1 CS 501 Spring 2008 CS 501: Software Engineering Lecture 13 System Architecture and Design I

1 CS 501 Spring 2008

CS 501: Software Engineering

Lecture 13

System Architecture and Design I


Administration

Quizzes

There are 4 quizzes, each with 2 questions. The final grade will be based on the best 6 questions out of 8.

Uncollected answer books are at 301 College Avenue.

Average grades:

Quiz 1 Q1 Quiz 1, Q2 Quiz 2 Q1 Quiz 2 Q2

7.6 6.2 6.8 8.0



Usability (continued)


Evaluation

• Making sure that a system is usable before launching it.

• Iterative improvements after launch.

• Categories of evaluation methods:

Analytical evaluation: without users

Measurements on operational systems

Empirical evaluation: with users


Evaluation

How do you measure usability?

Usability comprises the following aspects:

• Effectiveness – the accuracy and completeness with which users achieve certain goals Measures: quality of solution, error rates

• Efficiency – the relation between the effectiveness and the resources expended in achieving themMeasures: task completion time, learning time, clicks number

• Satisfaction – the users' comfort with and positive attitudes towards the use of the systemMeasures: attitude rating scales

From ISO 9241-11


Measurement

Basic concept: log events in the users' interactions with a system

Examples from a Web system

• Clicks (when, where on screen, etc.)

• Navigation (from page to page)

• Keystrokes (e.g., input typed on keyboard)

• Use of help system

• Errors

May be used for statistical analysis or for detailed tracking of individual user.


Evaluation based on measurements

Analysis of system logs

• Which user interface options were used?

• When was was the help system used?

• What errors occurred and how often?

• Which hyperlinks were followed (click through data)?

Human feedback

• Complaints and praise

• Bug reports

• Requests made to customer service


Evaluation with Users

Testing the system, not the users!

Stages of evaluation with users:

Preparation

Sessions conduct

Analysis of results

User testing is time-consuming, expensive, and essential.


Evaluation with UsersPreparation

• Determine goals of the usability testing

“Can a user find the required information in no more than 2 minutes?”

• Write the user tasks

“Answer the question: how hot is the sun?”

• Recruit participants

Use the descriptions of users from the requirements phase to detect potential users

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Usability Laboratory

Concept: monitor users while they use system

Evaluators User

one-way mirror

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Evaluation with UsersSessions Conduct

• Conduct the session– Usability Lab– Simulated working

environment

• Observe the user– Human observer(s)– Video camera– Audio recording

• Inquire satisfaction data

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Evaluation with users:Results analysis

• If possible, use statistical summaries.

• Pay close attention to areas where users

– were frustrated

– took a long time

– could not complete tasks

• Respect the data and users' responses. Do not make excuses for designs that failed.

• Note designs that worked and make sure they are incorporated in the final product.

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Usability: Design Tensions in Networked Systems

• Client computers and network connections vary greatly in capacity

• Client software may run on various operating systems. It may be current or an earlier version. What assumptions do you make about the user's computer and Web browser?

• Designers wish to control client software, e.g., Web browsers, but users wish to configure their own environments. This can be a factor in accessibility, e.g., which part of the system determines the font size.

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System considerations of user interface design

• Personal computer cycles are there to be used

• Any network transfer involves delay

• Shared systems have unpredictable performance

• Data validation often requires access to shared data

• Mobile code poses security risks

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Usability and Cost

• Good usability may be expensive in hardware or special software development

• User interface development may be a major part of a software development project

Programming environments provide powerful user interface toolkits

• Costs are multiplied if a user interface has to be used on different computers or migrate to different versions of systems

Web browsers provide a general purpose user interface where

others maintain the user interface software

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Changes in user interface design

Examples of change: 1995 to 2007

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1990

SEARCH I NSPEC Dat abase- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Type keywor ds and pr ess RETURN - - orent er a command

Def aul t i s ADJ : aci d f r ee- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Set #3: aci d adj f r e 0 r ecor ds I NSPEC Dat abase

Set #4: aci d adj f r ee 5 r ecor ds I NSPEC Dat abase

Set #5: aci d and paper 448 r ecor ds I NSPEC Dat abase

Set #6: deaci di f i cat i on 4 r ecor ds I NSPEC Dat abase- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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System Architecture and Design

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System Architecture and Design

The overall design of a system:

• Computers and networks (e.g., monolithic, distributed)

• Interfaces and protocols (e.g., http, ODBC)

• Databases (e.g., relational, distributed)

• Security (e.g., smart card authentication)

• Operations (e.g., backup, archiving, audit trails)

• Software environments (e.g., languages, source control tools)

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UML: System and Subsystem Modeling

Subsystem model

A grouping of elements that specifies what a part of a system should do.

Component (UML definition)

"A distributable piece of implementation of a system, including software code (source, binary, or executable) but also including business documents, etc., in a human system."

A component can be thought of as an implementation of a subsystem.

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UML Notation: Component & Node

orderform.java

A component is a physical and replaceable part of a system that conforms to and provides the realization of a set of interfaces.

Server

A node is a physical element that exists at run time and represents a computational resource, e.g., a computer.

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Components and Replaceability

Components allow system to be assembled from binary replaceable elements.

• A component is physical -- bits not concepts

• A component can be replaced by any other component(s) that conforms to the interfaces.

• A component is part of a system.

• A component provides the realization of a set of interfaces.

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System Architecture Example:Extensibility in Web Browsers

Web browsers provide a flexible user interface through an extensible architecture.

Protocols:HTTP, WAIS, Gopher, FTP, etc., proxies

Data types: helper applications, plug-ins, etc.

Executable code:CGI scripts at serverJavaScript at clientJava applets

Style sheets:

CSS, etc.

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Web Interface: Basic

Web serverWeb browser

• Static pages from server

• All interaction requires communication with server

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UML Notation: Deployment Diagram

WebBrowser

PersonalComp

WebServer

DeptServer

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UML Notation:Application Programming Interface (API)

API is an interface that is realized by one or more components.

WebServer

Get Post

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UML Notation: Interfaces

WebBrowser WebServer

HTTP

dependency

interface

realization

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Web User Interface: CGI Script

Web browser

• Scripts can configure pages

• Scripts can validate information

• All interaction requires communication with server

Data

CGIScripts

Web server

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UML Notation: CGI Interface Diagram

CGIScript

HTTP

Apache

CGI

ODBC

MySQL

These components might be located on a single node.

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Web User Interface: JavaScript

Data

CGIScripts

Web server

Web browser

• JavaScripts can validate information as typed

• Some interactions are local

• Server interaction constrained by web protocols

JavaScript

html

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UML Notation: Package

A package is a general-purpose mechanism for organizing elements into groups.

Note: Some authors draw packages with a different shaped box:

JavaScript

JavaScript

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Example: Web Browser

HTTP

JavaScript

HTMLRenderEach package represents a group of objects.

WebBrowser

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Web User Interface: Applet

Any server

Web serversWeb browser

• Any executable code can run on client

• Client can connect to any server

Applets

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Applet Interfaces

WebBrowser WebServer

HTTP

XYZServer

XYZInterface

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UML Diagrams and Specifications

For every subsystem, there is a choice of diagrams

Choose the diagrams that best model the system and are clearest to everybody.

In UML every diagram must have supporting specification

The diagrams shows the relationships among parts of the system, but much, much more detail is needed to specify a system explicitly.

For example, in the Applet Interface slide, at the very least, the specification should include the version of the protocols to be supported at the interfaces, the options (if any), and implementation restrictions.

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Components and Classes

Classes represent logical abstractions. They may be grouped into packages.

Components represent physical things. They may live on nodes.

Classes have attributes and operations directly. Components have operations that are reachable only through interfaces.

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System Design: Data Intensive Systems

Examples

• Electricity utility customer billing (e.g., NYSEG)

• Telephone call recording and billing (e.g., Verizon)

• Car rental reservations (e.g., Hertz)

• Stock market brokerage (e.g., Charles Schwab)

• E-commerce (e.g., Amazon.com)

• University grade registration (e.g., Cornell)

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Example: Electricity Utility Billing Transaction Types

Requirements analysis has identified several transaction types:

• Create account / close account

• Meter reading

• Payment received

• Other credits / debits

• Check cleared / check bounced

• Account query

• Correction of error

• etc., etc., etc.,

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Batch Processing DesignExample: Electricity Utility Billing

First attempt:

Data input Master fileTransaction Bill

Each transaction is handled as it arrives.

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Criticisms of First Attempt

Where is this first attempt weak?

• A bill is sent out for each transaction, even if there are several per day

• Bills are not sent out on a monthly cycle

• Awkward to answer customer queries

• No process for error checking and correction

• All activities are triggered by a transaction

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Batch Processing: Validation

Data input

Master file

Edit & validation

read only

errors

Batches of validated transactions

Batches of incoming transactions

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UML Deployment Diagram:Batch Processing Validation

MasterFile

EditCheck

ValidData

DataInput

RawData

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Batch Processing: Master File Update

Master fileupdate

Bills

Validated transactionsin batches

Sort by account

errors Reports

Instructions

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Interfaces to DataInput

DataInput

RawData

EditCheckErrorUpdateError

DataforCheck

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Benefits of Batch Updating

• All transactions for an account are processed together at appropriate intervals

• Backup and recovery have fixed checkpoints

• Better management control of operations

• Efficient use of staff and hardware

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Online Inquiry

Master file

read only

Customer Service

Customer Service department can read file, make annotations, and create transactions, but cannot change the master file.

New transaction

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Online Inquiry: Use Cases

CustomerServer

AnswerCustomer

NewTransaction

<<uses>>

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Data Intensive SystemsExample: A Small-town Stockbroker

• Transactions

Received by mail or over telephone

For immediate or later action

• Complex customer inquiries

• Highly competitive market

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A Database Architecture

Databases:

• Customer and account database

• Financial products (e.g., account types, pension plans, savings schemes)

• Links to external databases (e.g., stock markets, mutual funds, insurance companies)

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Real-time Transactions

Customer & account database

Products & services database

External services

Real-time transactions

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Real-time Transactions & Batch Processing

Customer & account database

Products & services database

External services

Real-time transactions

Batch processing

Data input

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Stock Broker: Interface Diagram

CustomerDBProductDB

OnLineTR BatchTR

External

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Practical considerations to include in Architecture and Specification

• Real-time service during scheduled hours with batch processing overnight

• Database consistency after any type of failure

two-phase commitreload from checkpoint + logdetailed audit trail

• How will transaction errors be avoided and identified?

• How will transaction errors be corrected?

• How will staff dishonesty be controlled?

These practical considerations may be major factors in the choice of architecture.

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Documents

1 CS 501 Spring 2008 CS 501: Software Engineering Lecture 13 System Architecture and Design I