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than "http:". If the user clicks on the anchor text (Philip's emailaddress), the browser will pop up a "send mail to philg@mit.edu"window.2.8 Picking a Programming EnvironmentNow you get to pick a programming environment for the rest of thesemester. If you've been building RDBMS-backed Internetapplications for some time, you can just use whatever you've beenusing. Switching tools is seldom a path to glory. If you haven't builtthis kind of software before, read on...Choosing an RDBMSThere are probably three reasonable choices for an RDBMS thissemester: (1) Microsoft SQL Server, (2) Oracle, and (3) PostgreSQL.For experienced Windows programmers, Microsoft SQL Server iseasy to install and administer. And if you expect to spend the rest ofyour professional life in a Microsoft environment you might as welllearn it.Concurrency is Oracle's strongest suit relative to its commercialcompetitors. In Oracle, readers never wait for writers and writersnever wait for readers. Suppose the publisher at a large site starts aquery at 12:00 PM summarizing usage by user. Oracle might have tospend an hour sifting through 200 GB of tracking data. The diskdrives grind and one CPU is completely used up until 1:30 PM.Further suppose that User #356712 comes in at 12:30 PM andchanges his email address, thus updating a row in the users table. Ifthe usage tracking query arrives at this row at 12:45 PM, Oracle willnotice that the row was last modified after the query started. Underthe "I" in ACID, Oracle is required to isolate the publisher from theuser's update. Oracle does this by reaching into the rollback segmentand producing data from user row #356712 as it was at 12:00 PMwhen the query started. Here's the scenario in a table:Time Publisher12:00PM12:30PM26Starts a 90-minute query --summarizing usage for precedingyearusage summary continues to chugawayPublic Web ApplicationUser #356712 updatesemail address from"joe@foobar.com" toPersistenceThe continued existence of data. A persistence mechanismis something that provides long-term data storage, evenwhen the application that created the data is no longerrunning. Examples include RDBMSes, XML documents, andflat-file databases.RDBMSRelational Database Management System. A computerprogram that lets you store, index, and retrieve tables ofdata. The simplest way to look at an RDBMS is as aspreadsheet that multiple users can update. The mostimportant thing that an RDBMS does is providetransactions.Request ProcessorPortion of a Web server program that decides how to handlean incoming request. A well-designed request processorenables a publisher to expose only abstract URLs, e.g.,"glossary" rather than "glossary.html". The job of the requestprocessor is to dig around in the file system and find adocument to deliver or a script to execute.RMSRichard M. Stallman. In 1976, developed Emacs, the world'sbest and most widely used text editor. Went on to developgcc, the most widely used compiler for the C programminglanguage. Won a $240,000 MacArthur fellowship in 1990.Stallman is the founder of the free software movement (seewww.fsf.org), and Project GNU, which gave rise to Linux.RobotIn the technologically optimistic portion of the 20th century,robots were intelligent anthropomorphic machines thatunderstood human speech, interpreted visual scenes, andmanipulated objects in the real world. In the technologicallyrealistic 21st century, robots are absurdly primitive programsthat do things like "Go look up this book title at threedifferent online bookstores and see who has the lowestprice; fail completely if any one of the online bookstores hasadded a comma to their HTML page." Also known asintelligent agents (an intellectually vacuous term but usefulfor getting tenure if you're a university professor). Somesimple but very useful examples of robots are the spiders orWeb crawlers that fill the content database at public searchengine sites such as AltaVista.ScalableA marketing term used to sell defective software toexecutives at big companies. Internet applications arefundamentally concerned with processing updates from323
OraclePerlnamed XYZ" without the programmer having to know howmany disk drives the computer has or what companymanufactured those drives. Examples of operating systemsare Unix and Windows XP. Examples of things that try to beoperating systems but mostly fail to fulfill the "coexistpeacefully" condition are Windows 98 and the MacintoshOS.Oracle is the most popular relational database managementsystem (RDBMS). It was developed by Larry Ellison'sOracle Corporation in the late 1970s.Perl is a scripting language developed by Larry Wall in 1986to make his Unix sysadmin job a little easier. It unifies abunch of capabilities from disparate older Unix tools. LikeUnix, Perl is perhaps best described as "ugly but fast anduseful". Perl is free, has particularly powerful stringprocessing operators, and quickly developed a largefollowing and therefore library for CGI scripting. For moreinfo, see www.perl.com or www.perl.org.Historical Note: Lisp programmers forced to look at Perlcode would usually say "if there were any justice in thisworld, the guys who wrote this would go to jail." In a rarecase of Lisp programmers getting their wish, in 1995 IntelCorporation persuaded local authorities to send RandalSchwartz, author of Learning Perl (O'Reilly 2001), to the BigHouse for 90 days (plus 5 years of probation, 480 hours ofcommunity service, and $68,000 of "restitution" to Intel).Sadly, however, it seems that Schwartz's official crime wasnot corrupting young minds with Perl syntax and semantics.Most Unix sysadmins periodically run a program called"crack" that tries to guess user passwords. When crack issuccessful, the sysadmins send out email saying "yourpassword has been cracked; please change it to somethingharder to guess." Obviously they do not need the passwordssince they have root access to all the boxes and can readany of the data contained on them. At a university, you getpaid about $50,000/year for doing this. In Oregon if you dothis for a multi-billion dollar company that has recentlydonated $100,000 to the local law enforcement authorities,you've committed a crime. Seehttp://www.lightlink.com/spacenka/fors/ for more on State ofOregon v. Randal Schwartz.12:45PM1:30PMusage summary arrives at User --#356712; Oracle reaches intorollback segment and pulls out"joe@foobar.com" for the report,since that's what the value was at12:30 PMusage summary report completes --"joe@yahoo.com"How would this play out in Microsoft SQL Server? When you'rereading, you take read locks on the information that you're about toread. Nobody can write until you release them. When you're writing,you take write locks on the information that you're about to update.Nobody can read or write until you release the locks. In the precedingexample, User #356712 would submit his request for the addresschange at 12:30 PM. The thread on the Web server would beblocked waiting for the read locks to clear. How long would it wait? Afull hour with a spinning/waving "browser still receiving information"icon in the upper right corner of the browser window. If you'rethoughtful you can program around this locking architecture in SQLServer but most Internet service operators would rather just installOracle than train their programmers to think more carefully aboutconcurrency.The open-source purist's only realistic choice for an RDBMS isPostgreSQL, available from www.postgresql.org. In some ways,PostgreSQL has more advanced features than any commercialRDBMS and it has an Oracle-style multi-version concurrency system.PostgreSQL is easy to install and administer but is not used byoperators of large services because there is no way to build a trulymassive PostgreSQL installation or one that can tolerate hardwarefailures.Most of the SQL examples in this book will use Oracle syntax. This ispartly because Oracle is the world's most popular RDBMS but mostlybecause Oracle is what we had running at MIT when we startedworking in this area back in 1994 and therefore we have whole filesystems full of Oracle code. Problem set supplements (see end ofchapter) may contain translations for ANSI SQL databases such asMicrosoft SQL Server and PostgreSQL.32227
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Page 5 and 6: end-users. We use every opportunity
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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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