lec-3-functional-design

8/7/2019 lec-3-functional-design

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Andrew IrelandSoftware Design F28SD2

Outline

Motivations

Data flow

Structuring

Detailed design


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Motivations

Many significant software systems fall into this

classification, e.g. transaction processing

Many legacy applications were developed usingfunction-oriented design


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Function-oriented Activities

Data flow design:

Model data flow through the system as a set of

data flow diagrams

Structural decomposition: Model the functionality of the system as a

hierarchy of sub-functions via the structure chart

notation

Detailed design: Provide detailed data and functional descriptions,

typically supported by the notion of a data

dictionary


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Example: Regression TestingTesting that code modifications have not regressed the

correctness of the system as a whole:

Also as a consequence of the introduction of new bugs,

program maintenance requires far more system testing

per statement written than any other programming.

Theoretically, after each fix one must run the entire batch

of test cases previously run against the system, to

ensure that it has not been damaged in an obscure way.

In practice, such regression testing must indeed

approximate this theoretical idea, and it is very costly."

Fred Brooks, The Mythical Man Month (p 122)


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DFD ExampleSource

code db

Test case

db

Expectedresults db

Run tests Compare

Gen Log

Gen

reports

Gen

reports

Gen

certifs

Validate

Regression

test db

and

or

s_file

t_file

e_file

a_resr_res

r_res

idid

l_file

r_file

c_file

report

certif

Data flow


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Exercise: Wanted PersonsConsider the following system requirements:

1. A user provides facial features based upon crime reports

2. If valid, then the feature list is used to retrieve a list of relevant

feature templates from a database. If the feature list is not valid

then an error message is sent to the user

3. The feature template list is used to search for matching offender

imagines which are stored within an offender records database

4. Any records for offenders already in prison are then pruned

5. From pruned list, a suspect list is dispatched to the user, while the

corresponding records are sent to a wanted persons database

Exercise: construct a DFD for the requirements above


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Structural Decomposition

For a given DFD there will exist choices as to

how it is implemented

The structure chart notation provides a means of

adding more structure to the design Each function (or procedure) is represented as a

rectangle

A high-level function is represented as a

hierarchy of sub-functions

Links between sub-functions are labelled with

input/output data, i.e. parameters or shareddata


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Structural Decomposition

A

y

A

y

x x

A

x y

A

x x

input output transform Co-ordinate


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Structural Decomposition Note that a structure chart shows the relationship

between sub-functions, but not the order in

which they are invoked

Typically a series of structure charts aredeveloped for a given DFD incrementally

expanding level by level, e.g.

A

B C

A

B C

D E F

x

x

x

z

v

u

r

s

t

z


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Structural DecompositionA

B C D

E F G H

K

I J

input outputprocess


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Structural DecompositionA

B C D

E F G H

K

I J

decision

making

data

processing


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Structure Chart: FirstRegression

tester

Initialize ReportTest

Source

code

Test

cases

Expected

results

Source

code id

Regression

results: regression log

defect report certificate

id

id

res

res


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Structure Chart: SecondRegression

tester


Sourcecode

Testcases

Expectedresults

Regression

log

id

idid

Validate Run tests Compare

id

res

Logger Generator

ida_res

r_res

r_res

r_res r_res

Defect

report

Certificate

r_res


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Structure Chart: ThirdRegression

tester


Sourcecode db

Testcase db

Regression

test db

id

idid

Validate Run tests Compare

id

res

Logger Generator

Gencert

Genreport

ida_res

r_res

r_res

r_res r_res

r_file c_file

Expectedresults db

report cert

l_files_file t_file e_file

r_res


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Detailed Design

Detailed design involves getting inside the

functional blocks represented with the structure

chart notation

At this level, Process Description Languages(PDL), i.e. pseudo code

A useful intermediate representation is a Data

Dictionary (DD) a structured, but informal,

recordofdata items and functional

DDs are useful to designers, programmers as

well as during maintenance


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Example: Data DictionaryEntity Type Description

id string Name of the source code base under test

res boolean The result of validating the name of the source code

base providedby the user

r_res vector The regression test results, i.e. the actual results vs

expectedresults

a_res vector The actual results obtainedby executing the code

base against the regression tests

test procedure Input: name of source code baseFunction: co-ordinates the execution of the regression

tests and the comparison with the expected results

Output: results of regression testing


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Sequential vs Concurrent

Note that no explicit reference has been made as

to how a design is implemented, i.e. sequential vs

concurrent

DFDs in particular do not impose a control

paradigm

A DFD representation of a function could be

implemented as a: sequence of sub-function invocations or

set of communicating processes


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