Quality Attributes of Web Software Applications

Quality Attributes of Web Software Applications - Transcript

focus

engineering Internet software

Quality Attributes of Web Software Applications
Jeff Offutt George Mason University

T
Web applications have very high requirements for numerous quality attributes This article discusses some of the technological challenges of building today s complex Web software applications their unique quality requirements and how to achieve them
0740 7459 02 17 00 2002 IEEE

he World Wide Web was originally designed to present information to Web surfers using simple sites that consisted primarily of hyperlinked text documents Modern Web applications run largescale software applications for e commerce information distribution entertainment collaborative working surveys and numerous other activities They run on distributed hardware platforms and heterogeneous
computer systems The software that powers Web applications is distributed is implemented in multiple languages and styles incorporates much reuse and third party components is built with cutting edge technologies and must interface with users other Web sites and databases Although the word heterogeneous is often used for Web software it applies in so many ways that the synonymous term diverse is more general and familiar and probably more appropriate The software components are often distributed geographically both during development and deployment diverse distribution and communicate in numerous distinct and sometimes novel ways diverse communication Web applications consist of diverse components including traditional and nontraditional software interpreted scripting languages plain HTML files mixtures of HTML and programs databases graphical images and complex user interfaces As such engineering an effective Web site requires large teams of people with very diverse skills and backgrounds These teams include programmers graphics designers usability engineers information layout specialists data communications and network experts and data base administrators This diversity has led to the notion of Web site engineering 1 The tremendous reach of Web applications into all areas of communication and commerce makes this one of the largest and most important parts of the software industry Yet a recent National Research Council study2 found that the current base of science and technology is inadequate for building systems to control critical software infrastructure The President s commission on critical infrastructure protection reached
March April 2002
IEEE SOFTWARE

25

Instead of sooner but worse it is often advantageous to be later and better

this same conclusion in the President s Information Technology Advisory Committee report 3 This inadequacy is particularly severe in the novel high risk area of Web application software Although Web software development uses cutting edge diverse technologies little is known about how to ensure quality attributes such as Web application reliability Unique aspects of Web application software Several factors inherent to Web development contribute to the quality problem Developers build Web based software systems by integrating numerous diverse components from disparate sources including custombuilt special purpose applications customized off the shelf software components and third party products In such an environment systems designers choose from potentially numerous components and they need information about the various components suitability to make informed decisions about the software s required quality attributes Much of the new complexity found with Web based applications also results from how the different software components are integrated Not only is the source unavailable for most of the components the executables might be hosted on computers at remote even competing organizations To ensure high quality for Web systems composed of very loosely coupled components we need novel techniques to achieve and evaluate these components connections Finally Web based software offers the significant advantage of allowing data to be transferred among completely different types of software components that reside and execute on different computers However using multiple programming languages and building complex business applications complicates the flows of data through the various Web software pieces When combined with the requirements to keep data persistent through user sessions persistent across sessions and shared among sessions the list of abilities unique to Web software begins to get very long Thus software developers and managers working on Web software have encountered many new challenges Although it is obvious that we struggle to keep up with the technology less obvious is our difficulty in

understanding just how Web software development is different and how to adapt existing processes and procedures to this new type of software Economic changes We evaluate software by measuring the quality of attributes such as reliability usability and maintainability yet academics often fail to acknowledge that the basic economics behind software production has a strong impact on the development process Although the field of software engineering has spent years developing processes and technologies to improve software quality attributes most software companies have had little financial motivation to improve their software s quality Software contractors receive payment regardless of the delivered software s quality and in fact are often given additional resources to correct problems of their own making So called shrink wrap vendors are driven almost entirely by time to market it is often more lucrative to deliver poor quality products sooner than high quality products later They can deliver bug fixes as new releases that are sold to generate more revenue for the company For most application types commercial developers have traditionally had little motivation to produce high quality software Web based software however raises new economic issues When I recently surveyed a number of Web software development managers and practitioners I found that companies that operate through the Web depend on customers using and most importantly returning to their sites Thus unlike many software contractors Web application developers only see a return on their investment if their Web sites satisfy customers needs And unlike many software vendors if a new company puts up a competitive site of higher quality customers will almost immediately shift their business to the new site once they discover it Thus instead of sooner but worse it is often advantageous to be later and better Despite discussions of sticky Web sites and development of mechanisms to encourage users to return 4 thus far the only mechanism that brings repeat users to Web sites has been high quality This will likely remain true for the foreseeable future In software development a process driver is a factor that strongly influences the

26

IEEE SOFTWARE

March April 2002

process used to develop the software Thus if software must have very high reliability the development process must be adapted to ensure that the software works well When I surveyed the important quality process drivers for traditional software developers always gave a single answer that stands far above the rest time to market But when I recently made the same survey of Web software development managers and practitioners they claim that time to market although still important is no longer the dominant process driver They see the three most important quality criteria for Web application success and thus the underlying software as 1 reliability 2 usability and 3 security Additional important criteria include 4 5 6 7 availability scalability maintainability and time to market

been teaching software testing in various forms for 15 years yet have always felt like I was selling something that nobody wants Many businesses commercial success depends on Web software however if the software does not work reliably the businesses will not succeed The user base for Web software is very large and expects Web applications to work as reliably as if they were going to the grocery store or calling to order from a catalog Moreover if a Web application does not work well the users do not have to drive further to reach another store they can simply point their browser to a different URL Web sites that depend on unreliable software will lose customers and the businesses could lose much money Companies that want to do business over the Web must spend resources to ensure high reliability Indeed they cannot afford not to Usability Web application users have grown to expect easy Web transactions as simple as buying a product at a store Although much wisdom exists on how to develop usable software and Web sites Jakob Nielsen s text9 being a classic example many Web sites still do not meet most customers usability expectations This coupled with the fact that customers exhibit little site loyalty means unusable Web sites will not be used customers will switch to more usable Web sites as soon as they come online Security We have all heard about Web sites being cracked and private customer information distributed or held for ransom This is only one example of the many potential security flaws in Web software applications When the Web functioned primarily to distribute online brochures security breaches had relatively small consequences Today however the breach of a company s Web site can cause significant revenue losses large repair costs legal consequences and loss of credibility with customers Web software applications must therefore handle customer data and other electronic information as securely as possible Software security is one of the fastest growing research areas in computer science but Web software developers currently face a huge shortfall in both available knowledge and skilled personnel

Companies that want to do business over the Web must spend resources to ensure high reliability

Of course this is hardly a complete list of important or even relevant quality attributes but it provides a solid basis for discussion Certainly speed of execution is also important but network factors influence this more than software does and other important quality attributes such as customer service product quality price and delivery stem from human and organizational rather than software factors That said the quality attributes I just listed track closely with those cited in other books and articles 1 5 8 suggesting wide agreement that successful Web software development depends on satisfying these quality attributes Reliability Extensive research literature and a collection of commercial tools have been devoted to testing ensuring assuring and measuring software reliability Safety critical software applications such as telecommunications aerospace and medical devices demand highly reliable software but although many researchers are reluctant to admit it most software currently produced does not need to be highly reliable I have

March April 2002

IEEE SOFTWARE

27

Availability

Designing and building Web software applications that scale well represents one of today s most interesting and important software development challenges

In our grandparents time if a shopkeeper in a small town wanted to take a lunch break he would simply put a sign on the front door that said back at 1 00 Although today s customers expect to be able to shop during lunchtime we do not expect stores to be open after midnight or on holidays But that only works for brick and mortar stores When customers can visit our stores online 2 00 a m in North America is the middle of the afternoon in Asia and national or religious holidays fall on different days in different countries On the Web customers not only expect availability 24 hours a day seven days a week they expect the Web site to be operational every day of the year 24 7 365 Even a 10 minute downtime can be damaging I recently purchased 50 worth of books online but switched companies because the first Web site gave a missing file error message Ten minutes later that Web site was operational again but had lost my sale Although this was only one sale many customers would never come back Availability means more than just being up and running 24 7 365 the Web software must also be accessible to diverse browsers In the seemingly never ending browser wars of the past few years some software vendors actively sought to make sure their software would not work under competitors browsers By using features only available for one browser or on one platform Web software developers become foot soldiers in the browser wars sometimes unwittingly As an example one major Web site at my organization uses shockwave flash which is only compatible with MS IE and Netscape under Windows Thus the many Unix and Netscape users in my building cannot view their own Web site To be available in this sense Web sites must adapt their presentations to work with all browsers which requires significantly more knowledge and effort on developers part Scalability A recent television advertisement showed a small group of young men and women nervously launching their Web site The celebration started when the site got its first hit but their faces quickly turned gray when the number of hits went into the thousands then millions As with a small corner store

as few as three or four people can create a commercial Web site but unfortunately or fortunately for the profit margin virtually an unlimited number of customers can visit the Web site We must therefore engineer Web software applications to be able to grow quickly in terms of both how many users they can service and how many services they can offer The need for scalability has driven many technology innovations of the past few years The industry has developed new software languages design strategies and communication and data transfer protocols in large part to allow Web sites to grow as needed Scalability also directly influences other attributes Any programming teacher knows that any design will work for small classroom exercises but large software applications require discipline and creativity Likewise as Web sites grow small software weaknesses that had no initial noticeable effects can lead to failures reliability problems usability problems and security breaches Designing and building Web software applications that scale well represents one of today s most interesting and important software development challenges Maintainability One novel aspect of Web based software systems is the frequency of new releases or the update rate Traditional software involves marketing sales and shipping or even personal installation at customers sites Because this process is expensive software manufacturers usually collect maintenance modifications over time and distribute them to customers simultaneously For a software product released today developers will start collecting a list of necessary changes For a simple change say changing a button s label the modification might be made immediately But the delay in releases means that customers won t get more complex and likely important modifications for months perhaps years Web based software however gives customers immediate access to maintenance updates both small changes such as changing the label on a button and critical upgrades can be installed immediately Instead of maintenance cycles of months or years Web sites can have maintenance cycles of days or even hours Although other

28

IEEE SOFTWARE

March April 2002

Internet

software applications have high maintenance requirements and some research has focused on on the fly maintenance10 for specialized applications frequent maintenance has never before been necessary for such a quantity of commercial software Another ramification of the increased update rate has to do with compatibility Users do not always upgrade their software hence software vendors must ensure compatibility between new and old versions Companies can control the distribution of Web software to eliminate that need though Web applications must still be able to run correctly on several Web browsers and multiple versions of each browser Another possible consequence of the rapid update rate is that developers may not feel the same need to fix faults before release they can always be fixed later I have seen no data to indicate this is happening Time to market This has always been a key business driver and remains important for Web software but it now shares the spotlight with other quality attributes Most of the software industry continues to give priority to first to market Given the other factors discussed here however the requirement for patience can and must impact the process and management of Web software projects Software researchers practitioners and educators have discussed these criteria for years but no type of application has had to satisfy all of these quality attributes at the same time Web software components are coupling more loosely than any previous software applications In fact these criteria have until recently been important to only a small fraction of the software industry They are now essential to the bottom line of a large and fast growing part of the industry but we do not yet have the knowledge to satisfy or measure these criteria for the new technologies used in Web software applications Technology changes The commercial use of the Internet and Web has grown explosively in the past five years In that time the Internet has evolved from primarily being a communications medium email files newsgroups and chat rooms to a vehicle for distributing information to a full fledged market channel for

Gif images Helpers Audio Video

Browser HTML forms

HTTP server

CGI programs

Database

Files

Client side

Server side

e commerce Web sites that once simply displayed information for visitors have become interactive highly functional systems that let many types of businesses interact with many types of users These changes have had an enormous impact on software engineering As the use of the Internet and Web has grown the amount type and quality of software necessary for powering Web sites has also grown Just a few years ago Web sites were primarily composed of static HTML files so called soft brochures usually created by a single Webmaster who used HTML JavaScript and simple CGI scripts to present information and obtain data from visitors with forms Figure 1 illustrates the early Web a typical client server configuration in which the client is a Web browser that people use to visit Web sites that reside on different computers the servers and a software package called a Web server sends the HTML files to the client HTML files contain JavaScripts which are small pieces of code that are interpreted on the client HTML forms generate data that are sent back to the server to be processed by CGI programs This very simple model of operation can support relatively small Web sites It uses small scale software offers very little security usually cannot support much traffic and offers limited functionality This was called a two tier system because two separate computers were involved The Web s function and structure have changed drastically particularly in the past 24 months yet most software engineering researchers educators and practitioners have not yet grasped how fully this change affects engineering principles and processes Web

Figure 1 First generation Web sites followed a client server model that suffices to support simple page viewing and limited traffic but did not scale well

March April 2002

IEEE SOFTWARE

29

Network

Middleware

Middleware

Client

Web server

Application servers

Database server

Figure 2 Modern Web sites generally follow an N tier model that by separating presentation from business logic supports much greater application complexity higher traffic and stronger site security

sites are now fully functional software systems that provide business to customer ecommerce business to business e commerce and many services to many users Instead of referring to visitors to Web sites we now refer to users implying much interaction Instead of Webmasters large Web sites must employ Web managers leading diverse teams of IT professionals that include programmers database administrators network administrators usability engineers graphics designers security experts marketers and others This team uses diverse technologies including several varieties of Java Java Servlets Enterprise JavaBeans applets and Java Server Pages HTML JavaScript XML UML and many others The growing use of third party software components and middleware represents one of the biggest changes The technology has changed because the old two tier model did not support the high quality requirements of Web software applications It fails on security being prone to crackers who only need to go through one layer of security on a computer that is by definition open to the world to provide access to all data files It fails on scalability and maintainability because as Web sites grow a two tier model cannot effectively separate presentation from business logic and the applications thus become cumbersome and hard to modify It fails on reliability whereas previous Web software generations relied on CGI programs usually written in Perl many developers have found that large complex Perl programs can be hard to program correctly understand or modify Finally it fails on availability because hosting a site on one Web server imposes a bottleneck any server problems will hinder user access to the Web site Figure 2 illustrates current Web site software Instead of a simple client server model the configuration has expanded first to a

three tier model and now more generally to an N tier model Clients still use a browser to visit Web sites which are hosted and delivered by Web servers But to increase quality attributes such as security reliability availability and scalability as well as functionality most of the software has been moved to a separate computer the application server Indeed on large Web sites a collection of application servers typically operates in parallel and the application servers interact with one or more database servers that may run a commercial database The client server interaction as before uses the Internet but middleware software that handles communication data translation and process distribution often connects the Web and application servers and the application and database servers New Web software languages such as Java are easier to modify and program correctly and permit more extensive reuse features that enhance maintainability reliability and scalability The N tier model also permits additional security layers between potential crackers and the data and application business logic The ability to separate presentation typically on the Web server tier from the business logic on the application server tier makes Web software easier to maintain and to expand in terms of customers serviced and services offered Distributed computing particularly for the application servers allows the Web application to tolerate failures and handle more customers and allows developers to simplify the software design Newer design models11 12 have extended these goals even further Java Server Pages JSPs and Enterprise JavaBeans EJBs let developers separate presentation from logic which helps make software more maintainable To further subdivide the work developers can create a software dispatcher that accepts requests on the Web server tier then

30

IEEE SOFTWARE

March April 2002

forwards the request to an appropriate hardware software component on the application tier Such design strategies lead to more reliable software and more scalable Web sites Of course the technology keeps changing with the latest major addition to the technology being Microsoft s NET As of this writing it is too early to say what type of affect NET will have although it does not seem to provide additional abilities beyond what is already available Clearly modern Web sites increased functionality creates a need for increasingly complex software system integration and design strategies and development processes This leads to two exciting conclusions
I

ple US universities graduate approximately 25 000 bachelors in computer science every year but industry recently estimated that it needs more than 200 000 IT professionals 2 Even with the current economic downturn the university output is not enough If universities could double the production of computer scientists we still could not put a dent in the need Most economists and business leaders currently believe last year s many layoffs and bankruptcies in the e commerce sector resulted from temporary problems and expect significant growth in the near future I optimistically accept this prognosis if it is wrong then this article will be irrelevant anyway We can only meet this need by
I I

I

One of the largest and fastest growing software industry segments finds itself in dire need of the high end software engineering practices and processes that researchers and educators have been developing and teaching for years The new models for Web based software production and deployment require that we adapt or replace many of the research solutions available now

I

retraining experienced engineers to work with the new technology applying knowledge and technology to increase efficiency thereby reducing the number of engineers needed and finding ways to let people with less education and skills contribute

Although Web software s immaturity poses significant risk to both industry and government it also represents an opportunity for software engineering researchers and educators

We are already seeing some progress in all three of these directions
I

These conclusions imply that we need significant research progress significant education and significant training in diverse software engineering areas The software that drives the Web has become a critical part of the world s infrastructure Although Web software s immaturity poses significant risk to both industry and government it also represents an opportunity for software engineering researchers and educators Planning for the future One more important issue remains the lack of engineers skilled in Web software development A recent study from the US National Research Council found that the current base of science and technology is inadequate for building systems to control critical software infrastructure 2 Much of Web software is built from existing systems and involves complicated analysis to effectively compose and integrate these components into systems Yet the combination of a shortage of skilled IT engineers and the large number of IT jobs means companies often resort to hiring engineers who have less skills and education than desired As a small exam

I

I

Training classes and university courses in Web software engineering technologies are increasing and experiencing very high enrollment The Web software engineering courses at George Mason University are over subscribed every semester with some students being nondegree professionals seeking to improve their marketability New textbooks tools languages and standards are emerging to make Web software engineering knowledge more accessible and easier to learn use and deploy For example several XML innovations in the past year have made it a more useful and accessible language while new development tools and refinements in the standards for JSPs and EJBs allow more software to be created with less effort Automated technologies have recently allowed nonprogrammers to contribute more to Web software development When HTML was first introduced an HTML writer needed to fully know the language and be proficient with a text editor to create Web pages Recent tools provide point and click ability to create
March April 2002
IEEE SOFTWARE

31

Web pages that can even be enhanced with dynamic HTML and JavaScripts while requiring very little knowledge of HTML and programming Achieving the high quality requirements of Web software represents a difficult challenge Although other segments of the software industry have already mastered some of these such as the need for reliability in telecommunications and network routing aerospace and medical devices they have typically done so by hiring the very best developers on the market using lots of resources time developers and testing or relying on old stable software and technologies Unfortunately these solutions will not work for Web applications There are simply not enough of the best developers to implement all of the Web software needed today and few but the largest companies can afford to invest extra resources in the form of time developers and testing Finally it should be obvious that old stable software technologies will not suffice as a base for the Web it relies on the latest cutting edge software technology Although the use of new technology involves some risk it allows us to achieve otherwise unattainable levels of scalability and maintainability Indeed technological innovations in just the past three years have greatly advanced the field both in breadth and in depth And research progresses New conferences appear almost monthly and traditional conferences feature more tracks and papers on Web software issues Every new PhD student it seems wants to write a thesis on some aspect of Web software engineering and more textbooks and classes teach Web software application material When George Mason University offered a graduate course in Web software engineering in Fall 2000 the class immediately became popular with a large waiting list

W

eb software engineering presents challenging and unique research problems We currently lack the knowledge to create Web software of sufficient complexity or quality and that can be updated quickly and reliably Although Web software engineering has significant differences from traditional software engineering we can adapt much of what we already know to understanding and resolving these differences Not only are we making extraordinary progress we are also bringing much research of the past 20 years to fruition Indeed this is an exciting time to be a software engineer

Acknowledgments
This work is supported in part by the US National Science Foundation under grant CCR 98 04111

References
1 T A Powell Web Site Engineering Beyond Web Page Design Prentice Hall Upper Saddle River N J 2000 2 F B Schneider Trust in Cyberspace National Academy Press Washington D C 1999 3 President s Information Technology Advisory Committee Information Technology Research Investing in our Future Technical Report National Coordination Office for Computing Information and Communications Washington D C 1999 www ccic gov ac report 4 D A Menasc Scaling for E Business Technologies Models Performance and Capacity Planning Prentice Hall Upper Saddle River N J 2000 5 E Dustin J Rashka and D McDiarmid Quality Web Systems Performance Security and Usability AddisonWesley Boston 2001 6 L L Constantine and L A D Lockwood Software for Use A Practical Guide to the Models and Methods of Usage Centered Design ACM Press New York 2000 7 S Murugesan and Y Deshpande Web Engineering A New Discipline for Development of Web Based Systems Web Engineering 2000 Lecture Notes in Computer Science 2016 Springer Verlag Berlin 2001 pp 3 13 8 N Kassem and the Enterprise Team Designing Enterprise Applications with the Java 2 Platform Enterprise Edition Sun Microsystems Palo Alto Calif 2000 9 J Nielsen Designing Web Usability New Riders Publishing Indianapolis Ind 2000 10 M E Segal and O Frieder On the Fly Program Modification Systems for Dynamic Updating IEEE Software vol 10 no 2 Mar 1993 pp 53 65 11 Wrox Multi Team Professional Java Server Programming J2EE edition Wrox Press Chicago 2000 12 A Scharl Evolutionary Web Development SpringerVerlag Berlin 2000

About the Author
Jeff Offutt is an Associate Professor of Information and Software Engineering at George Mason

University His current research interests include software testing analysis and testing of Web applications object oriented program analysis module and integration testing formal methods and software maintenance He received a PhD in Computer Science from the Georgia Institute of Technology He served as program chair for ICECCS 2001 and is on the editorial boards for IEEE Transactions on Software Engineering Journal of Software Testing Verification and Reliability and Journal of Software and Systems Modeling He is a member of the ACM and IEEE Computer Society Contact him at the Dept of Information and Software Engineering Software Engineering Research Lab George Mason University Fairfax VA 22030 4444 ofut ise gmu edu www ise gmu edu faculty ofut

For more information on this or any other computing topic please visit our Digital Library at http computer org publications dlib

32

IEEE SOFTWARE

March April 2002