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Software re-engineering

Reorganising and modifying existing software systems to make them more maintainable

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Objectives

To explain why software re-engineering is a cost-effective option for system evolution
To 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

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Topics covered

Source code translation
Reverse engineering
Program structure improvement
Program modularisation
Data re-engineering

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System re-engineering

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

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When to re-engineer

When system changes are mostly confined to part of the system then re-engineer that part
When hardware or software support becomes obsolete
When tools to support re-structuring are available

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Re-engineering advantages

Reduced 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

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Business process re-engineering

Concerned with re-designing business processes to make them more responsive and more efficient
Often reliant on the introduction of new computer systems to support the revised processes
May force software re-engineering as the legacy systems are designed to support existing processes

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Forward engineering and re-engineering

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The re-engineering process

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Re-engineering cost factors

The quality of the software to be re-engineered
The tool support available for re-engineering
The extent of the data conversion which is required
The availability of expert staff for re-engineering

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Re-engineering approaches

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Source code translation

Involves converting the code from one language (or language version) to another e.g. FORTRAN to C
May be necessary because of:

Hardware platform update
Staff skill shortages
Organisational policy changes

Only realistic if an automatic translator is available

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The program translation process

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Reverse engineering

Analysing software with a view to understanding its design and specification
May be part of a re-engineering process but may also be used to re-specify a system for re-implementation
Builds a program data base and generates information from this
Program understanding tools (browsers, cross-reference generators, etc.) may be used in this process

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The reverse engineering process

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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

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Program structure improvement

Maintenance tends to corrupt the structure of a program. It becomes harder and harder to understand
The program may be automatically restructured to remove unconditional branches
Conditions may be simplified to make them more readable

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Spaghetti logic

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Structured control logic

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Condition simplification

-- Complex condition

if not (A > B and (C < D or not ( E > F) ) )...

-- Simplified condition

if (A <= B and (C>= D or E > F)...

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Automatic program restructuring

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Restructuring problems

Problems 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 code
The understandability of data-driven programs may not be improved by re-structuring

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Program modularisation

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

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Module types

Data abstractions

Abstract data types where datastructures and associated operations are grouped

Hardware modules

All functions required to interface with a hardware unit

Functional modules

Modules containing functions that carry out closely related tasks

Process support modules

Modules where the functions support a business process or process fragment

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Recovering data abstractions

Many legacy systems use shared tables and global data to save memory space
Causes problems because changes have a wide impact in the system
Shared global data may be converted to objects or ADTs

Analyse common data areas to identify logical abstractions
Create an ADT or object for these abstractions
Use a browser to find all data references and replace with reference to the data abstraction

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Data abstraction recovery

Analyse common data areas to identify logical abstractions
Create an abstract data type or object class for each of these abstractions
Provide functions to access and update each field of the data abstraction
Use a program browser to find calls to these data abstractions and replace these with the new defined functions

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Data re-engineering

Involves analysing and reorganising the data structures (and sometimes the data values) in a program
May be part of the process of migrating from a file-based system to a DBMS-based system or changing from one DBMS to another
Objective is to create a managed data environment

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Approaches to data re-engineering

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Data problems

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

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Data migration

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Data problems

Data 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 literals
No data dictionary

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Data value inconsistencies

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Data conversion

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

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The data re-engineering process

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Key points

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

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Key points

Reverse engineering is the process of deriving the system design and specification from its source code
Program structure improvement replaces unstructured control constructs with while loops and simple conditionals
Program modularisation involves reorganisation to group related items
Data re-engineering may be necessary because of inconsistent data management