Software Engineering for Internet Applications - Student Community

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2.5 Why the Relational Database ManagementSystem?Why is the relational database management system (RDBMS) thedominant technology for persistence behind a Web server? There arethree main factors.The first pillar of RDBMS popularity is a declarative query languagecalled "SQL". The most common style of programming is notdeclarative; it is called "imperative" or "procedural". You tell thecomputer what to do, step by step:20• do this• do this• do this• if it is after March 17, 2023, do this, this, and then this;otherwise do this• do this ...Programs written in this style have two drawbacks. First, they quicklybecome complex and then can only be developed and maintained byprofessional programmers. Second, they contain a lot of errors. Forexample, the program sketched above may have quite a few bugs. Itis not after March 17, 2023. So we can't be sure that the stepsspecified in the THEN clause of the IF statement are error-free.An alternative style or programming is "declarative". We tell thecomputer what we want, e.g., a report of users who've beenregistered for more than one year but who haven't answered anyquestions in the discussion forum. We don't tell the RDBMS whetherto scan the users table first and then check the discussion forumtable or vice versa. We just specify the desired characteristics of thereport and it is the job of the RDBMS to prepare it.Stop someone in the street. Pick someone with fashionable clothingso you can be sure he or she is not a professional programmer. Askthis person "Have you ever programmed in a declarative computerlanguage?" Follow that up with "Have you ever used a spreadsheetprogram?" Chances are that you can find quite a few people who willtell you that they've never written any kind of computer program butyet they've developed fairly sophisticated spreadsheet models. Why?The spreadsheet language is declarative: "Make this cell be the sumof these three other cells." The user doesn't tell the spreadsheetTo the InstructorThank you for considering this textbook. This section is intended tohelp you use it effectively for students at the following levels:• juniors and seniors in Computer Science taking a one-termcram course in Internet application design (the MIT way)• juniors and seniors in Computer Science taking a one-year"Capstone" course in software engineering• seniors in Computer Science doing a capstone independentstudy project or bachelor's thesis• sophomores in Computer Science or non-majors spending asemester learning about building modern informationsystemsWith respect to these goals, we will treat the following issues: (1)what to do during lectures, (2) how to find clients for your students,(3) what to put on exams, (4) how to find and use alumni mentors,and (5) evaluation and grading.Before plunging into these issues, let's take a step back and reflecton the rationale for teaching this material at all.A step backWhy is software engineering part of the undergraduate computerscience curriculum? There are enough mathematical and theoreticalaspects of computer science to occupy students through a bachelor'sdegree. Yet most schools have always included at least some handsonprogramming. Why? Perhaps there is a belief that someone withan engineering degree ought to be able to engineer the sorts ofsystems that society demands. In the 1980s, users wanted desktopapplications. Universities adapted by teaching students how to builda computer program that interacted with a single user at a time,processing input from the mouse and keyboard and displaying resultsgraphically. Starting in the early 1990s, however, demand shiftedtoward server-based Internet applications. With 1000 userspotentially attempting the same action at the same instant, thetechnical challenge shifts to managing concurrency and transactions.Given stateless protocols such as HTTP, software engineers mustlearn to develop stateful user experiences. Given the ubiquitousnetwork and evolving standards for remote procedure calls, studentscan learn practical ways of implementing distributed computing.329
has permission to perform each task. Most task performersare humans but they can also be automated processes.WSDLWeb Services Description Language. A way for a Webserver to answer, in a machine-readable form, the question"what services do you provide?" with said services ultimatelyto be provided by SOAP. See http://www.w3.org/TR/wsdl.WYSIWYGWhat You See Is What You Get. A WYSIWYG wordprocessor, for example, lets a user work view an on-screendocument as it will appear on the printed page, e.g., withtext in italics appearing on-screen in italics. This approach tosoftware was pioneered by Xerox Palo Alto ResearchCenter in the 1970s and widely copied since then, notablyby the Apple Macintosh. WYSIWYG is extremely effectivefor structurally simple documents that are printed once andnever worked on again. WYSIWYG is extremely ineffectivefor the production of complex documents and documentsthat must be maintained and kept up-to-date over manyyears. Thus Quark Xpress and Adobe Frame facilitated atremendous boom in desktop publishing while MicrosoftFrontPage and similar WYSIWYG tools for Web pageconstruction have probably hindered development ofinteresting Web applications.XHTMLThe next generation of HTML, compliant with XMLstandards. Although it is very similar to the current HTML, itfollows a stricter set of rules, thus allowing for betterautomatic code validation. This structure also makes itpossible to embed other XML-based languages such asMathML (for equations) and SMIL (for multimedia) inside ofXHTML pages. More:www.wdvl.com/Authoring/Languages/XML/XHTML/XHTML-MPXHTML Mobile Profile. A strict subset of XHTML, used as amarkup language for wireless application development. Seethe Mobile chapter for more.XMLExtensible Markup Language, a simplified version of SGMLwith enhanced features for defining hyperlinks. As withSGML, it solves the trivial problem of defining a syntax forexchanging structured information but doesn't do any of thehard work of getting users to agree on semantic structure.328program in what order to perform the computation, merely thedesired result.The declarative language of the spreadsheet created an explosion inthe number of people who were able to develop working computerprograms. Through the mid-1970s organizations that worked withdata kept a staff of programmers. If you wanted some analysisperformed you'd call one into your office, explain the assumptionsand formulae to be used, then wait a few days for a report. In 1979Dan Bricklin (MIT EECS '73) and Bob Frankston (MIT EECS '70)developed Visicalc and suddenly most of the people who'd beenhollering for programming services were able to build their ownmodels.With an RDBMS the metaphoric little strips of paper pushed underthe door are declarative programs in the SQL language. (See SQLfor Web Nerds at http://philip.greenspun.com/sql/ for the languageitself.)The second pillar of RDBMS popularity is isolation of important datafrom programmers' mistakes. With other kinds of databasemanagement systems it is possible for a computer program to makearbitrary changes to the data set. This can be convenient forapplications such as computer-aided design systems with verycomplex data structures. However if your goal is to preserve a dataset over a 25-year period, letting arbitrarily buggy imperativeprograms make arbitrary changes isn't a good idea. The RDBMSlimits programmers to uttering very simple statements of the formINSERT, DELETE, and UPDATE. Furthermore, if you're unhappywith the contents of your database you can simply review all thestrips of paper that were pushed under the door. Each strip willcontain an SQL statement and the name of the program orprogrammer that authored the strip. This makes it easy to correctmistakes and reform offenders.The third and final pillar of RDBMS popularity is good performancewith many thousands of simultaneous users. This is more a reflectionon the refined state of commercial development of systems such asIBM DB2, Oracle, Microsoft SQL Server, and the open-sourcePostgreSQL, than an inherent feature of the RDBMS itself.2.6 The StepsWhen building any Internet application you're going to go through thefollowing steps:21
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• object-create• object-display
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The trees chapter of SQL for Web Ne
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);274-- ordering within a form, low
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and start the high-level document f
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Example Ontology 2: FlyingWe want a
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systems. What would a knowledge man
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spreadsheet". Other users can comme
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Chapter 15Metadata (and Automatic C
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{site url}{site description}en-usCo
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drawing on the intermodule API that
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At this point you have something of
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• description• URL for a photo
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Here's a raw SOAP request/response
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Chapter 14Distributed Computing wit
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conduct programmer job interviews h
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Most admin pages can be excluded fr
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content that should distinguish one
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Chapter 13Planning ReduxA lot has c
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the Internet-specific problem of no
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wouldn't see these dirty tricks unl
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12.8 Exercise 4: Big BrotherGeneral
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than one call to contains in the sa
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A third argument against the split
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way 1 1/16One might argue that this
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absquatulate 612bedizen 36, 9211cry
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What if the user typed multiple wor
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Chapter 12S E A R C HRecall from th
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long as it is much easier to remove
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features that are helpful? What fea
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made it in 1938)? Upon reflection,
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environment, we identify users by t
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those updates by no more than 1 min
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Balancer and mod_backhand, a load b
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translation had elapsed--the site w
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It seems reasonable to expect that
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11.1.5 Transport-Layer EncryptionWh
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such as ticket bookings would colla
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give their site a unique look and f
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It isn't challenging to throw hardw
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Chapter 11Scaling GracefullyLet's l
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10.15 Beyond VoiceXML: Conversation
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Consider that if you're authenticat
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In this example, we:194• ask the
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As in any XML document, every openi
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(http://www.voicegenie.com). These
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Chapter 10Voice (VoiceXML)questions
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9.15 MoreStandards information:•
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9.14 The FutureIn most countries th
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9.10 Exercise 7: Build a Pulse Page
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9.6 Keypad HyperlinksLet's look at
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text/xml,application/xml,applicatio
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Protocol (IP) routing, a standard H
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