Module 6

Architectural Patterns

Properties of patterns

• Addresses a recurring design problem that arises in specific design situations and presents a solution to it
• Document existing well proven design experience
• Identify and specify abstractions at the high level
• Helps to build complex systems
• Manage software complexity

A set of guidelines that help to get a good design are:
1. Avoid rigidity (Hard to change)
2. Avoid Fragility (Whenever the design changes, it should not break)
3. Avoid Immobility (Cannot be reused)

OO Design Principles

• Open Close:

  • Open to extension and close for modification
  • Template and strategy pattern

• Dependency Inversion:

  • Decouple two module dependencies
  • Adapter pattern

• Liskov's Substitution: Superclass can be replaced by subclass

• Interface Segregation: Interfaces are made only for a particular purpose

• Single responsibilies: One class for only one task

Three part schema for a pattern

Context

• A scenario or situation where design problem arises
• Ideally the scenario should be generic but it might not always be possible
• Example
  • Developing messaging solution for mobile applications
  • Developing software for a man machine interface

Problem

• Start with a problem statement that captures the theme
• Completed by forces
  • A requirement
  • A constraint
  • A desirable property
• The forces can complement or contradict. This can be used to choose what force needs to be taken into consideration and what should not be

Solution

• It is a configuration to balance the forces mentioned earlier
  • Like structure with components and relationships
  • Run time behavior
• Structure: Addresses static part of the solution
• Run time: Behavior while running

Problem Categories

Layering Pattern

Example

Layers

• Services of Layer J are only used by Layer J + 1
• No further direct dependencies between layers
• Each layer can have many components
  

The following is used to define the layer abstraction criteria
- Most generic components are in the lowest layer and domain specific components are in the top layer
- More stable components are in lower layers.
- Distance from hardware is also used in determining the layers

Construction of layer

• Use black box approach to specify layer interface
• Structure each layer
  • Identify components in each layer
  • Bridge or strategy pattern can help

Examples

Summary

Pipe and Filter Pattern

Filter

• Has interfaces from which a set of inputs flow in and set of outputs flow out
• Processing step is in a filter component
• Independent entities
• Does not share state with other filters

Pipes

• Data is passed through pipes between adjacent filters
• Stateless data stream
• Source end feeds filter input and sink receives output

3 Part schema

A simple case

• Passive Filters
  
• Active Filters
  
• Buffered Pipe for synchronizing
  

Components of Pipe and Filter

Implementation Steps

Pros and Cons

• No intermediate files necessary
• Filter addition replacement and reuse
• Rapid prototyping
• Concurrent execution

• Certain analysis possible like deadlock Throughput and latency checks

• Data transformation overhead

• Error handling is a challenge
• Sharing state is expensive
• Lowest common denominator on data transmission determines the overall throughput

Examples

• Most PaaS providers have message oriented service orchestration
• Azure has queing services
• Amazon SQS is used to communicate between EC2 instances

Blackboard Pattern

Title	Details
Context	A set of heterogeneous specialized modules which dynamically change their strategies as a response to unpredictable events
Problem	When there is no deterministic solutions to process
	Solutions to partial problems require representation
	No predetermined strat is present to solve a problem
	Dealing with uncertain knowledge
Forces	A complete search of solution is not possible
	Different algos used for partial solutions
	One algorithm uses results of another algo
	No strict sequence between algos

Examples

• Speech recognition
• Vehicle Identification
• Robot Control
• Modern Machine Learning algos for complex task
• OCR based text recognition

Components

• Central data structures
• Control plan encapsulates information necessary to run the system
• Domain KS are concerned with the solving of domain specific problems
• Control KS adapt the current control plan to the current situation

Pros and cons

Distributed System Patterns

Title	Description
Context	Complex environment of systems to take advantage of computing power via clusters
	A software may be available only on a specific computer
	Due security we might have to run different parts in different systems
	Some services are provided by business partners over internet
Problem	To build a sw system as a set of decoupled interoperating components rather than a monolith
	Require a flexible means of interprocess communication
Forces	It should be possible to distribute components during deployment
	Need to exchange add or remove components at run time
	Architecture should hide system specific and implementation specific details from users of components and services

Broker Pattern

A broker component helps in achieving better decoupling of clients and servers
- Servers: Register themselves with the broker and make their services available
- Clients: access the functionality of servers by sending requests via the broker

The broker:
- Locating the appropriate server and forwarding a request to that server
- Transmitting results and exceptions back to the client

Implementation

• Define an object model or use an existing one
• Decide which kind of component interoperability the system should offer
• Specify the APIs the broker component provides for collaborating
• Use proxy objects to hide implementation details from clients and servers
• Design the broker component in parallel with steps 3 and
• Develop IDL compilers

CORBA

• CORBA is the oldest amongst the middleware technologies used in today's IT world
• CORBA stands for Common Object Request Broker Architecture
• CORBA supports Dynamic Invocation Interface (DII)

Remote Method Invocation

• Sun's Java RMI is based on the transparent Broker variant pattern
• Client side proxy and server side invoker have to be created manually by am additional compilation step
• The service interface is written in Java
• RMI is limited to usage in Java
• To establish interoperability RMI IIOP is provided

Benefits

• Location independence
• Type system transparency (Marshaling and unmarshaling are done)
• Isolation
• Separation of concern
• Resource management
• Portability

Liability

• Error handling
• Overhead since developers do not know about the location of objects
• Performance
• Lower fault tolerance (server / broker failures)

Model View Controller

MVC Dynamics

MVC Initialization

AJAX application as MVC

Benefits

• Multiple views can be done for the same model
• Synchronized views
• Pluggable views and controllers
• Exchangeability of look and feel
• Framework potential

Liabilities

• Increased complexity
• Potential execssive number of updates
• Inefficiency of data access in view
• Difficulty of using MVC with modern user interface tools

Micro Kernel

Context

The development of several applications that use similar programming interfaces that build on the same core functionality

Problem

• Developing software for an application domain that needs to cope with a broad spectrum of similar standards and technologies
• Continuous evolution, platform must be extensible, portable and adaptable

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Tags: !SoftwareArchitecturesIndex