Software Architecture: Patterns and Styles

• Software Architecture consists of architectural patterns and styles
• Nacha Chondamrongkul is an Assistant Professor at the School of IT, Mah Fah Luang University

Agenda:

• Architectural patterns
• Architecture Decisions
• Architectural patterns provide a way to structure systems from interactive to distributed

The Architecture

• Architects look at previous architectures when designing new ones

Architectural Pattern

• Architectural patterns are solutions for recurring problems and are synonymous with architectural styles
• Architectural patterns are similar to building styles like Gothic, Pyramid, or Greek
• They specify the fundamental structure of an application

Overview of Relation:

• Basic Architecture is the basis of Architecture Patterns
• Architecture Pattern provides solutions that resolve multiple problems
• "Most system software has multiple patterns"
• Architecture Tactic includes Architecture Pattern
• Architecture Tactic can focus on a specific quality attribute
• Quality attributes can allow you to achieve the desired outcome
• “In order to address multiple required quality attributes"

Categories of Pattern

• Patterns are grouped into 4 categories.
• “From mud to structure” helps avoid "sea" of components and supports decomposing system tasks into subtasks.
• Distributed systems include the Broker pattern, which provides a complete infrastructure for distributed applications.
• Interactive Systems support structuring software systems with human-computer interaction.
• Adaptable Systems support extension of applications and adapting to evolving technology and functional requirements.

From Mud to Structure

• There are 3 patterns in this category:
  • Layer
  • Pipe & Filters
  • Blackboard

Layer Pattern

• Helps structure applications so they can be decomposed into subtasks at particular abstraction levels.
• Suitable for large systems that require decomposition.
• Higher-level operations handle higher-level issues by relying on lower-level operations.
• Lower-level operations handle lower-level issues and adaptors and interfaces can be used as connectors.
• It allows you to structure a system into different layers
• Design the upper layers by delegating subtasks to lower layers, while supporting subtasks, until it reaches the highest level
• The solution is extremely simple from a high-level viewpoint.
• Example: VAT calculation software

Layer Pattern Example:

• User visible element: Button, Textfield, windows
• Specific Application Module: Calculate VAT 7%
• Common Service Level: Basic mathematic operation +, -, *, /
• Operating System: Driver for monitor, keyboard, CPU
• Hardware: Monitor, keyboard, CPU

Layer vs Tier

• Tier and Layer are often confused
• The layer pattern places components within layers according to their generality.
• More general-purpose components (most reusable) are in the lower layers, with a focus on reusing.
• A tier is a physical machine or server that hosts one or more layers and tiers are not the same thing as layers.

Jacobson's Layer Architecture:

• Proposed layer architecture (Jacobson 1997)
• Application-Specific layer contains elements specific to the application.
• Business-Specific layer contains elements specific to a business or application domain (e.g., financial systems).
• These can be reusable across applications.
• Business-Independent layer contains elements independent of business or application domain, such as UI frameworks or databases.

Usage of Layer

• Network OSI 7 Layer Model
• Virtual Machine – JVM
• API – Application Programming Interface
• MS Windows
• Information System

Layer Pattern - Consequence:

• Reuse of Layer: Achieved with well-defined abstraction and documentation.
• Exchangeability: Achieved with standardized interfaces and adaptors that connect components between layers.
• Dependencies Kept Local: Achieved with standardized interfaces and adaptors that connect components between layers.

Pipe & Filter

• Provides structure for processing a stream of data.
• Suitable for systems that process data streams.
• These system youtube that detects internet speed to adjust video quality
• It contains : Data sources, Filter 1, Filter 2, Filter 3 and a Data sink

Pipe & Filter – Push vs. Pull:

• Push: Data source source pushing to sink downstream
• Pull: Filters reading from source to sink upstream

Flexibility of Pipe & Filter

• Flexibility by filter exchange to rearrange and create new fitlers
• Reuse of filter to reuse components
• Efficiency by parallel processing to start fitler component in parralel

Known uses of Pipes & Filters:

• Unix
• LASSPTool
• streaming video/audio over internet

Blackboard

• Useful for problems lacking deterministic solution strategies as it acts as Specialized subsystems whose knowledge allows them to assemble a solution
• Each subsystem specializes in solving a part of the overall task.
• This is useful when we do not know the exact solution

What it is useful for

• Suitable for problems that do not have feasible deterministic solutions for transformation of raw data into high-level data structures:
  • Diagrams
  • Tables
  • English phrases
  • Vision
  • Image recognition
  • Speech recognition

Blackboard Sample

• Speech recognistion is a sample
• It leverages control to build a "solution" using segmented Knowledge sources

Consequence of blackboard

• Supports modifiability and maintainability
• There are difficulties for testing due to not having a deterministic algorithm
• There is no good solution that is guarantee
• It typically has low efficiency

Distributed System

• The broker pattern is used to structure distributed software systems with decoupled components and allows interaction by remote service invocations
• A broker component coordinates communication

Broker

• Suitable for distributed and heterogeneous systems with independently cooperating components.
• A Broker acts like a directory (yellow book) and messenger, servers resgister in the broker
• They forward requests from the cient
• They encapsulate connectivity, which can hide IP addresses to create VPNs
• Example : Bangkok bridge Tokyo

The Broker facilitates:

• Implementation of service by registering itself with broker to respond back
• Supports un/register server, transfer of service, location of server and communication with other brokers
• Implements functionality to encapsulate system
• Implement the client functions, and the request sent

Known Uses of Brokers:

• World Wide Web (WWW)
• CORBA - Common Object Request Broker Architecture
• The network specific functionality it enables involves mediating local brokers

Consequences of Broker

• Portability of system
• Changeability/Extensibility of components.
• Location Transparency.
• Suffers from Restricted efficiency due to indirect communication because if broker faces it crashes it cannot access

Interactive System:

• For systems with high degree of user interaction and usability enhancement.
• There are 2 patterns within this group
  • Model View Controller (MVC)
  • Presentation Abstraction Control

Model View Controller (MVC):

• Suitable for interactive applications with a flexible human-computer interface.
• Divides the system into 3 parts:
  • Model: Core functionality & data
  • View: Information displayed to user (Output)
  • Controller: Handles user input (Input)
• View & Controller together are the user interface.
• The model is indepedent from output to input
• It displays info and propagates change, and often works controller 1 to 1 where the view translates events

Functional Core

• Has model to provide function, register dependancies to controller, and notify data changes
• Translate events and send the request for propogation and disables buttons
• Separate human-computer interaction from core functionality

Consequence of MVC:

• Multiple Views of the same Model.
• Exchangeability of look-and-feel.
• Framework potential.
• Increased Complexity.
• Potential Inefficiency with data access in view if updates are frequent.

Decision:

• The decision to develop a UI in mobile as opposite to web, a UI in Authetication as compared to 2 factor Authetication
• Architects take responsiblity to guding technology and decisions

Architecture Description Record

• ADR is a document that captures and communicates important decisions made during the software architecture design process.
• ADRs provide the way to document standardized context, consideration, and rationale behind a specific architectural decision.
• Context is needed to find which issues we need to motivate for a change
• We need to find the best solution and why we choose it
• The consequences are the pros and cons for why it becomes easier and difficult