above example, we might have:

<name>ref

5

BA A B

“ready_to_send”

“ready_to_receive”

“finished”

“shutdown”

SD Initialize SD Shutdown

Notice these are “handshakes” between A and B.

We will specify the SD “Exchange” later.6

Note that a sub-SD does not synchronizetimelines.

i.e. either of A or B is free to leave sub-SDExchange without the other leavingsimultaneously.

Here’s another example to clarify the point.

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BA A B

“hello”

SD Unsynchronised_Ref SD Sub-1

“goodbye”

Sub-1

“what?”

ref

8

Possible executions are either:1. hello2. what3. goodbye (A leaves Sub-1 late)or1. hello2. goodbye (A leaves Sub-1 early)3. what?

9

User System

“username”

“password?”

SD normal_log_in

“my_password”

Precondition: Power is on, operating system is

active, log-in menu is visible.

Postcondition: Power is on, operating system isactive, user is logged in under own profile, user’s

desktop is visible, log-in menu is not visible.

10

We introduce SD interaction operators

• alt : alternative choice of sections• par : parallel execution of several sections• loop : iterative execution of a section• opt : optional section that could be omitted• (exc : exception section to handle errors.)

11

Interaction Operator alt

An operator (possibly with a Boolean guard)used to define two or more alternatives, at most one of which will be taken.

Below, users u1 and u2 compete for Printer p.

Either u2 wins (top) or u1 wins (bottom)12

u1 : User u2 : User p : Printer

SD Alternatives

“print_1”

“print_2”

alt “accept_2”

“accept_1”

13

The only possible executions or traces for SDAlternatives are either :1. print_12. print_23. accept_2or1. print_12. print_23. accept_1

14

Interaction Operator par

An operator used to define two or more sections, all of which will be executed simultaneously

Compare par with alt!

Below, u1 and u2 both request aprint job in parallel and both are accepted.

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u1:User u2:User p:Printer

SD Parallel

par“accept_1”

“print_1”

“print_2”“accept_2”

16

This time there are 6 possible executions . These represent all possible interleavings of the two subsections of par.

1. print_1 1. print_1 1. print_2 1.print_12. print_2 2. accept_1 2. accept_2 2.print_23. accept_1 3. print_2 3. print_1 3.accept_24. accept_2 4. accept_2 4. accept_1 4.accept_1

1. print_2 1. print_22. print_1 2. print_13. accept_1 3. accept_24. accept_2 4. accept_1

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Interaction Operator loop

An operator (possibly with a Boolean guard, noguard = [true]) used to define a section thatmay be iterated finitely or infinitely many times.

Guard evaluated on each iteration. As well asBoolean guards we can bound the number of iterations.

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Keywords :(1) loop <m, n>, loop at least m times and at most n

times, for fixed integer constants m, n.(2) loop <m, inf>, loop finitely often, but at least m

times. ( *not* infinitely often).(3) loop <inf, inf> loop at least infinitely

many times.(4) loop <n> = loop <n, n> .(5) loop = loop <1, inf> .(6) [ while <Boolean expression> ]

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Important note: The parameters m, n are fixed constants, they are not variables which can be changed.

In the following example, the user polls aprinter until the printer becomes ready. When it becomes ready the printer prints the file.

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Loop <0, inf>

alt “ready?”

“busy”

“ready?”

“ready?”

“yes”

“print(file)”

“printing”

u:User p:Printer

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Interaction Operator Opt

An expression, possibly with a Boolean guard, [ <Boolean expression> ]

(no guard is the same as [true]) used to define an optional section which may or may not be executed (non -deterministic).

In the next example, A sends to B and may or may not get confirmation in time t< maxdelay before the nextsend.

22

opt “received”

“ok”

“send”

“send”

a:A b:B

[ t < maxdelay ]

23

Other Interaction Operators

neg – traces which are defined to be impossible

region – a critical region, i.e. traces cannot be interleaved by other events.

assert – all traces that involve the assertion being false are impossible (??)

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3. Introduction to Live Sequence Charts LSC

The Play-Engine is an MS Windows-based tool for Live Sequence Charts:1. managing a requirements project of LSCs,2. graphically editing correct LSCs, (play-in),3. simulating a set of LSCs (play-out),4. analysing LSCs (model checking).

Menu bar

Tool bar

ProjectExplorerTree

Work area

An LSC Application GUI

Basic structure

Use cases

Requirements

structure

behaviour

Code

Systemmodel

LSCs (or MSCs or sequence diagrams)

object model diagrams +statecharts

Java, C++, etcTraditional system development

Basic structure

Use cases

Requirements

structure

behaviour

Code

Systemmodel

LSCs (or MSCs or sequence diagrams)

object model diagrams +statecharts

Java, C++, etcPlay-in/out in theDevelopmentLifecycle

played behaviour

play-outplay-in