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FEUP, Ademar Aguiar

Arquitectura de Sistemas de Software, 2005-06

FEUP ● Ademar Aguiar ● Arquitectura de Sistemas de Software, 2005-06 1

Arquitectura de Sistemas de Software Mestrado em Engenharia InformáticaLicenciatura em Engenharia Informática e Computação

Ademar AguiarUniversidade do Porto & INESC Portoademar.aguiar at fe.up.pt


Estilos de arquitectura

adaptado de http://www.ugrad.cs.ubc.ca/~cs410/lectures/slides/cs410-ArchitecturalStyle1.ppte http://www.ugrad.cs.ubc.ca/~cs410/lectures/slides/cs410-ArchitecturalStyle2.ppt

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Architectural style (Buildings)

18th Century Georgian Architecture“Old Corner Bookstore”Boston MA

Early Gothic ArchitectureNotre Dame, Paris


Styles and patterns

Identifying styles (and patterns) can help codify and share knowledge/expertise• Architectural Styles [Shaw and Garlan, Software Architecture, Prentice Hall 96]

• Design Patterns [Gamma, Helm, Johnson, Vlissides, Design Patterns, Addison Wesley 95] [Buschmann et. Al, Pattern-oriented Software Architecture: A System of Patterns, John Wiley & Sons 96]

• Code Patterns [Coplien, Advanced C++ Programming Styles and Idioms, Addison-Wesley 91]

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Architectural styles

A style consists of:• a set of component types (e.g., process, procedure) that perform some

function at runtime• a topological layout of the components showing their runtime relationships• a set of semantic constraints• a set of connectors (e.g., data streams, sockets) that mediate

communication among components

Styles provide guidance for architectural design based on the problem domain and the context of use

• Recurring (and proven) architectural design• Definition of common vocabulary

Each style has:• components, connectors, key characteristics, strengths and

weaknesses, variants and specializations.From Chapter 5, Software Architecture in Practice, p. 94


A catalog of architectural styles

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Call-and-Return style

Goal: achieve modifiability and scalability

Substyles:• Main-program-and-subroutine

- Decompose program hierarchically. Each component gets control and data from its parent.

• Remote procedure call- Components are distributed across a network. Remote call is between two computers, but

otherwise looks like a procedure call.

• Object-oriented or abstract data type

• Layered


Object-oriented (ADT) substyle

Objects help achieve modifiability by encapsulating internal secrets from environment

Access to objects is only through methods (constrained forms of procedure calls)

Object-oriented paradigm is distinguished from ADT by inheritance and polymorphism

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Example: cruise control system

A cruise-control system exists to maintain the speed of a car, even over varying terrain, when turned on by the driver.

When the brake is applied, the system must relinquish speed control until told to resume.

The system must also steadily increase or decrease speed to reach a new maintenance speed when directed to do so by the driver.Throttle

System on/off

Engine on/off

Pulses from Wheel

Accelerator

BrakeIncrease/Decrease

Speed

Resume Speed

Clock From Model Problems in Architecture, Shaw


Cruise control: Input/output

System on/off: If on, cruise-control should maintain the car speed.

Engine on/off: If on, denotes car engine is turned on; cruise-control is only active if the engine is on.

Pulses from wheel: A pulse is sent for every revolution of the wheel.

Accelerator: Indication of how far the accelerator has been pressed.

Brake: On when the brake is pressed; cruise-control temporarily reverts to manual control if the brake is pressed.

Increase/Decrease Speed: Increase or decrease the maintained speed; only applicable if the cruise-control system is on.

Resume: Resume the last maintained speed; only applicable if the cruise-control system is on.

Clock: Timing pulse every millisecond.

Throttle: Digital value for the engineer throttle setting.

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Layered substyle

Goals: modifiability and portability

Optimally, each layer communicates only with its neighbors

Sometimes, must layer bridgefor performance: decreases benefits of style

Core

Basic Utility

Subsystem

Subsystem

User Interface


Layered substyle: Example

Outline of a design used on a mobile robot

Level 1 is robot control

Levels 2-3 deal with input from real world

Level 4 manages robot’s view of world

Level 5 manages robot navigation

Levels 6-7 schedule and plan robot’s activities

Top-level is UI and supervisory

Supervisor

Global Planning

Control

Navigation

Real-world Modelling

Sensor Integration

Sensor Interpretation

Robot Control

Environment

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Data-Flow style

Data-flow

BatchSequential

Pipes andFilters

• components are independent programs• each step runs to completion

before the next starts• each batch of data is transmitted as

a whole between stepsClassic data-processing approach

Goals: achieve reuse and modifiability


Pipe-and-Filter substyle

Incremental transformation of data by successive components

• Filters transform data using little contextual information and retain no state between instantiations

• Pipes are stateless and are used to move streams of data betweenfilters

A pipe’s source end connects to a filter’s output port; sink end connects to filter’s input port

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Pipe-and-Filter substyle...

Pipes and filters run non-deterministically until no more computations or transmissions are possible

+ Simple to reason about, eases maintenance, enhances reuse, can easily enhance performance by parallelizing or distributing

- Interactive apps are difficult, filters can’t cooperate to solve a problem, performance is often bad


Cruise control: Pipe-and-Filter style

Design a pipe-and-filter solution given the following filters:• System State: determines overall status. If engine on and cruise control on,

then On, else Off• Clock Pulses: Transforms pulses to numeric value

• Calculate Desired Speed (CDS): determine current desired speed based on system state, whether brake is applied, and cruise state

• Calculate Throttle

• Split: takes an input and creates two output streams

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Heterogeneous Use

Systems are generally built from multiple styles

Three kinds of heterogeneity:• Locationally heterogeneous: run-time structures reveal different styles in

different areas

• Hierarchically heterogeneous: a component of one style, when decomposed is structured according to another style

• Simultaneously heterogeneous: different styles depending on point of view


Data-Centered styles

Two-substyles: Repository and Blackboard

When a system can be described as a centralized data store that communicates with a number of clients

Shared DataClient

Client Client

In a (passive) repository, such asshown on left, data might bestored in a file. In an activerepository, such as a blackboard,the blackboard notifies clients whendata of interest changes (so therewould be control from datato clients)

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Blackboard: Example

Captain is the overall supervisor

Map navigator is a high-level path planner

Pilot is a low-level path planner and motor controller

Perception subsystem takes data from sensors and integrates it

Perception Subsystem

Blackboard

lookout

captain

mapnavigator

pilot


Independent components style

We’ll look at the implicit invocation (event) substyle

Goals: achieve modifiability by decoupling various parts of the computation

Approach: have independent processes or objects communicate through messages

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Event systems

Individual components announce data they wish to share (publish) with others

Other components may register an interest in the kind of data (subscribe). They are invoked when the data appears

How does this achieve modifiability?


Implementing event systems

Several different ways to achieve an event system:• Callbacks

• Event Handling in Java Swing

• Observer design pattern

• Event notification pattern

• Message Manager

• (this is not a complete list)

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Event handling in Java Swing

Some examples:User closes a window WindowListenerTable selection changes ListSelectionListenerUser clicks a button ActionListener

Event Source

Event Listener

Event Listener

Event ListenerEventObject


Java Swing…

public class MyClass implements ActionListener {public void actionPerformed( ActionEvent e ) { … }

}

someComponent.addActionListener( instanceOfMyclass );

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Observer design pattern


Limitations of observer

the basic implementation described in the pattern has some limitations

• can’t tell why an object is being told to update (except by digging through code)

• one size fits all: an observer might not be interested in all changes in the subject

• an observer has only one routine that dispatches updates

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Event notification pattern

Dirk Riehle created the Event Notification Pattern to address some of these problems (http://www.riehle.org/papers/1996/tapos-1996-event.html)

An event represents an abstract state change of an object (a subject). The event notification mechanism notifies dependent objects (observers) when a change has occurred; the dependent objects can then update themselves (sometimes through explicit invocation)


Event notification pattern: How-To

Make abstract state changes for an object (a subject) visible by providing state change objects as part of its interface

A dependent object ( an observer) uses event stub objects to represent its dependencies as first-class objects

Event stubs are linked to state change objects via event link objects

Event link objects propagate change

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Event notification pattern: Basic design

Observer

OpForEv1(..)OpForEv2(..)

Subject

StateChg1(..)StateChg2(..)

EventStuboperationobserver

Forward(..)

observer->operation(..)

StateChangeAnnounce(..)Register(..)

Unregister(..)

For all registeredstubs:stub->Forward(..)

1..n

1..n 1..n

Overallimplicitinvocation

Indirect invocation


Event notification pattern: Refined design

Observer

OpForEv1(..)OpForEv2(..)

Subject

StateChg1(..)StateChg2(..)

EventStuboperationobserver

Forward(..)

StateChangeAnnounce(..)Register(..)

Unregister(..)

For all registeredstubs:stub->Forward(..)

1..n

1..n 1..n

Overallimplicitinvocation

Indirect invocation

EventLink

Forward(..)

inherits

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Using the event notification pattern

We will break into groups and try out the event notification pattern in class


Message manager

Sometimes you want even more flexibility

For instance, if building a programming environment, perhaps you want to be able to add tools on the fly and have them respond to events

A message manager can help• Receive messages and route them to registered parties

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Heterogeneous use

Systems are generally built from multiple styles

Three kinds of heterogeneity:• Locationally heterogeneous: run-time structures reveal different styles in

different areas

• Hierarchically heterogeneous: a component of one style, when decomposed is structured according to another style

• Simultaneously heterogeneous: different styles depending on point of view


Characterizing and comparing styles

The following categories are useful in comparing and characterizing styles:

• What kinds of components and connectors are used in the style?

• What are the control structures?

• How is data communicated?

• How do data and control interact?

• What kind of reasoning does the style support?

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Components & Connectors

“A component is a unit of software that performs some function at runtime.” [p.105]

“A connector is a mechanism that mediates communication, coordination, or cooperation among components” [p. 105]


Control Issues

How does control pass among components?

Topology: • What is the topology of a batch-sequential data-flow style?

• What is the topology of a main-program-and-subroutine style?

Other topologies include star, arbitrary… Topology maychange at runtime.

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Control Issues...

Synchronicity: How interdependent are the component’s actions upon each other’s control states?

• E.g., lockstep - state of one component implies state of all others

• E.g., synchronous - components synchronize regularly; other state relationships are unpredictable


Control Issues...

Binding Time: When is the partner in a communication established?• Program-write time?

• Compile-time?

• Invocation-time?

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Issues...

Data Issues:• Topology• Continuity: Continuous vs. sporadic; volume• Mode: Passed, shared, copy-out-copy-in, etc.• Binding Time: Same as control issue

Control/Data Interaction Issues:• Shape of control and data topologies• Directionality: does data flow in direction of control?

Type of Reasoning


Estilos de arquitectura – 2

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Architectural Styles & Properties

Important concepts• Component, Connectors or Relationships• View

- A partial aspect of a software architecture that shows specific properties of a software system.

- 4 views of Software Architecture: Logical View, Process View, Physical View, Development View

• Functional Property- Deals with particular aspect of a system’s functionality.- Related to Specified Functional requirement.- Seldom explicitly stated in SAs., and mostly assumed implicitly.- Stating explicitly with Functional Properties help in traceability of an architectural

element to its functional requirements.• Non-Functional Property

- Not covered by its functional description.- Addresses aspects like reliability, compatibility, maintainability, interoperability ,

changeability, efficiency, testability and reusability of a software system.

Advantages and disadvantages


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