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Software Architecture
Frank Tsui, Orland Karam, and Barbara Bernal, Essential of Software Engineering, 3rd edition, Jones & Bartett Learning. Section 7.2.Hans Van Vliet, Software Engineering, Principles and Practice, 3rd edition, John Wiley & Sons, 2008. Chapter 11.M. Shaw and D. Garlan, Software Architecture: Perspectives on an Emerging Discipline, Prentice Hall, 1996.
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Learning Objectives
To understand the concept of software architectures along with its motivation
To be able to characterize some important software architecture styles
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Outline
What’s software architecture? Elements of software architecture Architectural styles
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Architecture in Civil Engineering
Arch Beam Suspension Cable-stayed
Design at the highest level Example: Bridge architectures
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Architecture in Civil Engineering
Arch Beam Suspension Cable-stayed
Design at the highest level Example: Bridge architectures
Why do civil engineers talk about bridge architectures?
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What Is Architecture?
“Arch” used as prefix or suffix Meaning: Main, chief, principal, most important, highest,
fundamental, basic, extreme or most characteristic, ruler, leader
Examples:Archangel, archenemy, archduke, archbishopMatriarch, patriarch, monarch
Definition in Civil Engineering The art and science of designing and erecting buildings Fundamental underlying design of buildings Style and method of design and construction of buildings
Buildings and other structures CS 4311
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What Is Software Architecture?
Framework, mechanism, way, shape, form, means, organization, arrangement
The Big Picture!
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Software Architecture
The structure or structures of a software system software components externally visible properties of those components the relationships between them
The set of principle design decisions made about a software system System structure Functional behavior Interaction Nonfunctional properties Implementation System goals, stakeholders, non-technical considerations, etc.
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Outline
What’s software architecture? Elements of software architecture Architectural styles
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Elements of Software Architecture
Components Connectors Configuration topologies System models
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Software Components
Elements that encapsulate processing and data in a system’s architecture
An architectural entity that encapsulates a subset of the system’s functionality and/or data, restricts access to that subset via an explicitly defined interface,
and had explicitly defined dependencies on its required execution
context.
Often represented as boxes in diagrams.
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Connectors
Elements that are tasked with effecting and regulating interactions among components.
Application independent. Often represented as lines in diagrams.
component
connector
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Configuration Topology
• A set of specific associations between the components and connectors of a software system’s architecture.
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System Models
An artifact that captures some or all of the design decisions that comprise a system’s architecture.
Architectural modeling is the reification and documentation of those design decisions.
Different models: structural vs. behavioral static vs. dynamic Scope
Q: Key constraints of layered model?
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Outline
What’s software architecture? Elements of software architecture Architectural styles
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Architectural Styles
A named collection of architectural design decisions that: are applicable in a given development context, constrain architectural design decisions that are
specific to a particular system with that context, and elicit beneficial qualities in each resulting system.
Not about: specific systems, components, interaction mechanisms, or configurations.
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An Architectural Style Example
Physically separate the software components used to request services from the components that provide the services, to allow for proper distribution and scaling up, both in the number of service providers and service requesters.
Make the service providers unaware of the requesters’ identity to allow the providers to service transparently many, possibly changing requestors.
Insulate the requesters from one another to allow for their independent addition, removal, and modification. Make the requesters dependent only on the service providers.
Allow for multiple service providers to emerge dynamically to off-load the existing providers should the demand for services increase above a given threshold.
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More on Architectural Styles
Advantages Each style emphasizes one or more of the quality attributes,
such as usability, availability, security, performance, maintainability, flexibility, portability, etc.
Remarks: Each style has its own advantages and drawbacks. An application may require several architectural views. Choosing the right views is a key factor in having a good
design.
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The Repertoire of Architecture Styles
Client-Server Architecture Style
Application context: Clients request services from a server Centralization of computation and data at the server Separation of user interface tasks from computation and
data management and storage
Q: Any examples?
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Style: Client-Server
Components: Clients and server Connectors: Remote procedure call, network protocols Data elements: Parameters and return values are sent by the
connectors Topology: Two-level, with multi-clients Qualities yielded: Centralization of computation and data at the
server, with the information made available to remote clients. A single powerful server can service many clients.
Cautions: Network bandwidth.
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Style: Three-Tier or N-Tier
Application context: Client-Server architecture style Multi-level services are needed
Q: Examples?
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Style: Layered
Application context: Ordered layered services are necessary Insulate upper layer from lower layer
Q: Examples?
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Layered (Cont.)
Components: Layers, offering a set of services to other layers, typically comprising several programs or subprograms
Connectors: Procedure call Data elements: Parameters and return values are sent by the
connectors Topology: Linear, for strict virtual machine; a directed acyclic graph
in looser interpretations Qualities yielded: Clear dependence structure; Software at upper
levels immune to changes of implementation within lower levels as long as the service specification are invariant; Software at lower levels fully independent of upper levels.
Cautions: Efficiency
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Style: Repository
Application context: Collaborate and communicate through a shared
repository A.k.a. shared state, blackboard, whiteboard Q: Examples?
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Repository (Cont.)
Components: ? Connectors: ? Data elements: ? Topology: ? Qualities yielded: ? Cautions: ?
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Style: Batch Sequential
Application context: A series of dedicated and independent components
that transforms data in a sequential manner. Each component needs to finish its task before the
next one can start. Q: Examples?
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Batch Sequential (Cont.)
Components: Independent programs responsible for a particular task or transformation
Connectors: Procedure call Data elements: Intermediate data produced or transformed Topology: Linear or ordered Qualities yielded: Solve complex task by dividing it into
several easier tasks; obtain modifiability/maintainability, reusability, and flexibility
Cautions: Error handling; efficiency
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Style: Pipes and Filters
Application context: A series of independent, sequential transformations
on ordered data Consists of filters transforming data and pipes
transferring data Q: Examples?
% ps | grep bash | sort | more
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More Architectural Styles
Client server Three-tier Layered Repository Batch sequential Pipe and filter
Event driven Interpreter Peer-to-peer Plug-in Service-oriented Publish-subscribe Rule-based Mobile code …
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In-Class and Assignment:
Study two architectural styles per team Prepare a 10-minute presentation for the next class You will present:
What is it? What problem does it solve? How does it solve the problem? Who are the actors (i.e., main elements)? How do they relate to each other? What’re the strengths and drawbacks? Example applications? Include your sources (e.g., websites)
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