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5861: Software Architecture


Lecture 9

Dr. Michael Whalen

Fall 2010

10/14/2010

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Topics for Today


Questions / Comments from Last Week

◦
Project Phase III Questions / Examples


Architectural Patterns & Pattern
Languages


Guest Lecture: Software Product Line
Engineering


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Project Phase III

1.

Version History

2.

Table of Contents

3.

Introduction

4.1

Purpose and Scope


4.2

Audience

4.3

Status


4.4

Architectural Design Approach

5.

Glossary


6.

System Context


6.1

System Environment


6.2

Overview of Requirements

6.3

System Scenarios


7.

Architectural Forces

7.1

Goals

7.2

Constraints

7.3

Architectural Principles

8.

Architectural Views for Top
-
Level
System

◦
Pick Two views

9.

Architectural Views for
Subsystem

◦
Pick Two Views


10.

Appendices

10.1

Appendix: Decisions and
Alternatives

10.3

Appendix: References


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Do this

Refer to

Phase II

Do this

Refer to

Phase II

Do this


Do this

Optional

What do I do
HERE?

How do I bound my (sub)system?


Several questions about what to call a
subsystem and when is it architecturally
interesting?


To make it concrete, I’ll pick on
Ziad

a
little


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Choosing a subsystem to analyze
for Android


The Android SDK
allows one to build
applications for the
Google Android
phone platform


It is all in Java, so the
SDK is a (large!) suite
of Java classes that act
as a framework for
Android applications

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Choosing a subsystem to analyze
for Android


View

class is the base class for all GUI widgets

◦
All controls, canvases, windows, etc. are derived from
View
.

◦
Layout and composition is performed by
ViewGroup

subclass



Question 1: Do Android
Views
(the View class and its
descendents) form an architecturally interesting
subsystem to analyze?


Question 2: Is it enough to look at

the
View
class and its descendents?

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Android
Views:
Interesting?


Q1: Yes.


Q2: No


The Android
view
architecture encompasses
more than just the subclasses of the
View
class

◦
Layout can be externalized to XML resources

◦
View class can also process UI
events
generated by
the
platform (i.e.
it also acts as
controller
)

◦
Usually this event processing is performed by
event
listener
classes that attach to views


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So which
parts
are interesting?


Do not describe all 50+ subclasses of
View



Interesting parts (to me) are:

◦
Top
-
level interface for view objects and how this
allows a MVC
-
style architecture for the GUI

◦
View compositing hierarchy
–

how are child views laid
out and composed?

◦
View creation: either programmatic or via the XML
interface

◦
Use of
event listeners

to specialize view event
processing

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Which viewpoints are interesting?

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Which viewpoints are interesting?


Well, certainly functional


Depending on XML creation interface,

◦
Deployment:
The XML view descriptions are
stored as separate resource files that must be
bundled with the application. How does this
work?

◦
Development:
describe the module structure
for views and their dependencies to other
portions of the Android API.


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ARCHITECTURAL
PATTERNS

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Architecture & Design Patterns


A
design pattern is
a
solution

to a
problem
that occurs repeatedly in a variety of
applications



Documents both how problem is solved and (to
some extent) why

◦
Why is this idea problematic?

◦
What is the rationale for the solution?



Solution (pattern) must be non
-
obvious

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Architecture & Design Patterns


Each pattern has a name



Use of each pattern has documented
consequences

◦
May be both positive and negative



When is a pattern “architectural”?

◦
Short answer: it depends

◦
Longer answer (in the form of a question): how
central is it to the architecture being described?

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A Pattern Language


Describe a
system in terms
of interacting
patterns


Focus is on
describing
relationships
between
patterns

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Patterns and Views

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Example: Layers Pattern

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Pattern: Layers

Consider a system in which high
-
level components depend on low
-
level
components to perform their functionality, which further depend on even lower
-
level components and so on. Decoupling the components in a vertical manner is
crucial in order to support modifiability, portability, and reusability. On the other
hand components also require some horizontal structuring that is orthogonal to
their vertical subdivision.


To achieve these goals, the system is structured into layers so that each layer
provides a set of services to the layer above and uses the services of the layer
below. Within each layers all constituent components work at the same level of
abstraction and can interact through connectors. Between two adjacent layers a
clearly defined interface is provided. In the pure form of the pattern, layers
should not be by
-
passed: higher
-
level layers access lower
-
level layers only through
the layer beneath.

Layers


Examples:

◦
Network stacks: OSI,
TCP/IP

◦
OS Organization

◦
Virtual Machines

◦
Robotics

◦
Planning Systems

◦
Avionics


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Indirection Layer


Provide
a unified interface to a set of interfaces
in a subsystem without damaging the
generic
form of the sub system.


Also called Façade pattern


Image from: http://www.renaissance.co.il/ivbug/meeting87/Architecture%20and%20design%20patterns.ppt

Data and Layer Patterns

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22

Pipe and Filter Pattern

Target
Data

Transformation Program

Source Data

…

…

…

…

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Data Repository

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Can be passive or active;
passive repo stores data;
active repo broadcasts
changes in data to interested
clients (Pub / Sub)

Blackboard usage is for splitting
processing of complex problem
into subtasks that can be
simultaneously solved. A control
client monitors & coordinates the
blackboard

Coordinator

Adaptation Patterns

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Language Interpretation
-
based patterns

Generic
Interface

Patterns

Deferred Binding Patterns

Microkernel

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Microkernel Pattern

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Mechanism implementing primitives

Resource Policies

Client Services

Microkernel Pattern

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From: people.cs.uchicago.edu/~mark/51050/lectures/lecture.6/lecture.6.ps

Reflection


Reflection allows runtime discovery of
interfaces and dynamic calls to discovered
services.


Built into programming

languages

◦
Java

◦
Python

◦
Ruby


Also built into

middleware

◦
COM/DCOM

◦
CORBA

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Reflection & Modification


Some tools allow reflection interface to
modify
class
or even
language
definitions

◦
Re
-
program system on
-
the
-
fly!

◦
These are often called
Metaobject

protocols


Easy to create derivative languages using
this functionality

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Interceptor Pattern

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Language Patterns in More Detail

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User Interaction Patterns

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Model
-
View
-
Controller


Benefits

◦
Supports independent

update of GUI (view)

and business logic

(model)

◦
Allows display of

same data multiple ways

◦
Better testability


Examples: Built into

most object
-
oriented

UI frameworks

◦
Java, MFC (C++), Cocoa (Mac), .NET

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Presentation Abstraction Control

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Component Interaction Patterns

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Method Call /
RPC

Observer
Notification

Mechanisms
for Response

Component Interaction Patterns II

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Broker Pattern

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What Have We Learned?


Some Examples of Architectural Patterns


What a Pattern Language is


How a PL can be used to relate patterns


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Further Reading


Pattern
-
oriented Software Architecture:
System of Patterns
by Frank
Buschmann

et.
al

◦
Currently 5 volumes!

◦
Grounded in examples of real systems

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