Architecture Representation

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

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

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

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

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

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.

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)

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.

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

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

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

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

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.