CO7206 System Reengineering 4.2 Software Reengineering€¦ · Document generation System...

©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 28 Slide 1

CO7206System Reengineering

4.2 Software Reengineering

Most slides are Slides by Ian Sommerville


ObjectivesTo explain why software re-engineering is a cost-effective option for system evolutionTo describe the activities involved in the software re-engineering process To distinguish between software and data re-engineering and to explain the problems of data re-engineering


Software re-engineering

Reorganising and modifying existing software systems to make them more maintainable

"the examination of a subject system to reconstitute it in a new form and the subsequent implementation of the new form.

[ElliotChikofsky and JamesCross, Reverse Engineering and Design Recovery: A Taxonomy, IEEE Software 7(1):13-17, 1990.]


Topics coveredSource code translationReverse engineeringProgram structure improvementProgram modularisationData re-engineering


Re-structuring or re-writing part or all of a legacy system without changing its functionalityApplicable where some but not all sub-systems of a larger system require frequent maintenanceRe-engineering involves adding effort to make them easier to maintain. The system may be re-structured and re-documented

System re-engineering


When system changes are mostly confined to part of the system then re-engineer that partWhen hardware or software support becomes obsoleteWhen new ways of accessing are needed, but its functionality remains When tool support is are available

When to re-engineer


Re-engineering advantagesReduced risk• There is a high risk in new software development. There may be

development problems, staffing problems and specification problems

Reduced cost• The cost of re-engineering is often significantly less than the

costs of developing new software


Business process re-engineeringConcerned with re-designing business processes to make them more responsive and more efficientOften reliant on the introduction of new computer systems to support the revised processesMay force software re-engineering as the legacy systems are designed to support existing processes


Forward engineering and re-engineering

Understanding andtransformation

Existingsoftware system

Re-engineeredsystem

Design andimplementation

Systemspecification

Newsystem

Software re-engineering

Forward engineering


Forward engineering and re-engineering

“Forward engineering is the traditional process of moving from high-level abstractions and logical, implementation-independent designs to the physical implementation of a system." [ElliotChikofsky and JamesCross, Reverse Engineering and Design Recovery: A Taxonomy, IEEE Software 7(1):13-17, 1990.]


The re-engineering process

Reverseengineering

Programdocumentation

Datareengineering

Original data

Programstructure

improvement

Programmodularisation

Structuredprogram

Reengineereddata

ModularisedprogramOriginal

program

Source codetranslation


Re-engineering cost factorsThe quality of the software to be re-engineeredThe tool support available for re-engineeringThe extent of the data conversion which is requiredThe availability of expert staff for re-engineering


Re-engineering approaches

Automated restructuringwith manual changes

Automated sourcecode conversion

Restructuring plusarchitectural changes

Automated programrestructuring

Program and datarestructuring

Increased cost


Source code translationInvolves converting the code from one language (or language version) to another e.g. FORTRAN to CMay be necessary because of:• Hardware platform update• Staff skill shortages• Organisational policy changes

Only realistic if an automatic translator is available


The program translation process

Automaticallytranslate code

Design translatorinstructions

Identify sourcecode differences

Manuallytranslate code

System to bere-engineered


Re-engineeredsystem


Reverse engineeringAnalysing software with a view to understanding its design and specificationMay be part of a re-engineering process but may also be used to re-specify a system for re-implementationBuilds a program data base and generates information from thisProgram understanding tools (browsers, cross-reference generators, etc.) may be used in this process


Reverse engineering“Reverse engineering is the process of analyzing a subject system with two goals in mind:

(1) to identify the system's components and their interrelationships; and,

(2) to create representations of the system in another form or at a higher level of abstraction."



The reverse engineering process

Data stucturediagrams

Program stucturediagrams

Traceabilitymatrices

Documentgeneration

Systeminformation

store

Automatedanalysis

Manualannotation



Reverse engineering“Design recovery is a subset of reverse engineering in which domain knowledge, external information, and deduction or fuzzy reasoning are added to the observations of the subject system." The objective of design recovery is to identify meaningful higher-level abstractions beyond those obtained directly by examining the system itself. [ElliotChikofsky and JamesCross, Reverse Engineering and Design Recovery: A Taxonomy, IEEE Software 7(1):13-17, 1990.]


Reverse engineering

Reverse engineering often precedes re-engineering but is sometimes worthwhile in its own right• The design and specification of a system may

be reverse engineered so that they can be an input to the requirements specification process for the system’s replacement

• The design and specification may be reverse engineered to support program maintenance and reengineering


Program structure improvementMaintenance tends to corrupt the structure of a program. It becomes harder and harder to understandThe program may be automatically restructured to remove unconditional branchesConditions may be simplified to make them more readable


Program Restructuring“Restructuring is a transformation from one form of representation to another at the same relative level of abstraction." The new representation is meant to preserve the semantics and external behaviour of the original.



Spaghetti logicStart: Get (Time-on, Time-off, Time, Setting, Temp, Switch)

if Switch = off goto offif Switch = on goto ongoto Cntrld

off: if Heating-status = on goto Sw-offgoto loop

on: if Heating-status = off goto Sw-ongoto loop

Cntrld: if Time = Time-on goto onif Time = Time-off goto offif Time < Time-on goto Startif Time > Time-off goto Startif Temp > Setting then goto offif Temp < Setting then goto on

Sw-off: Heating-status := offgoto Switch

Sw-on: Heating-status := onSwitch: Switch-heatingloop: goto Start


Structured control logicloop

-- The Get statement finds values for the given variables from the system’s-- environment.

Get (Time-on, Time-off, Time, Setting, Temp, Switch) ;case Switch of

when On => if Heating-status = off thenSwitch-heating ; Heating-status := on ;

end if ;when Off => if Heating-status = on then

Switch-heating ; Heating-status := off ;end if;

when Controlled =>if Time >= Time-on and Time < = Time-off then

if Temp > Setting and Heating-status = on thenSwitch-heating; Heating-status = off;

elsif Temp < Setting and Heating-status = off thenSwitch-heating; Heating-status := on ;

end if;end if ;

end case ;end loop ;


Another Spaghetti logicSTART:

GOTO MAMMALSDOG:

WALK THE DOGGOTO CAT

MAMMALS:GOTO DOG

FISH:FEED THE FISHCOVER THE BIRDGOTO FROG

CAT:PUT OUT THE CATGOTO FISH-AND-FOWL

FISH-AND-FOWL:GOTO FISH

FROG:SING TO THE FROG

EXIT.


Another structured control logic

START:CALL FUNCTION DOGCALL FUNCTION CATCALL FUNCTION FISHCALL FUNCTION BIRDCALL FUNCTION FROGEXIT.

DOG:WALK THE DOGRETURN

CAT:PUT OUT THE CATRETURN

FISH:FEED THE FISHRETURN

BIRD:COVER BIRD CAGERETURN

FROG:SING TO THE FROGRETURN


Condition simplification

-- Complex conditionif not (A > B and (C < D or not ( E > F) ) )...

-- Simplified conditionif (A <= B and (C>= D or E > F)...


Automatic program restructuring

Graphrepresentation

Programgenerator

Restructuredprogram

Analyser andgraph builder

Program to berestructured


Restructuring problemsProblems with re-structuring are:• Loss of comments• Loss of documentation• Heavy computational demands

Restructuring doesn’t help with poor modularisation where related components are dispersed throughout the codeThe understandability of data-driven programs may not be improved by re-structuring


Program modularisationThe process of re-organising a program so that related program parts are collected together in a single moduleUsually a manual process that is carried out by program inspection and re-organisation


Data re-engineeringInvolves analysing and reorganising the data structures (and sometimes the data values) in a programMay be part of the process of migrating from a file-based system to a DBMS-based system or changing from one DBMS to anotherObjective is to create a managed data environment


Approaches to data re-engineeringApproach DescriptionData cleanup The data records and values are analysed to improve their quality.

Duplicates are removed, redundant information is deleted and a consistentformat applied to all records. This should not normally require anyassociated program changes.

Data extension In this case, the data and associated programs are re-engineered to removelimits on the data processing. This may require changes to programs toincrease field lengths, modify upper limits on the tables, etc. The data itselfmay then have to be rewritten and cleaned up to reflect the programchanges.

Data migration In this case, data is moved into the control of a modern databasemanagement system. The data may be stored in separate files or may bemanaged by an older type of DBMS.


Data problemsEnd-users want data on their desktop machines rather than in a file system. They need to be able to download this data from a DBMSSystems may have to process much more data than was originally intended by their designersRedundant data may be stored in different formats in different places in the system

Data migration

Databasemanagement

system

Logical andphysical

data models

describes

File 1 File 2 File 3 File 4 File 5 File 6

Program 2 Program 3

Program 4 Program 5 Program 6 Program 7

Program 1

Program 3 Program 4 Program 5 Program 6

Program 2 Program 7

Program 1

Becomes


Data problemsData naming problems• Names may be hard to understand. The same data may have

different names in different programs

Field length problems• The same item may be assigned different lengths in different

programs

Record organisation problems• Records representing the same entity may be organised

differently in different programs

Hard-coded literalsNo data dictionary


Data value inconsistenciesData inconsistency DescriptionInconsistent defaultvalues

Different programs assign different default values to the same logical dataitems. This causes problems for programs other than those that created thedata. The problem is compounded when missing values are assigned adefault value that is valid. The missing data cannot then be discovered.

Inconsistent units The same information is represented in different units in differentprograms. For example, in the US or the UK, weight data may berepresented in pounds in older programs but in kilograms in more recentsystems. A major problem of this type has arisen in Europe with theintroduction of a single European currency. Legacy systems have beenwritten to deal with national currency units and data has to be converted toeuros.

Inconsistent validationrules

Different programs apply different data validation rules. Data written byone program may be rejected by another. This is a particular problem forarchival data which may not have been updated in line with changes todata validation rules.

Inconsistentrepresentationsemantics

Programs assume some meaning in the way items are represented. Forexample, some programs may assume that upper-case text means anaddress. Programs may use different conventions and may therefore rejectdata which is semantically valid.

Inconsistent handlingof negative values

Some programs reject negative values for entities which must always bepositive. Others, however, may accept these as negative values or fail torecognise them as negative and convert them to a positive value.


Data conversionData re-engineering may involve changing the data structure organisation without changing the data valuesData value conversion is very expensive. Special-purpose programs have to be written to carry out the conversion


The data re-engineering process

Entity namemodification

Literalreplacement

Data definitionre-ordering

Datare-formatting

Default valueconversion

Validation rulemodification

Dataanalysis

Dataconversion

Dataanalysis

Modifieddata

Program to be re-engineered

Change summary tables

Stage 1 Stage 2 Stage 3


Key pointsThe objective of re-engineering is to improve the system structure to make it easier to understand and maintainThe re-engineering process involves source code translation, reverse engineering, program structure improvement, program modularisation and data re-engineeringSource code translation is the automatic conversion of of program in one language to another


Key pointsReverse engineering is the process of deriving the system design and specification from its source codeProgram structure improvement replaces unstructured control constructs with while loops and simple conditionalsProgram modularisation involves reorganisation to group related itemsData re-engineering may be necessary because of inconsistent data management

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