1. Architecture Representation

2. Outline Goals of Architecture Representation
Foundations of Software Architecture Representation
Architecture Description Languages
Design language elements
First-class connectors
Modules and Components
Applying ADLs
Summary

3. Outline (Cont’d) Modeling the problem and the solution domains Views
Problem domain models
Solution domain models
Views
Objectives and purpose models
Behavioral/functional models
Information/data models
Models of form
Nonfunctional/performance models
Summary

4. A Model for Representing a Software Architecture
Goals of architecture representation – a representation of a system that can be used to:
Design a system
Analyze a design
Generate a system
Foundations of software architecture representation – the elements for describing an architecture are:
Components
Connectors
Architectural constraints

5. A Model for Representing a Software Architecture (Cont’d)
Architectural description languages – ADLs are (machine readable) design languages used to describe a system that possess design language properties like:
Composition
Abstraction
Reusability
Configuration
Heterogeneity
Analysis

6. Foundations of Software Architecture
Three types of constructs are necessary in representing software architectures:
Elements
Form
Rationale
Their elements are classified as:
Data
Processing
Connecting

7. Foundations of Software Architecture (Cont’d)
The architectural form is composed of weighted (by importance or necessity) properties and relationships.
Properties may be used to define constraints.
Relationships may be used to show how different architectural elements interact.
Rationales capture the motivation behind various decisions; they are inferences that can be structured as an argument with the design decision being the conclusion.

8. Fundamental Software Design Views (Cont’d)
Architectural styles can be sequential or parallel
In the sequential style the connecting elements are procedure calls and parameters
In the parallel style the connecting element is a shared representation of the data (a repositiory)

9. Architecture Description Languages
Programming language structures are inadequate for describing architectural elements.
Furthermore, they provide no way to model a strict separation of concerns between architectural-level design issues and detail design issues.
Languages in general serve the purpose of describing complex relationships among primitive elements and element combinations.

10. Architecture Description Languages (Cont’d)
Having identified semantic constructs it makes sense to define a language around them.
Common architectural description elements include:
(Pure) computation (processing elements) – simple input/output relations with no retained state
Memory (data elements) – shared collections of persistent structured data
Manager – manage state and closely related operations
Controller – governs the sequence of operations
Link – transmit information between elements

11. Common Component Interactions
Procedure call
Data flow
Implicit invocation
Message passing
Shared data
Instantiation

12. Design Language Elements -- Albin
Components – the primitive semantic elements
Operators – functions that combine components
Abstraction rules – allow for the definition of named expressions of components and operators
Closure rules – determines which abstractions can be added to the classes of primitive components and operators
Specification – associates semantics to syntactic forms

13. Design Language Elements – Shaw and Garlan
Components – the modules that compose the architectural level of design
Operators – the inter-component connection mechanisms
Patterns – design templates that solve a particular set of problems (a framework)
Closure – defines the conditions under which a particular assembly of components and operators may also be used as an atomic component
Specification – associates semantics such as functionality and other quality attributes to the syntactic forms

14. Six Classes of Properties that Should Characterize an ADL
Composition
Abstraction
Reusability
Configuration
Heterogeneity
Architectural Analysis

15. Composition
An ADL should allow for the description of a system as a composition of components and connectors.
It must support the ability to split a system or module into two modules
It must support the ability to synthesize or combine modules to create new forms.
These splitting or synthesis operations should be independent of implementation design decisions (choice of algorithms, data structures, connecting technology, etc.)
A composition of elements must be allowed to be viewed as a single component.
It must be possible to operate on the individual components of the composition.

16. Abstraction
An ADL should allow a designer to focus on high-level concerns without having to think in terms of programming level constructs.
Architectural abstractions are patterns of programming language constructs.
An architect can think in terms of components and connectors focusing on the architectural concerns of modifiability, reliability, performance, etc. without having to map them to specific programming language elements.

17. Reusability
It should be possible to modularize a specification written in a particular ADL so that the components can be used in other systems.
These specifications are reusable patterns of components.
For example, a certain client-server pattern might identify a database server component with a particular database table structure (a reusable data model).
Each physical database implementation might look different but each would have the same core data and enforce the same semantic rules.

18. Configuration
It should be possible with an ADL to describe a composite structure separately from its elements so that the composition can be reasoned about as an atomic element.
It should support the dynamic reconfiguration of a system in terms of restructuring compositions without knowing their internal structure.
For example, in a client/server system, an indefinite number of clients may be executing at any time.

19. Heterogeneity
This is the ability to mix architectural styles within a single architectural specification.
At one level, the architecture may exhibit a particular pattern of compositions, but the structure of each composition may follow a different pattern.
An ADL should allow different compositions to be compiled to different languages.

20. Architecture Analysis
An ADL should support the ability to analyze an architecture.
Analysis of architecture includes both automated and non automated reasoning about quality attributes of a system.
ADL research aims to automate this analysis by providing machine-readable specifications of specific quality attribute requirements and then checking to see if the architecture specification conforms to it.

21. First-Class Connectors
Connectors should be considered equal to components.
Connectors often embody the nonfunctional quality attributes and bring form to the architecture.

22. Modules and Components
“A module is a unit whose structural elements are powerfully connected among themselves and relatively weakly connected to elements of other units. Clearly there degrees of connection; thus gradations of modularity.” – Baldwin
Programming languages are insufficient for representing architectural level designs because they lack explicit connecting elements.
Connecting elements are just as important from the architectural level of design as processing and data elements.
The ability to abstract the implementation of a connecting element within an architecture specification is what makes an ADL so powerful.

23. An ADL Example – C2 SADL
The C2 Software Architecture Description Language is intended for designing a flexible, extensible component- and message- based system that has a graphical user interface.
A C2-based system is structured as a hierarchy of concurrent components that communicate via connectors, which are message-routing mechanisms.
A component can have at most two connectors – a top connector and a bottom connector, which attach to components either up one level in the hierarchy or down one level.
The top connector of one component is coupled with the bottom connectors of other components, and vice versa.

24. An ADL Example – C2 SADL (Cont’d)
There is no limit to the number of connectors that ma be coupled with a single connector.
A C2 component is only aware of the components above it, not those below.
Requests go up and notifications go down.
There is one component at the top.
The C2 architectural style relies on a programming language neutral event mechanism, such as a message queue.

25. An ADL Example – C2 SADL (Cont’d)
C2 SADL is composed of three parts:
Interface Definition Notation (IDN)
Supports textual specification of C2 component interfaces
Consists of specifications for parameters, methods, behavior, and context
Architecture Description Notation (ADN)
Supports textual specification of a C2 architecture
Architecture Construction Notation (ACN)
Supports textural specification of architecture changes

26. Applying ADL’s ADLs are not commonly used. Most are experimental.
Understanding the problems that ADLs attempt to solve, can help an architect to think about the solution to a particular problem.
Understanding ADLs helps the architect to focus on the nature of connections between components.

27. Summary
The component level of design allows us to reason about the properties of the system without first constructing the entire system.
The fundamental language of specifying architecture design is composed of elements, form, and rationale.
Elements are processing, data, and connecting
Form is composed of weighted properties and relationships.
Rationales capture the reasons why certain design decisions were made.

28. Summary (Cont’d)
Architecture Description Languages (ADLs) are high-level languages for describing the component view of a software system.
ADLs have not been applied much in practice.
UML is often used as an ADL even though it is inadequate.
The dependency (design) structure matrix is still one of the most powerful representation tools.