1. Software Architecture

2. Software Architecture
Describes overall system organization and structure in terms of its major constituents and their interactions
Promotes independence of modules
Decoupled modules can be developed and tested independently
Enhances ability to change
Poorly architected software is difficult to change
Reduces cost/time to implement
Helps formalize, organize, prioritize decisions

3. Architecture Defined
If architecture is strategic design, then the rest of the design is the tactical design.
Architecture establishes the context for further design and implementation
architecture
design
implementation
CODE
Architecture focuses on significant design decisions that have a lasting impact on the performance, reliability, cost, and resilience of the system

4. Architecture Defined IEEE Software architecture encompasses
Emphasize that the rationale for the architectural decisions is very important.
IEEE
Software architecture is the fundamental organization of a system, including its components, their relationships to each other and the environment, and the principles governing its design and evolution
Software architecture encompasses
Selection of the structural elements and interfaces by which a system is composed
Decomposition of subsystems into constituent structural and behavioral elements
Behavior as specified in collaborations among those elements
Architectural style that guides this organization

5. Example 1

6. Example 2

7. Architectures Follow Patterns
Common solution to a common problem
Codifies specific knowledge collected from experience in a domain

8. Examples of Architectural Patterns
Repository
Data-flow
Centralized Control
Client-Server
Event Driven
Layered Systems
Object Oriented
Domain Specific

9. Repository Sub-systems exchange data
A shared central database or repository can be accessed by all sub-systems
Each sub-system maintains its own database elements and passes data to those for use by other sub-systems
When large amounts of data are to be shared, the repository model of sharing is most commonly used

10. Repository Advantages
Efficiency: no need for inter-subsystem communication for data transfer
Transparency: sub-systems need not be concerned with how data is produced. Centralized management
Standard: sharing model is published as the repository schema
Disadvantages
Sub-systems must agree on a repository data model (compromise)
Data evolution is difficult/expensive
Difficult to distribute efficiently

11. Data-flow
Functional transformations process their inputs to produce outputs
May be referred to as pipe and filter model (UNIX shell)
When transformations are sequential, this is a batch sequential model which is extensively used in data processing systems
Not directly suitable for interactive systems

12. Centralized Control
A control sub-system takes responsibility for managing the execution of other sub-systems
Call-return model
Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems
Manager model
Applicable to concurrent systems. One component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement

13. Model-View-Controller
Variant of centralized control architecture;
Controller processes user commands which manipulate data in the model, and
calls on various views.
Controller (interact with
user, perform commands
View (display
model for user)
Model (store data,
e.g. text)

14. Client-server
Distributed system model that shows how data and processing is distributed across a range of components
Set of stand-alone servers that provide specific services such as printing, data management, etc.
Set of clients that call on these services
Network that allows clients to access servers

15. Client-server Advantages Distribution of data is straightforward
Makes effective use of networked systems. May require cheaper hardware
Easy to add new servers or upgrade existing servers
Disadvantages
No shared data model so sub-systems use different data organization - data interchange may be inefficient
Redundant management in each server
No central register of names and services - it may be hard to find out what servers and services are available

16. Event-driven
Driven by externally generated events where the timing of the event is outside of the control of the sub-systems which process the event
Two main event-driven models
Broadcast models. An event is broadcast to all sub-systems. Any sub-system which can handle the event may do so
Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing

17. Layered Systems Model the interfacing of sub-systems
Organises the system into a set of layers (or abstract machines) each of which provide a set of services
Supports incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affected
However, often difficult to structure systems in this way V e r s i o n m a n a g e m e n t O b j e c t m a n a g e m e n t D a t a b a s e s y s t e m O p e r a t i n g s y s t e m

18. Object Oriented
Structure the system into a set of loosely coupled objects with well-defined interfaces
Object-oriented decomposition is concerned with identifying object classes, their attributes and operations
When implemented, objects are created from these classes and some control model used to coordinate object operations

19. Domain-Specific: e.g., Compiler
Compiler model is a well-known example although other models exist in more specialized application domains
Lexical analyser
Symbol table
Syntax analyser
Syntax tree
Semantic analyser
Code generator
Generic compiler model may be organized according to different architectural models

20. Exercises
What architectural style would you use, and why, for the following:
Software controlling an elevator
Software automating a fantasy baseball league
Software automating a library’s book and patron information and tasks such as check-in/check-out
A company’s payroll software

21. Examples of Architectural Patterns
Repository
Data-flow
Centralized Control
Client-Server
Event Driven
Layered Systems
Object Oriented
Domain Specific

22. Example 3