Chapter 12
Analysis Modelling
Analysis Modelling :
Where to Begin?
o the FAST working document provides a “statement
of scope” for the software
o the working document must be “parsed” to extract
data and function domain information
1
Building the Analysis Model
o
odata
data model
model
ï
ï data
dataobjects
objects
ï
ïrelationships
relationships
ï
ï ERDs
ERDs
o
ofunctional
functional model
model
ï
ï data
datatransforms
transforms
ï
ïDFDs
DFDs
o
obehavioural
behavioural model
model
ï
ïevents
eventsand
andstates
states
ï
STDs
ï STDs
The Structure of the Model
Data
Dictionary
State-Transition
Diagram
2
The Data Model
o A data object is a representation of almost any
composite information that must be understood by
software
o Attributes define the properties of a data object and
take on one of three different characteristics
o Relationships connect data objects to one another in a
variety of different ways
The ERD : Relationships
orders
displays
Book
stocks
sells
returns
Bookstore
Relationships between objects
3
The ERD : Cardinality & Modality
Cardinality and modality
The Functional Model : The DFD
data/control flow notation
process
external
entity
data
item
event flow,
control item
data/control store
4
Data Flow Hierarchy
Flow Modelling Notes
o all arrows, with the possible exception of arrows to/from
stores should be labelled
o information or control continuity (sometimes called
“balancing”) is maintained between levels of the flow
model
o a suggested “expansion ratio” between one level and the
next is 1:5
o if a bubble does a number of different things, it needs
further refinement
o the flow model itself is not enough, other notation must
be developed for a complete description of requirements
5
Getting Started
Example
Manufacturing cell software controls a robot by
generation of position coordinates that are transmitted
to the robot. An operator inputs commands that cause
the manufacturing cell software to read positioning
and control commands from an NC command file.
Components to be assembled are held in parts fixtures
that activate robot control functions once each fixture
contains a part ...
Getting Started (Cont’d)
Using nouns to isolate external entities, data items and stores
Manufacturing cell software controls a robot by generation
of position coordinates that are transmitted to the robot. An
operator inputs commands that cause the manufacturing
cell software to read positioning and control commands
from an NC command file. Components to be assembled
are held in parts fixtures that activate robot control
functions once each fixture contains a part ...
Use verbs to help isolate processes (bubbles)
6
Creating a Context Diagram
Level 0 Flow Model (also called a “context diagram”
DFD Questions
Q : How does the NC unit software transform input to
output?
A : Lower DFD levels will provide details.
Q : What are “operator commands”?
A : Another notational tool- the data dictionary- will help
to describe.
Q : Where are processing details?
A : A PSPEC (“structured English”) and other notational
tools are used.
7
Refining to Level 1
Refining to Model More Detail
8
Flow Modelling : Q & A
Q : How far do we take the flow model ... What should a
bubble represent?
A : For now, we’ll say that each bubble should correspond
to an independent software module. Later we’ll define
more explicit terms.
Q : How do we know what goes on inside a bubble?
A : Lower level flow models provide part of the answer, a
processing narrative provides the rest.
Flow Modelling : Q & A (Cont’d)
Q : What does a value on an arrow really mean?
A : The data dictionary provides a detailed description of
a composite data item.
Q : Aren’t there a number of different ways to draw the
flow model?
A : Yes, just as there are different ways to write an
English language description or code the program.
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Process & Control Models
Process Model
Data Conditions
10
The Control Model
o the control flow diagram is “superimposed” on the
DFD and shows events that control the processes
noted in the DFD
o control flows- events and control items- are noted by
dashed arrows
o a vertical bar implies an input to or output from a
control spec (CSPEC)- a separate specification that
describes how control is handled
The Control Model (Cont’d)
o a dashed arrow entering a vertical bar is an input to
the CSPEC
o a dashed arrow leaving a process implies a data
condition
o a dashed arrow entering a process implies a control
input read directly by the process
o control flows do not physically activate/deactivate the
processes- this is done via the CSPEC
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Level 1 DFD for Photocopier
Level 1 CFD for photocopier
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Control Flow Diagram
Control Specification (CSPEC)
13
State-transition for photocopier
Guidelines for Building a CSPEC
o list all sensors that are “read” by the software
o list all interrupt conditions
o list all “switches” that are actuated by the operator
o list all data conditions
o recalling the noun-verb parse that was applied to the
software statement of scope, review all “control
items” as possible CSPEC inputs/outputs
o describe the behaviour of a system by identifying its
states; identify how each state is reach and defines the
transitions between states
o focus on possible omissions ... A very common error
in specifying control, e.g., ask : “Is there any other
way I can get to this state or exit from it?”
14
Process Specification (PSPEC)
Processing Narrative
15
Using PDL
The Data Dictionary
o
o aa quasi-formal
quasi-formal grammar
grammar for
for describing
describing the
the content
content
of
of data
data that
that the
the software
software will
will process
process and
and create
create
o
o aa notation
notation for
for describing
describing control
control data
data and
and the
the values
values
that
that control
control data
data can
can take,
take, e.g.,
e.g., “on”
“on” or
or “off”
“off”
o
o aa repository
repository that
that also
also contains
contains “where-used”
“where-used”//“how
“how
used”
information
used” information
o
o aa notation
notation that
that can
can be
be represented
represented manually,
manually, but
but isis
best
bestdeveloped
developedusing
usingCASE
CASEtools
tools
16
Building a Data Dictionary
Name :
Aliases:
Where used:
How used:
Description:
Format:
the primary name of the composite data
item
other names for the data item
data transforms (processes) that use the
composite data item
the role of the data item (input, output,
temporary storage, etc.
a notation for representing content
(presented on next slide)
specific information about data types,
pre-set values (if known)
Data Dictionary Notation
Notation
Meaning
=
is composed of
+
and
[|]
either-or
{ }n
n repetitions of
( ... )
optional data
* ... text ... *
delimits a comment
17
Data Dictionary Example
Writing the Software Specification
18
Specification Guidelines
o use a layered format that provides increasing detail as
the “layers” deepen
o use consistent graphical notation and apply textual
terms consistently (stay away from aliases)
o be sure to define all acronyms
o be sure to include a table of contents; ideally, include
an index and/or a glossary
o write in a simple, unambiguous style (see “editing
suggestions” on the following pages)
o always put yourself in the reader’s position, “Would I
able to understand this if I wasn’t intimately familiar
with the system?”
Specification Guidelines
Be on the lookout for persuasive connectors, ask why?
Keys : certainly, therefore, clearly, obviously, it follows
that ...
Watch out for vague terms
keys : some, sometimes, often, usually, ordinarily, most,
mostly ...
When lists are given, but not completed, be sure all items
are understood
keys : etc., and so forth, and so on, such as
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Specification Guidelines (Cont’d)
Be sure stated ranges don’t contain unstated assumptions
e.g., Valid codes range from 10 to 100. Integer? Real?
Hex?
Beware of vague verbs such as handled, rejected,
processed, ...
Beware “passive voice” statements
e.g., The parameters are initialized. By what?
Beware “dangling” pronouns
e.g., The I/O module communicated with the data
validation module and its control flag is set. Whose
control flag?
Specification Guidelines
When a term is explicitly defined in one place, try
substituting the definition for other occurrences of the term
When a structure is described with a picture, try to redraw
the picture to emphasize different elements of the structure
When symbolic equations are used, try expressing their
meaning in words
When a calculation is specified, work at least two examples
Look for statements that imply certainty, then ask for proof
keys; always, every, all, none, never
Search behind certainty statements- be sure restrictions or
limitations are realistic
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Example: SafeHome
Establishing Connection
21
Developing relationship
Level 0 DFD for SafeHome
22
Level 1 DFD
Level 2 DFD – monitor sensor
23
Level 1 CFD for SafeHome
State-Transition Diagram
24
Process activation table
25