1. 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, Chapter 11.
M. Shaw and D. Garlan, Software Architecture: Perspectives on an Emerging Discipline, Prentice Hall, 1996.
CS 4311

2. Learning Objectives
To understand the concept of software architectures along with its motivation
To be able to characterize some important software architecture styles
CS 4311

3. Outline What’s software architecture?
Elements of software architecture
Architectural styles
CS 4311
CS 4311

4. Architecture in Civil Engineering
Design at the highest level
Example: Bridge architectures
Arch
Beam
Suspension
Cable-stayed
CS 4311

5. Architecture in Civil Engineering
Why do civil engineers talk about bridge architectures?
Design at the highest level
Example: Bridge architectures
Arch
Beam
Suspension
Cable-stayed
CS 4311

6. 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, archbishop
Matriarch, 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

7. What Is Software Architecture?
Framework, mechanism, way, shape, form, means, organization, arrangement
The Big Picture!
CS 4311

8. 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.
CS 4311

9. Outline What’s software architecture?
Elements of software architecture
Architectural styles
CS 4311
CS 4311 9

10. Elements of Software Architecture
Components
Connectors
Configuration topologies
System models
CS 4311

11. 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.
CS 4311

12. Connectors
Elements that are tasked with effecting and regulating interactions among components.
Application independent.
Often represented as lines in diagrams.
component
connector
CS 4311

13. Configuration Topology
A set of specific associations between the components and connectors of a software system’s architecture.
CS 4311

14. 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?
CS 4311

15. Outline What’s software architecture?
Elements of software architecture
Architectural styles
CS 4311
CS 4311 15

16. 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.
CS 4311

17. 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.
CS 4311

18. Example: Client-Server
CS 4311

19. 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.
CS 4311

20. Client-Server Architecture Style
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?
CS 4311

21. 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.
CS 4311

22. Style: Three-Tier or N-Tier
Application context:
Client-Server architecture style
Multi-level services are needed
Q: Examples?
CS 4311

23. Style: Layered Application context: Q: Examples?
Ordered layered services are necessary
Insulate upper layer from lower layer
Q: Examples?
CS 4311

24. 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
CS 4311

25. Style: Repository Application context:
Collaborate and communicate through a shared repository
A.k.a. shared state, blackboard, whiteboard
Q: Examples?
CS 4311

26. Repository (Cont.) Components: ? Connectors: ? Data elements: ?
Topology: ?
Qualities yielded: ?
Cautions: ?
CS 4311

27. 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?
CS 4311

28. 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
CS 4311

29. 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
CS 4311

30. 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 …
CS 4311

31. 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)
CS 4311