Foundations for the Study of Software Architecture - Department of ...

Why?
Foundations for the Study of
Software Architecture
? Referenced by the main books and papers
? Good history of the software engineering
context
? Relationships to other fields
Dewayne E. Perry and Alexander L. Wolf
ACM SIGSOFT Software Engineering Notes vol. 17 no 4
October 1992 Pag.. 40-52
Presented by Letizia Jaccheri
Structure of the Paper
Abstract
? Abstract
? Introduction
? Intuition, Context, and Motivation
? Model of Software Architecture
? Examples
? Benefits
? Conclusions
? Architecture
{
? Elements: components and connectors
? Form: properties and relationships among the
elements (constraints on the elements)
? Rationale: system constraints, which most
derive from requirements
}
1

Introduction (History)
Introduction (Benefits)
? 1970s Software Design
? 1980s CASE tools, process, formal
specifications
? 1990 architecture
? 2000 COTS, components, etc.
? Framework for satisfying requirements
? Technical basis for design and as the
managerial basis for cost estimation and
process management
? Basis for reuse
? Dependency and consistency analysis
Intuition
? Computing Hardware Architecture
? Network Architecture
? Building Architecture
Computing Hardware Architecture
? Both generic and
specific
? RISC
? Pipeline
? Small number of design
elements
? Scale is achieved by
replication of elements
? Multi-processors
? Similarities and
differences with
software architectures
? Types and instances
2

Network architecture
Civil engineering
? Star networks
? Ring network
? Manhattan street
? Similarities and differences with software
architectures
? Distributed network
? Message passing
? Multiple views
? To understand different
aspects
? Architectural styles
? Form for codification that
can be used descriptively
or prescriptively
? Style and engineering
? Style and materials
? Principles and material
properties
Italian Romanic, marble
Italian Gothic, Milan
3

Trondheim and Torino
Royal Garden, Trondheim
The context
Requirements
? Requirements
? Architecture
? Design
? Implementation
Are concerned with the determination of the
information, processing, and the
characteristics of that information and
processing needed by the user of the system
Note that the notion of requirements presented
here is an idealistic one…
? Computer science engineering
4

Architecture
Design
Is concerned with the selection of architectural
elements, their interactions, and the
constraints on those elements and their
interactions necessary to provide a
framework in which to satisfy the
requirements and serve as a basis for the
design
Is concerned with the modularization and the
detailed interfaces of the design elements,
their algorithms and procedures, and the data
types needed to support the architecture and
to satisfy the requirements
Implementation
…
Is concerned with the representations of the
algorithms and the data types that satisfy the
design, architecture, and requirements.
? The different parts of a particular product are
by no means as simple as this
characterization. There is a continuum of
possible choices of models, abstractions,
transformations, and representations. We
simplify this continuum into four discrete parts
primarily to provide an intuition about how
architecture is related to the requirements
and design of a software system
5

Motivations
From motivations to goals
? Evolution
? What: Systems evolve and are adapted to new users
? Problem: Systems are resistant to changes
? Causes: erosion (violation) and drift (insensibility)
? Customization
? What: building similar systems
? Problem: when building new systems, recreate design
elements for each system
? Causes: there are no standard templates for architectural
styles
? Prescribe the architectural constraints to the desired
level
? …determine the desired level of generality or particularity,
define what is necessity and what is luxury, …
? Separate aesthetics from engineering
? Load bearing versus decoration
? Express different aspects of the architecture in an
appropriate manner
? Perform dependency and consistency analysis
Model of Software Architecture
? Software architecture = {Elements, Form,
Rationale}
Elements
? Processing
? Data
? Connecting
6

Form
Rationale
? Weighted Properties
? are used to constrain the choice of architectural
elements
? Relationships
? are used to constrain the placement of architectural
elements
? to indicate alternatives
? Motivations for choices
? In software architecture, the rationale
explicates the satisfaction of the system
constraints. These constraints are
determined by considerations ranging from
basic functional aspects such as economics,
performance, and reliability.
Architectural style
Process/data/connector
? Examples: distributed, multi process,
blackboard, etc.
? Encapsulates important decisions about
elements and relationships
? We observe that if a process view of an
architecture is provided, the resulting
emphasis is on the data flow though the
processing elements and on some aspects of
the connections between the processing
elements with respect to the data elements.
7

Examples
Compiler
? Compiler (Multi phase architectural style)
? Lexical analysis acts on characters in a
source text to produce tokens for syntactic
analysis. Syntactic analysis produces
phrases that are either definition phrases or
use phrases. Semantic analysis produces
correlated phrases. Optimization produces
hints for code generation.
? Processing elements {Lexer,parser,
semantor, optimizer, code generator}
? Data elements {characters, tokens, phrases,
correlated phrases, annotated phrases,
object code}
A1: sequential
A1: sequential (processing view)
? Connecting elements: procedure call and
parameters
Characters
Lexer
Tokens (NT)
Parser
Phrases (NT + AST)
Correlated phrases (NT + ASG)
Code
Generator
Optimizer
Correlated phrases (NT + AASG)
Semantor
Correlated phrases (NT + ASG)
8

A1: sequential (data view)
Benefits
lexer
Has-all-tokens
parser
? Analysis
? Reuse
Code generator
Has all optimized
annotations
optimizer
Code generator
Has all
correlated phrases
semantor
Has-all-phrases
Conclusions
? Definitions by others
? …we feel…
? Zachman
? Shaw (see our model problems)
A2: parallel processes connected by means
of a shared internal representation
9