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An Introduction to Software Architecture Pejman Salehi. Table of content. Introduction Architecture Business Cycle Architectural Structures and Views Quality Attributes Achieving Qualities Attribute Driven Design. Introduction. Definition.

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An introduction to software architecture pejman salehi l.jpg

An Introduction to Software ArchitecturePejman Salehi


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Table of content

  • Introduction

  • Architecture Business Cycle

  • Architectural Structures and Views

  • Quality Attributes

  • Achieving Qualities

  • Attribute Driven Design



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Definition

The software architecture of a program or computing system is the structure or structures of the system, which comprise software elements, the externally visible properties of those elements, and the relationships among them.


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Externally Visible vs. Internal Properties

Externally visible properties are what assumption other elements can make of an element

  • Provided services (and interface to access those services)

  • Performance

  • Fault handling

  • Shared resource usage

    SA intentionally abstracts away internal properties of elements (to better encounter complexity)


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Some Points

  • Every software system has an architecture (SA ≠ specification of SA)

  • Specification of architecture can comprise more than one structure

  • Behavior of elements and relationships are defined in SA (abstractly)

  • The definition do not talk about Good and Bad architectures: SA evaluation methods

  • In the literature:

    • Component = Element

    • Connector = Relationship


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Why is architecture important?

  • Handling complexity

  • Communication among stakeholders

    • Requirements and concerns of stakeholders

    • Time

    • Budget

    • Other Resources


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Why is architecture important? (cont)

  • Early Design Decisions

    • Constraints implementation and implementers

    • Organizational structure

    • Enables predicting and ensuring quality attributes

    • Makes it possible to reason about and manage change

    • Helps evolutionary prototyping (risk reduction)


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Why is architecture important? (cont)

  • SA is a transferable, reusable model

    • Software product lines

    • Component-based development

    • Automatic generation of lower-level models


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Hazards

  • With regards to SA changes are categorized to

    • Local (a single component)

    • Non-local (a few components)

    • Architectural (architectural style)

  • Once decided architecture is extremely hard to change

  • It impossible to reach to some quality attribute if architecture disallows


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Software Arch. vs. System Arch.

  • System Arch. is the overall architecture of system including hardware and software architecture

  • In assuring quality attributes the architect needs to think about system architecture too (e.g. performance or reliability)

  • But architect has more freedom in software architecture than hardware (hardware choices is less under the architects control)







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Architectural Structures and Views

In construction, there are blueprints of

  • Plan

  • Different sides of construction

  • Electrical wiring

  • Plumbing

    Each of these views specifies a single entity (i.e. the construction) from a different perspective (used by a different person, for a different goal).

    Similarly there are different structures and views in SA.


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Structures and Views (cont)

  • Structures is a set of coherent elements and the relations among them. For each structure these we can specify:

    • Types of elements

    • Types of relations

    • A set of syntactic constraints

    • Semantics of the diagram

    • Rationale, principles, and guidelines

    • For what purposes it is useful

  • View is a representation of software architecture based on an structure as written by the architect and read by stakeholders (an instance of the structure)

  • SA is documented by a number of views.


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Categorization of Structures

  • Module Structures

  • Component and Connector Structures

  • Allocation Structures


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1 Module Structures

  • Elements: modules (units of implementation). Modules are a code-based way of considering the system

  • Specifies:

    • Functional responsibility of modules

    • Other elements a module is allowed to use

    • Generalization and specialization relations

  • Run-time operation of software is not a concern from this view


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1.1 Decomposition Structure

  • Elements: modules in a hierarchy

  • Relations: is a sub-module of, shares secret with

  • Function Example:

    • Contributes to system's modifiability, by ensuring that likely changes fall within the scope of at most a few small modules.


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1.2 Uses Structure

  • Elements: modules, procedures, or resources on the interfaces of modules

  • Relations: uses: one unit uses another if the correctness of the first requires the presence of a correct version.

  • Function Example:

    • Allows incremental development


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1.3 Layered Structure

  • Is a subclass of uses structure

  • Elements: layers: a coherent set of related functionality

  • Relations: uses (ideally layer n may only use the services of layer n – 1), provides abstraction to

  • Function Example:

    • Layers are often designed as abstractions (virtual machines) that hide implementation specifics below from the layers above, engendering portability.


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1.4 Class Structure

  • Elements: classes /objects

  • Relations: inherits from, is an instance of

  • Function Example:

    • Allows us to reason about reuse and the incremental addition of functionality


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2 Component and Connector Structures

  • Elements: run-time components (principal units of computation) and connectors (communication vehicle among components.)

  • Specifies:

    • Major executing components and how they interact

    • Major shared data-stores

    • Which part of system is replicated

    • Flow of data through the system

    • What parts can run in parallel

    • How can system structure change as it executes


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2.1 Process Structure

  • Elements: processes or threads

  • Relations: attachment (that allow communication, synchronization, and/or exclusion operations)

  • Function Example:

    • Engineering a system's execution performance and availability.


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2.2 Shared Data or Repository Structure

  • Elements: data stores, data producers, and data consumers

  • Relations: data-flow

  • Function Example:

    • To ensure good performance and data integrity.


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2.3 Client-Server Structure

  • Elements: clients and servers

  • Relations: protocols and message passing infrastructure.

  • Function Example:

    • Separation of concerns (supporting modifiability)

    • Load balancing (supporting runtime performance)


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2.4 Concurrency Structure

  • Elements: Component

  • Relations: Logical Thread.

  • Function Example:

    • managing the issues associated with concurrent execution.

      • resource contention


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3 Allocation Structures

  • Show the relationship between the software and the elements in one or more external environment in which software is created and executed.

  • Specifies:

    • The processor that executes each software element

    • The file that stores each software element during development

    • Assignments of software to development team


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3.1 Deployment Structure

  • Shows how software is assigned to hardware

  • Elements: software (usually a process from a component and connector view), hardware entities, and communication pathways

  • Relations: is-allocated-to and migrates-to (for dynamic allocations)

  • Function Example:

    • Allows reasoning about performance, data integrity, availability, and security.


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3.2 Implementation Structure

  • Shows how software elements (usually modules) are mapped to the file structure(s) in the system's development, integration, or configuration management environments.

  • Elements: any logical unit (e.g. module)

  • Relations: implemented in

  • Function Example:

    • management of development activities and build process


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3.3 Work Assignment Structure

  • Assigns responsibility for implementing and integrating the modules to the appropriate development teams

  • Elements: any logical unit (e.g. module), team structure

  • Relations: is assigned to

  • Function Example:

    • The architect will know the expertise required on each team

    • The means for factoring functional commonalities and assigning them to a single team, rather than having them implemented by everyone who needs them.


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Notes

  • Each structure is useful on its own right but not all structures are used in all projects.

  • Structures are not independent and must be considered together

    • e.g. relationship of modules with components (many to many)

    • Some structures may be the same in some systems

    • Some structures may be combined (e.g. all component and connector structures may be combined in a single structure)




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Traditional Classification of Requirements

  • Requirements

    • Functional

    • Non-Functional (Quality Attributes)

      A popular software myth: first we build a software that satisfies functional requirements, then we will add or inject non-functional requirements to it.

  • This idea leads to loss of resources and finally poor quality.

  • So we should design for qualities from the very beginning (architecture level).


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Functionality and Architecture

  • Functionality and quality attrs are orthogonal [in theory]. But not all qualities are achievable to any level desired with any functionality.

  • Functionality may be achieved in many ways (it is not so architectural.)

  • Architecture is a means of achieving quality attributes by structuring functionality into elements.


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Architecture and Qualities

  • Achieving qualities must be considered throughout design (including SA), implementation, and deployment.

  • Qualities have both architectural and non-architectural aspects. For example

    • In usability: selecting form elements vs. supporting undo operation

    • Performance: amount of communication among components vs. algorithms


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Architecture an Qualities

  • Quality attributes are not independent and may be achieved in isolation.

  • Positive Correlation; e.g.

    • Modifiability and Buildability (in many cases)

  • Negative Correlation (conflict); e.g.

    • Reliability vs. Security

    • Performance vs. All Other Qualities


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Classification of Quality Attributes

  • Qualities of the system: availability, modifiability, performance, security, testability, and usability.

  • Business qualities (such as time to market) that are affected by the architecture.

  • Architecture qualities, such as conceptual integrity.


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System Quality Attributes

  • Availability (related to Reliability)

  • Modifiability (includes Protability and Reusability, Scalability)

  • Performance

  • Security

  • Testability

  • Usability (includes Self-Adaptability and User-Adaptability)


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Business Qualities

  • Time to market

  • Cost and benefit

  • Predicted lifetime of the system

  • Targeted market

  • Rollout schedule

  • Integration with legacy systems


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Qualities of the Architecture

  • Conceptual Integrity

    • Conceptual integrity is the most important consideration in system design. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas. [Brooks 75]

  • Correctness and Completeness

  • Buildability


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System Quality Attributes

  • System quality attributes have been of interest to the software community at least since the 1970s

  • Shortcoming of the previous work:

    • The definitions for an attribute are not operational.

      • Modifiability with regards to which aspect?

    • Which quality a particular aspect belongs to.

      • Is a system failure an aspect of availability, an aspect of security, or an aspect of usability?

    • Each attribute community has developed its own vocabulary.

      • Performance community events, security community attacks, availability community failures, and usability community user input may actually refer to the same occurrence.


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Quality Attribute Scenario

  • Solution:

    • Quality Attribute Scenarios as a means of characterizing quality attributes

  • consists of six parts:

    • Source of stimulus.

    • Stimulus.

    • Environment.

    • Artifact.

    • Response.

    • Response measure.


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Quality Attribute Scenario (cont.)

  • General Scenario


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Quality Attribute Scenario (cont.)

  • Sample Scenario



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The Whole Story

Business

Requirements

Quality

Requirements

Tactics Selection

Tactics Implementation:

Architectural Patterns


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Tactics

  • A tactic is a design decision that influences a quality attribute.

  • e.g. using redundancy to increase availability

  • Tactics can be refined to other tactics to become more concrete; e.g. redundancy: redundancy of data + process




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Patterns

  • A pattern is a common abstract solution to a common abstract problem that

    • Can be tailored to a given situation

    • Has predefined characteristics

  • Abstraction level of patterns

    • Business

    • Analysis

    • Architecture

    • Design

    • Implementation (Idioms)

    • Test Patterns (or guideline to testing patterns)


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Relationship of Tactics to Patterns

  • An architect usually chooses a pattern or a collection of patterns designed to realize one or more tactics.

  • However, each pattern implements multiple tactics, whether desired or not.


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Famous Pattern (Style) categories

Data-centered

  • Repository

  • Blackboard (publisher-subscriber)

  • Structural solution to integrability of data

  • Scalability

  • Modifiability

  • Client

    Client

    Shared Data

    Client

    Client


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    Famous Pattern (Style) categories

    Dataflow

    • Bach sequential

    • Pipes and filters

  • Reusability

  • Modifiability

  • Not interactive

  • Poor performance


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    Famous Pattern (Style) categories

    Virtual Machine

    • Interpreter (e.g. Adaptive Object Model)

  • Portability

  • Simulation

  • Adaptability

  • Low performance


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    Famous Pattern (Style) categories

    Call and Return

    • Main program and sub-routine

      • Modifiability

    • Remote procedure call

      • Performance tuning

    • Object-oriented or abstract data type

      • Modifiability

      • Reuse

    • Layered

      • Modifiability

      • Portability


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    Famous Pattern (Style) categories

    Independent components

    • Communicating Processes

      • Scalability

    • Event Systems

  • Modifiability


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    Example: ATM Software

    Develop 3 different architectures for ATM software and compare them regarding fulfillment of quality attributes.

    ATM = Automatic Teller Machine

    User operations:

    • Insert card and enter PIN

    • Withdraw money

    • Check Balance






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    Attribute-Driven Design

    • bases the decomposition process on the quality attributes.

    • quality requirements should be expressed as a set of system-specific quality scenarios.

    • Steps

      • Choose the module to decompose

      • Refinement

        • Choose the architectural drivers

        • Choose an architectural tactic and pattern

        • Instantiate modules and allocate functionality

        • Define interfaces of the child modules

        • Verify and refine use cases and quality scenarios

      • Repeat the steps above for every module


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