Software Engineering for Internet Applications - Student Community

More documents

Recommendations

Info

LogoAd Bannerthe presentation layer with the HTTP service layer and spreading theload among multiple CPUs for one layer will automatically spread itfor the other.Navigation/Context BarThe multi-CPU box versus multiple-separate-box decision here isagain based on whether or not the presentation layer holds state. Ifno session state is held by the running presentation scripts, it is moreeconomical to add CPUs inside separate physical computers.SectionLinksFooterCONTENTAREAContent in gray is derived from the master template. Note thatdoesn't mean that it is static or not page-specific. If a template is anASP or JSP fragment it can execute arbitrarily complex computerprograms to generate what appears within its portion of the page.Content in aqua comes from the per-page template.This sounds inefficient due to the large number of file system probes.However, once a system is in production it is easy for the Web serverto cache, per-URL, the results of the file system investigation. In fact,the Web server could cache all of the templates in its virtual memoryfor maximum speed. The reason that one wouldn't do this duringdevelopment is that it would make debugging difficult. Every time youchanged a template you'd have to restart the Web server or clear thecache in order to view the results of the change.7.5 Intermodule APIsRecall from the User Registration and Management chapter that wewant people to be accountable for their actions within an onlinecommunity. One way to enhance accountability is by offering a "user11.1.4 HTTP ServiceHTTP service per se is so simple that it hardly warrants itsown layer. A high performance pure HTTP server programsuch as Zeus (see www.zeus.com) can handle more than6000 requests per second and saturate a 100Mbps networklink on a single 500MHz Celeron processor. As of January2002 it would cost roughly $25,000 per month to pay for all thebandwidth that a single-CPU HTTP server could consume.Why then would anyone ever need to deploy multiple CPUs tosupport HTTP service? The reason is that HTTP servers areusually packaged with software to support computationallymore expensive layers. For example the Oracle RDBMSserver, capable of supporting the persistence layer and theabstraction layer, also includes the necessary software forinterpreting Java Server Pages and performing HTTP service.If you were running a popular service directly from Oracleyou'd probably need more than one CPU. More commonexamples are Web servers such as IIS and AOLserver thatare capable of handling the presentation and HTTP servicelayers from the same operating system process. If your scriptsinvolve a lot of template parsing it is easy to overload a singleCPU with the demands of the Web server/script interpreter.If no state is being stored in the HTTP Service layer it ischeapest to add CPUs in separate physical boxes. HTTP isstateless and user interaction is entirely mediated by theRDBMS. Therefore there is no reason for a CPU serving apage to User A to want to communicate with a CPU serving apage to User B.142207
such as ticket bookings would collapse if they had to support thesesearches as well.If separate physical CPUs are to be employed in the abstractionlayer, should they all come in the same box or will it work just as wellto rack and stack cheap 1-CPU machines? That rather depends onwhere state is kept. Remember that HTTP is a stateless protocol.Somewhere the server needs to remember things such as"Registered User 137 wants to see pages in the French language","Unregistered user who started Session 6781205 has placed thehardcover edition of The Cichlid Fishes in his or her shopping cart."In a multi-process multi-computer server farm it is impossible toguarantee that a particular user will always be returned to the samerunning computer program, if for no other reason than you want theuser experience to be robust to failure of an individual physicalcomputer. If session state is being kept anywhere other than in acookie or the persistence layer (RDBMS), your application serverprograms will need to communicate with each other constantly tomake sure that their ad hoc database is coherent. In that case itmight make sense to get an expensive multi-CPU machine to supportthe application server. However, if all the layers are stateless exceptfor the persistence layer, the application server layer can be handledby multiple cheap 1-CPU machines. At orbitz.com, for example, racksof cheap computers are loaded with identical local copies of the fareand schedule database. Each time a user clicks to see the options fortraveling from New York to London, one of those application servermachines is randomly selected for action.11.1.3 Presentation LayerComputer programs in the presentation layer pull information fromthe persistence layer (RDBMS) and merge those results with atemplate appropriate to the user's preferences and client software. Ina Web application these computer programs are doing an SQL queryand merging the results with an HTML template for delivery to theuser's Web browser. Such a program is so simple that it is oftenreferred to as a "script". You can think of the presentation layer as"where the scripts execute".The most common place for script execution is within the operatingsystem process occupied by the Web server. In other words thescript language interpreter is built into the Web server. Examples ofthis architecture are Microsoft Internet Information Server (IIS) andActive Server Pages, AOLserver and its built-in Tcl interpreter,Apache and the mod_perl add-in. If you've chosen to use one ofthese popular styles of Web development you've chosen to merge206contributions" page that will show all contributions from a particularuser. Wherever a person's name appears within the application it willbe a hyperlink to this user contributions page.Given that all site content is stored in relational database tables, themost obvious way to start writing the user contributions page script isby looking at the SQL data models for each individual module. Thenwe can write a program that queries a few dozen tables to find allcontributions by a particular user.A drawback to this approach is that we now have code that maybreak if we change a module's data model yet this code is not withinthat module's subdirectory and this code is probably being authoredby a programmer other than the one maintaining the individualmodule.Let's consider a different application: email alerts. Suppose that yourcommunity offers a discussion forum and a classified ad system,coded as separate modules. A user wishes to get a daily summary ofactivity in both areas. Each module could offer a completely separatealerts mechanism. However, this would mean that the user would gettwo email messages every night when a single combined email wasdesired. If we build a combined email alert system, however, we havethe same problem as with the user history page: shared code thatdepends on the data models of individual modules.Finally let's look at the site administrator's job. The site administratoris probably a busy volunteer. He or she does not want to waste 20mouse clicks to see today's new content. The site administrator oughtto be able to view recently contributed content from all modules on asingle page. Does that mean we will yet again have a script thatdepends on every table definition from every module?Here's a hint at a solution. On the photo.net site each module definesa "new stuff" procedure, which takes the following arguments:• since_when -- the date of the earliest content we'reinterested in• only_from_new_users_p -- a boolean indicating whetheror not we want to limit the report to contributions from newusers (useful for site administration because new users arethe ones who don't understand community standards andnorms)143
Page 1 and 2:
SoftwareEngineering forInternetAppl
Page 3 and 4:
Signature: ________________________
Page 5 and 6:
end-users. We use every opportunity
Page 7 and 8:
• availability of magnet content
Page 9 and 10:
• we want to see if a student is
Page 11 and 12:
you supply English-language queries
Page 13 and 14:
What to do during lecturesWe try to
Page 15 and 16:
The one-term cram courseWhen teachi
Page 17 and 18:
332• spend a term learning how to
Page 19 and 20:
Once we've taught students how to b
Page 21 and 22:
has permission to perform each task
Page 23 and 24:
UDDIUnixcustomer's credit card. If
Page 25 and 26:
thousands of concurrent users. This
Page 27 and 28:
OraclePerlnamed XYZ" without the pr
Page 29 and 30:
LDAPLinuxbits per color, a vastly s
Page 31 and 32:
FilterFirewallFlat-fileGIF318functi
Page 33 and 34:
when there is an educational dimens
Page 35 and 36:
system. The authors of the core pro
Page 37 and 38:
Sign-OffsTry to schedule comprehens
Page 39 and 40:
scheduling goals that both you and
Page 41 and 42:
Client Tenure In Job (new, mid-term
Page 43 and 44:
ReferencesEngagement ManagementSQL*
Page 45 and 46:
Decision-makers often bring senior
Page 47 and 48:
presentation to a panel of outsider
Page 49 and 50:
300always been written by programme
Page 51 and 52:
17.3 Professionalism in the Softwar
Page 53 and 54:
Try to make sure that your audience
Page 55 and 56:
Chapter 17WriteupIf I am not for my
Page 57 and 58:
Suppose that an RDBMS failure were
Page 59 and 60:
analysis programs analyzing standar
Page 61 and 62:
at 9 hours 11 minutes 59 seconds pa
Page 63 and 64:
found" will result in an access log
Page 65 and 66:
15.18 Time and MotionThe team shoul
Page 67 and 68:
select 227, 891, 'algorithm', curre
Page 69 and 70:
create table km_object_views (objec
Page 71 and 72:
• object-create• object-display
Page 73 and 74:
The trees chapter of SQL for Web Ne
Page 75 and 76:
);274-- ordering within a form, low
Page 77 and 78:
and start the high-level document f
Page 79 and 80:
Example Ontology 2: FlyingWe want a
Page 81 and 82:
systems. What would a knowledge man
Page 83 and 84:
spreadsheet". Other users can comme
Page 85 and 86:
Chapter 15Metadata (and Automatic C
Page 87 and 88:
{site url}{site description}en-usCo
Page 89 and 90:
drawing on the intermodule API that
Page 91 and 92: At this point you have something of
Page 93 and 94: • description• URL for a photo
Page 95 and 96: Here's a raw SOAP request/response
Page 97 and 98: Chapter 14Distributed Computing wit
Page 99 and 100: conduct programmer job interviews h
Page 101 and 102: Most admin pages can be excluded fr
Page 103 and 104: content that should distinguish one
Page 105 and 106: Chapter 13Planning ReduxA lot has c
Page 107 and 108: the Internet-specific problem of no
Page 109 and 110: wouldn't see these dirty tricks unl
Page 111 and 112: 12.8 Exercise 4: Big BrotherGeneral
Page 113 and 114: than one call to contains in the sa
Page 115 and 116: A third argument against the split
Page 117 and 118: way 1 1/16One might argue that this
Page 119 and 120: absquatulate 612bedizen 36, 9211cry
Page 121 and 122: What if the user typed multiple wor
Page 123 and 124: Chapter 12S E A R C HRecall from th
Page 125 and 126: long as it is much easier to remove
Page 127 and 128: features that are helpful? What fea
Page 129 and 130: made it in 1938)? Upon reflection,
Page 131 and 132: environment, we identify users by t
Page 133 and 134: those updates by no more than 1 min
Page 135 and 136: Balancer and mod_backhand, a load b
Page 137 and 138: translation had elapsed--the site w
Page 139 and 140: It seems reasonable to expect that
Page 141: 11.1.5 Transport-Layer EncryptionWh
Page 145 and 146: give their site a unique look and f
Page 147 and 148: It isn't challenging to throw hardw
Page 149 and 150: Chapter 11Scaling GracefullyLet's l
Page 151 and 152: 10.15 Beyond VoiceXML: Conversation
Page 153 and 154: Consider that if you're authenticat
Page 155 and 156: In this example, we:194• ask the
Page 157 and 158: As in any XML document, every openi
Page 159 and 160: (http://www.voicegenie.com). These
Page 161 and 162: Chapter 10Voice (VoiceXML)questions
Page 163 and 164: 9.15 MoreStandards information:•
Page 165 and 166: 9.14 The FutureIn most countries th
Page 167 and 168: 9.10 Exercise 7: Build a Pulse Page
Page 169 and 170: 9.6 Keypad HyperlinksLet's look at
Page 171 and 172: text/xml,application/xml,applicatio
Page 173 and 174: Protocol (IP) routing, a standard H
show all

Software Engineering for Internet Applications - Student Community

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?