Ch 03 1

Software Engineering Principles

Ch 03 2

Outline• Principles form the basis of methods,

techniques, methodologies and tools

• Seven important principles that may be used in all phases of software development

• Modularity is the cornerstone principle supporting software design

• Case studies

Ch 03 3

Some definitions• Methods:

– General guidelines that govern the execution of some activity; methods are rigorous, systematic, and disciplined approaches.

• Technique:– Techniques and methods are used interchangeably.– Techniques are more technical and mechanical than

methods; often they also have more restricted applicability.

– Methods and techniques are packaged to form methodology.

• Methodology:– used to promote a certain approach to solving a problem

by pre-selecting the methods and techniques to be used.

• Tools:– Developed to support the application of techniques,

methods, and methodologies.

Ch 03 4

A visual representation

Principles

Methodologies

Principles

Methods and techniques

Methodologies

Tools

• Principles apply to process and product• Principles become practice through methods and techniques

Ch 03 5

Key principles

• Rigor and formality• Separation of concerns• Modularity• Abstraction• Anticipation of change• Generality• Incrementality

Ch 03 6

Rigor and formality• Software engineering is a creative

design activity, BUT

• It must be practiced systematically

• Rigor is a necessary complement to creativity that increases our confidence in our developments

• Formality is rigor at the highest degree– software process driven and evaluated by

mathematical laws

Ch 03 7

Examples: product

• Mathematical (formal) analysis of program correctness– By specifying the program using a formal

specification (Z, B-method, Temporal logic, ..) and check specific properties.

• Systematic (rigorous) test data derivation:– Test data that covers most significant cases.

Ch 03 8

Example: process

• Rigorous documentation of development steps helps project management and assessment of timeliness

Ch 03 9

Separation of concerns• Tackles with the complexity of large

systems, by separating different issues to concentrate on one at a time (Divide & Conquer)

• Supports parallelization of efforts and separation of responsibilities.

• Separation of concern at functional and non-functional activities:– Functional (views): function, data, network,

deployment, process, etc.

– Non-functional (qualities): correctness, efficiency, reliability, etc.

Ch 03 10

Example: process / product

• Process: go through phases one after the other (as in waterfall)

– Applies separation of concerns by separating activities with respect to time

• Product: keep product requirements separate– functionality– performance– user interface and usability

Ch 03 11

Modularity

• A complex system may be divided into simpler pieces called modules

• A system that is composed of modules is called modular

• Supports application of separation of concerns– when dealing with a module we can ignore

details of other modules

Ch 03 12

Cohesion and coupling

• Each module should be highly cohesive– module must be understandable as a meaningful

unit (i.e., it provides a single functionality)

– Elements (i.e., functions) of a module are closely related to one another, in terms of sharing similar attributes.

• Modules should exhibit low coupling– modules have low interactions with others– understandable separately– More maintainable, low change propagation

Ch 03 13

A visual representation

(a)(b)

high coupling low coupling

Ch 03 14

Abstraction• Identify the important aspects of a

phenomenon and ignore its details

• Special case of separation of concerns

• The type of abstraction to apply depends on purpose

• Example : the user interface of a watch (its buttons) abstracts from the watch's internals for the purpose of setting time; other abstractions needed to support repair

Ch 03 15

Abstraction ignores details

• Example: equations describing complex circuit (e.g., amplifier) allows designer to reason about signal amplification

• Equations may approximate description, ignoring details that yield negligible effects (e.g., connectors assumed to be ideal)

Ch 03 16

Abstraction yields models

• For example, when requirements are analyzed we produce a model of the proposed application

• The model can be a formal or semiformal description

• It is then possible to reason about the system by reasoning about the model

Ch 03 17

An example

• Programming language grammar described through an abstract syntax (e.g., statement, expression, term) ignores details of different programs that can be generated by the same grammar.

– abstraction ignores details such as precision of number representation or addressing mechanisms

Ch 03 18

Abstraction in process

• When we do cost estimation we only take some key factors into account

• We reuse the results of previous similar cases for the new cost estimation, and ignore the differences among similar cases.

Ch 03 19

Anticipation of change• A basis for software evolution that requires

anticipating potential future changes

• Examples: – Set up a configuration management environment for

the project to take care of different changes in future (vs. having a fix system configuration).

– Anticipate the expansion of a company by considering reserve fields for the attributes of different stakehoders in the company.

– In desinging MicroProcessors, several registered are reserved for future expansions (versions) of the processor.

Ch 03 20

Generality• While solving a problem, try to discover if it is

an instance of a more general problem whose solution can be reused in other cases

• Carefully balance generality against performance and cost

• Sometimes a general problem is easier to solve than a special case

• E.g., defining a general tree traversal algorithm that receives a tree and a method of traversal (pre-order, in-order, post-order) to traverse every tree. As opposed to define three different methods. Even more general, to desing an Iterator algorithm.

Ch 03 21

Incrementality• Process proceeds in a stepwise fashion

(increments)

• Examples (process)

– deliver subsets of a system early to get early feedback from expected users, then add new features incrementally

– deal first with functionality, then turn to performance and other (non-functional) qualities.

– deliver a first prototype and then incrementally add effort to turn prototype into product

Ch 03 22

Case study: compiler

• Compiler construction is an area where systematic (formal) design methods have been developed

– e.g., BNF for formal description of language syntax

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Ch 03 23

Separation of concerns: examples

• Functional qualities:– When designing different components of a

compiler, you concentrate on one at a time. For example, in designing lexical scanner to tokenize program’s text, you don’t consider the method of parsing (e.g., LR, LL, or LALR parser).

• Non-functional qualities:– When designing optimal register allocation

algorithms (runtime efficiency) no need to worry about runtime diagnostic messages (user friendliness)

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Ch 03 24

Modularity example

• Compilation process decomposed into phases– Lexical analysis

– Syntax analysis (parsing)

– Code generation

• Phases can be associated with modules

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Ch 03 25

Representation of modular structure

Boxes represent modules, andarrows represent flow of data

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Lexicalanalysis

Parsetree

Sourcecode

Symboltable

Objectcode

“Tokenized”code

Parsing Codegeneration

Lexicaldiagnostics

Syntax diagnostics

Languagegrammar

Ch 03 26

Module decomposition may be iterated

Intermediatecode generationParse

tree

Objectcode

Codegenration

Intermediatecode

Symbol table

Machine codegeneration

further modularization of code-generation module

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Ch 03 27

Abstraction

• Applied in many cases– abstract syntax to neglect syntactic

details such as begin…end vs. {…} to bracket statement sequences

– intermediate machine code (e.g., Java Bytecode) for code portability

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Ch 03 28

Anticipation of change

• Consider possible changes of:

– source language (due to standardization committees)

– target processor

– I/O devices

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Ch 03 29

Generality

• Parameterize with respect to target machine (by defining intermediate code)

• Develop compiler generating tools (compiler compilers) instead of just one compiler:– Given a language Grammar the tool

produces a language to parse

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Ch 03 30

Incrementality

• Incremental development– deliver first a kernel version for a

subset of the source language, then increasingly larger subsets

– deliver compiler with little or no diagnostics/optimizations, then add diagnostics/optimizations

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Ch 03 31

Case study (system engineering): elevator system

• In many cases, the "software engineering" phase starts after understanding and analyzing the "systems engineering” issues

• The elevator case study illustrates the point

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Ch 03 32

Rigor & formality (1)

• Quite relevant: it is a safety critical system

– Define requirements• must be able to carry up to 400 Kg. (safety alarm

and no operation if overloaded)

• emergency brakes must be able to stop elevator within 1 m. and 2 sec. in case of cable failures

– Later, verify their fulfillment

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Ch 03 33

Separation of concerns

• Try to separate– safety– performance– usability (e.g, button illumination)– Cost

• Although some are strongly related– cost reduction by using cheap material can

make solution unsafe

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Ch 03 34

A modular structure

Elevator

Control apparatus

B3

B2

B1

buttons at floor i

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Ch 03 35

Module decomposition may be iterated

Elevator Engine

BrakesCabin

Internal Buttons

Control apparatus

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Ch 03 36

Abstraction

• The modular view we provided does not specify the behavior of the mechanical and electrical components– they are abstracted awayC

ase

Stud

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leva

tor

Ch 03 37

Anticipation of change, generality

• Make the project parametric with respect to the number of elevators (and floor buttons)

Elevators

Controlapparatus

Floorbuttons

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