Architectural Design Establishing the overall structure of a software system Objectives To introduce architectural design and to discuss its importance To explain why multiple models are required to document a software architecture To describe types of architectural model that may be used

What is Architecture? A high-level model of a thing Describes critical aspects of the thing Understandable to many stakeholders Allows evaluation of the thing’s properties before it is built Provides well understood tools and techniques for constructing the thing from its blueprint Which aspects of a software system are architecturally relevant? How should they be represented most effectively to enable stakeholders to understand, reason, and communicate about a system before it is built? What tools and techniques are useful for implementing an architecture in a manner that preserves its properties?

What is Software Architecture? A software system’s blueprint Its components Their interactions Their interconnections Informal descriptions Boxes and lines Informal prose A shared, semantically rich vocabulary Remote procedure calls (RPCs) Client-Server Pipe and Filer Layered Distributed Object-Oriented

From Requirements to Architecture From problem definition to requirements specification Determine exactly what the customer and user want Specifies what the software product is to do From requirements specification to architecture Decompose software into modules with interfaces Specify high-level behavior, interactions, and non-functional properties Consider key tradeoffs Schedule vs. Budget Cost vs. Robustness Fault Tolerance vs. Size Security vs. Speed Maintain a record of design decisions and traceability Specifies how the software product is to do its tasks

Another View: The Twin Peaks Model

Focus of Software Architectures Two primary foci System Structure, Behavior, Interactions, Topology, Data, and key Properties Correspondence between requirements and implementation A framework for understanding system-level concerns Global rates of flow Communication patterns Execution Control Structure Scalability Paths of System Evolution Capacity Throughput Consistency Component Compatibility

Why Software Architecture? A key to reducing development costs Component-based development philosophy Explicit system structure A natural evolution of design abstractions Structure and interaction details overshadow the choice of algorithms and data structures in large/complex systems Benefits of explicit architectures A framework for satisfying requirements Technical basis for design Managerial basis for cost estimation & process management Effective basis for reuse Basis for consistency, dependency, and tradeoff analysis Avoidance of architectural erosion

What is the Problem? This is a simple software system!

The Usual Tool: Design Abstraction We have to do better!

Architectural Abstraction Components Connectors Events

What is Software Architecture? Definition: A software system’s architecture is the set of principal design decisions about the system Software architecture is the blueprint for a software system’s construction and evolution Design decisions encompass every facet of the system under development Structure Behavior Interaction Non-functional properties

Other Definitions of Software Architecture Perry and Wolf Software Architecture = { Elements, Form, Rationale } what how why Shaw and Garlan Software architecture [is a level of design that] involves the description of elements from which systems are built, interactions among those elements, patterns that guide their composition, and constraints on these patterns. Kruchten Software architecture deals with the design and implementation of the high-level structure of software. Architecture deals with abstraction, decomposition, composition, style, and aesthetics.

Architectural design process System structuring The system is decomposed into several principal sub-systems Communications between these sub-systems are identified Control modelling A model of the control relationships between the different parts of the system is established Modular decomposition The identified sub-systems are decomposed into modules

Key Architectural Concepts Three canonical building blocks components connectors configurations A sub-system is a system in its own right whose operation is independent of the services provided by other sub-systems A module is a system component that provides services to other components but would not normally be considered as a separate system

Components A component is a unit of computation or a data store Components are loci of computation and state clients servers databases filters layers ADTs A component may be simple or composite composite components describe a (sub)system an architecture consisting of composite components describes a system of systems

Components Elements that encapsulate processing and data in a system’s architecture are referred to as software components Definition A software component is 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 has explicitly defined dependencies on its required execution context Components typically provide application-specific services

Software components In most engineering disciplines, systems are designed by composing existing components that have been used in other systems Software engineering has been more focused on original development It is now recognized that to achieve better software more quickly at lower cost we need to adopt a design process that is based on systematic reuse

Components Components provide a service without regard to where the component is executing or its programming language A component is an independent executable entity that can be made up of one or more executable objects The component interface is published and all interactions are through the published interface Components can range in size from simple functions to entire application systems

A software component: Implements some functionality Has explicit dependencies through provided and required interfaces Communicates through its interfaces only Has structure and behavior that conforms to a component model

LEGO analogy Set of building blocks in different shapes and colors Can be combined in different ways Composition through small stubs in one and corresponding holes in another building block  LEGO blocks are generic and easily composable LEGO can be combined with LEGO, not with e.g. MEGA Bloks

Component abstractions Functional abstraction The component implements a single function such as a mathematical function Casual groupings The component is a collection of loosely related entities that might be data declarations, functions, etc. Data abstractions The component represents a data abstraction or class in an OO language Cluster abstractions The component is a group of related classes that work together System abstraction The component is an entire self-contained system

Why CBSE? CBSE increases quality, especially evolvability and maintainability CBSE increases productivity CBSE shortens development time

Hiding of component internals Black box: only specification is known Glass box: internals may be inspected, but not changed Grey box: part of the internals may be inspected, limited modification is allowed While box: component is open to inspection and modification

Development process in CBSE Two separate development processes: Development of components Development of systems out of components Separate process to assess components

CBSE system development process Requirements: also considers availability of components (as in COTS) Analysis and design: very similar to what we normally do Implementation: less coding, focus on selection of components, provision of glue code Integration: largely automated Testing: verification of components is necessary Release: as in classical approaches Maintenance: replace components

Component assessment Find components Verify components Store components in repository

Component development process Components are intended for reuse  Managing requirements is more difficult More effort required to develop reusable components More effort in documentation for consumers

Component development process Requirements: combination of top-down (from system) and bottom-up (generality) Analysis and design: generality is an issue, assumptions about system (use) must be made Implementation: largely determined by component technology Testing: extensive (no assumptions of usage!), and well-documented Release: not only executables, also metadata

Software Connector Architectural element that models Interactions among components Rules that govern those interactions Simple interactions Procedure calls Shared variable access Complex & semantically rich interactions Client-server protocols Database access protocols Asynchronous event multicast Each connector provides Interaction duct(s) Transfer of control and/or data

Where are Connectors in Software Systems? Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Benefits of First-Class Connectors Separate computation from interaction Minimize component interdependencies Support software evolution At component-, connector-, & system-level Potential for supporting dynamism Facilitate heterogeneity Become points of distribution Aid system analysis & testing

An Example of Explicit Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

An Example of Explicit Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Software Connector Roles Communication Coordination Conversion Facilitation

Connector Types Procedure call Data access Event Stream Linkage Distributor Arbitrator Adaptor

A Framework for Classifying Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Procedure Call Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Event Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Data Access Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Linkage Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Stream Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Arbitrator Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Adaptor Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Distributor Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

How Does One Select a Connector? Determine a system’s interconnection and interaction needs Software interconnection models can help Determine roles to be fulfilled by the system’s connectors Communication, coordination, conversion, facilitation For each connector Determine its appropriate type(s) Determine its dimensions of interest Select appropriate values for each dimension For multi-type, i.e., composite connectors Determine the atomic connector compatibilities

Configurations/Topologies An architectural configuration or topology is a connected graph of components and connectors that describes architectural structure proper connectivity concurrent and distributed properties adherence to design heuristics and style rules Composite components are configurations

Scope of Software Architectures Every system has an architecture. Details of the architecture are a reflection of system requirements and trade-offs made to satisfy them Possible decision factors Performance Compatibility with legacy software Planning for reuse Distribution profile Current and Future Safety, Security, Fault tolerance requirements Evolvability Needs Changes to processing algorithms Changes to data representation Modifications to the structure/functionality

Example Architecture – Compiler Sequential Parallel

CASE toolset architecture

Version management system

Packing robot control system

Film and picture library

Architecture in Action: Product Line Motivating example A consumer is interested in a 35-inch HDTV with a built-in DVD player for the North American market. Such a device might contain upwards of a million lines of embedded software. This particular television/DVD player will be very similar to a 35-inch HDTV without the DVD player, and also to a 35-inch HDTV with a built-in DVD player for the European market, where the TV must be able to handle PAL or SECAM encoded broadcasts, rather than North America’s NTSC format. These closely related televisions will similarly each have a million or more lines of code embedded within them.

Growing Sophistication of Consumer Devices

Families of Related Products

The Necessity and Benefit of PLs Building each of these TVs from scratch would likely put Philips out of business Reusing structure, behaviors, and component implementations is increasingly important to successful business practice It simplifies the software development task It reduces the development time and cost it improves the overall system reliability Recognizing and exploiting commonality and variability across products

Reuse as the Big Win Architecture: reuse of Product families: reuse of Ideas Knowledge Patterns engineering guidance Well-worn experience Product families: reuse of Structure Behaviors Implementations Test suites…

Added Benefit – Product Populations

The Centerpiece – Architecture

Analogies to Software Architecture Hardware architecture small number of design elements scale by replication of (canonical) design elements Network architecture focus on topology only a few topologies considered e.g., star, ring, grid Building architecture multiple views styles

Architectural models Different architectural models may be produced during the design process Each model presents different perspectives on the architecture Static structural model that shows the major system components Dynamic process model that shows the process structure of the system Interface model that defines sub-system interfaces Deployment model shows the relationship between system elements and hosts

An Example Static Model Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

An example Deployment Model Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Key points The software architect is responsible for deriving a structural system model, a control model and a sub-system decomposition model Large systems rarely conform to a single architectural model Key architectural concepts are components, connectors, and configurations

Bibliography The material of this class come from the Software Engineering class of Ian Sommerville, Nenad Medvidovic, and Han Van Vliet.