2.6.3. Structuring SDs

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2.6.3. Structuring SDs

• Normally a use case scenario is too long and
• complex to fit on a single (A4?) SD.
• We need to hierarchically structure SDs and
• decompose them into
• sub-SDs
• called by
• SD references.

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• A reference is a pointer to another SD.
• References may not be circular or recursive.

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• Let us give a typical example. A simple data
• exchange between two systems can be
• divided into 3 phases
• set-up communication/initialize
• exchange data
• close-down communication
• We can specify the use case successful data
• exchange as follows.

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SD Successful_ Data_Exchange
B
A
ref
Initialize
ref
Exchange
ref
Shutdown
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• The symbol
• denotes a sub-SD called name. Thus every
• SD has a symbolic name also. The keyword
• ref stand for reference.
• Then for the above example, we might have

ref
ltnamegt
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SD Shutdown

SD Initialize
B
A
A
B
ready_to_send
finished
shutdown
ready_to_receive
Notice these are handshakes between A and
B. We will specify the SD Exchange later.
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• Note that a sub-SD does not synchronize
• timelines.
• i.e. either of A or B is free to leave sub-SD
• Exchange without the other leaving
• simultaneously.
• Heres another example to clarify the point.

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SD Sub-1
SD Unsynchronised_Ref
B
A
A
B
ref
hello
Sub-1
goodbye
what?
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• Possible executions are either
• hello
• what
• goodbye (A leaves Sub-1 late)
• or
• hello
• goodbye (A leaves Sub-1 early)
• what?

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SD normal_log_in
User
System
Precondition Power is on, operating system
is active, log-in menu is visible.
username
password?
my_password
Postcondition Power is on, operating system
is active, user is logged in under own profile,
users desktop is visible, log-in menu is not
visible.
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• 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.)

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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)

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SD Alternatives
u1 User
u2 User
p Printer
print_1
print_2
alt
accept_2
accept_1
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• The only possible executions or traces for SD
• Alternatives are either
• print_1
• print_2
• accept_2
• or
• print_1
• print_2
• accept_1

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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 a
• print job in parallel and both are accepted.

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SD Parallel
u1User
u2User
pPrinter
par
print_1
accept_1
print_2
accept_2
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• 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_1
• 2. print_2 2. accept_1 2. accept_2 2.print_2
• 3. accept_1 3. print_2 3. print_1 3.accept_2
• 4. accept_2 4. accept_2 4. accept_1 4.accept_1
• 1. print_2 1. print_2
• 2. print_1 2. print_1
• 3. accept_1 3. accept_2
• 4. accept_2 4. accept_1

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Interaction Operator loop

• An operator (possibly with a Boolean guard, no
• guard true) used to define a section that
• may be iterated finitely or infinitely many
• times.
• Guard evaluated on each iteration. As well as
• Boolean guards we can bound the number of
• iterations.

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• Keywords
• loop ltm, ngt, loop at least m times and at most n
  times, for fixed integer constants m, n.
• loop ltm, infgt, loop finitely often, but at least
  m times. ( not infinitely often).
• loop ltinf, infgt loop at least infinitely
• many times.
• (4) loop ltngt loop ltn, ngt .
• (5) loop loop lt1, infgt .
• (6) while ltBoolean expressiongt

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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 a
• printer until the printer becomes ready.
• When it becomes ready the printer prints
• the file.

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uUser
pPrinter
Loop lt0, infgt
alt
ready?
busy
ready?
ready?
yes
print(file)
printing
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Interaction Operator Opt

• An expression, possibly with a Boolean guard,
• ltBoolean expressiongt
• (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 tlt maxdelay before the
  next
• send.

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aA
bB
send
opt
received
ok
t lt maxdelay
send
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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
• managing a requirements project of LSCs,
• graphically editing correct LSCs, (play-in),
• simulating a set of LSCs (play-out),
• analysing LSCs (model checking).

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Menu bar
Tool bar
Project Explorer Tree
Work area
An LSC
Application GUI
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Traditional system development
Java, C, etc
Code
Basic structure
System model
structure
Use cases
behaviour
Requirements
LSCs (or MSCs or sequence diagrams)
object model diagrams statecharts
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Play-in/out in the Development Lifecycle
Java, C, etc
Code
Basic structure
Use cases
System model
structure
played behaviour
behaviour
play-in
play-out
Requirements
LSCs (or MSCs or sequence diagrams)
object model diagrams statecharts
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