Software Engineering for Internet Applications - Student Community

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stored. Suppose by contrast you were poking around in an unfamiliardatabase and encountered this table definition:create table address_book (address_book_idinteger primary key,user_id not null referencesusers,first_names varchar(30),last_name varchar(30),emailvarchar(100),email2varchar(100),line1varchar(100),line2varchar(100),city varchar(100),state_province varchar(20),postal_code varchar(20),country_code char(2) referencescountry_codes(iso),phone_home varchar(30),phone_work varchar(30),phone_cell varchar(30),phone_other varchar(30),birthdate date,days_in_advance_to_remind integer,date_last_reminded date,notes varchar(4000));Note the use of ISO countryThe author's source code comments codes, constrained by referencehave been stripped out yet it is to a table of valid codes, toreasonably clear that this table exists represent country in the tableabove. You don't want recordsto support an online address book.with "United States", "US", "us",Moreover the purpose of each"USA", "Umited Stares", etc.column can be inferred from its These are maintained by thename. Quite a few columns will be ISO 3166 Maintenance agency,NULL for each address book entry from which you can downloadbut not so many that the table will be the most current data in textabsurdly sparse. Because NULL format. Seecolumns take up so little space in the http://www.iso.ch/iso/en/prodsservices/iso3166ma/index.html.database you shouldn't decidebetween skinny and fat based on presumed data storage efficiency.Skinny is good when you are storing wildly disparate data on eachuser, such that you'd expect more than 75 percent of columns to beNULL in a fat data model. Skinny can result in strange-looking SQLqueries and data dictionary opacity.80for the person typedate_of_birth, titlefor the language typesyntax_example, garbage_collection_p (whether thelanguage has automatic storage allocation like Lisp ormemory leaks like C), strongly_typed_p,type_inferencing_p, lexical_scoping_p, date_first_usedfor the publication typethis is for storing references to books and journal articles soyou want all the fields that you'd expect to see whenreferencing something; include also an abstract fieldfor the data structure typecomplexity_for_insertion, complexity_for_retrieval (varcharscontaining "O(1)", "O(N)", etc.)for the system typeexamples of systems are "Multics", "Xerox Alto", "TCP/IP","MIT Lisp Machine", "Apple Macintosh", "Unix", "World WideWeb". Includes fields for date_of_conception, date_of_birth,organization_name, open_source_p. No need to includefields for the names of developers because we can insteaduse links to objects of type person to represent primedevelopers or promoters.for the problem typeexamples of problems are "traveling salesman", "diningphilosophers", "sort", "query for inclusion in sorted list". We'llwant elements for storing initial conditions and solutioncriteria. In general, objects of type problem will be linked toobjects of type algorithm (algorithms that solve theproblem), publication (papers that set forth the problem),and person (people who were involved in stating or solvingthe problem)for the algorithm typeexamples include "Quicksort" and "binary search" elementsfor pseudo_code and high_level_explanation. In general,objects of type algorithm will be linked to objects of typeproblem (what need the algorithm addresses), publication(papers that describe the algorithm or implementations of it),and person (people who were involved in developing thealgorithm)For an example of what a completed system of this nature might looklike, visit Paul Black's Dictionary of Algorithms, Data Structures, andProblems at http://www.nist.gov/dads/.269
systems. What would a knowledge management system for auniversity computer science department look like?Let's assume that we'll have at least the following types in ourontology:268• person• publication• data structure• system• algorithm• problem• languageFor each of these types, we will define a table and call a row in oneof those tables an "object". To say that "John McCarthy developedthe Lisp programming language", the author would create twoobjects: one of type language and one of type person. Why notlink to the users table instead? John McCarthy might not be aregistered user of the system. Some of the people you'll bereferencing, e.g., John Von Neumann, are dead.Each object comprises a set of elements. An element is stored in acolumn. For every object in the system we want to record thefollowing elements:• name (a short description of the thing)• overview (a longer description)• who created this object• when they created it• who modified it last• when they modified it• who has the right to modify it• who has the right to view it• does it need approval?• has it been approved?• if so, by whom and when?• if so, under what section?In addition to these housekeeping elements, we will define typespecificelements:5.3 User GroupsOne of the most powerful constructs in an online community is a usergroup. A group of users might want to collaborate on publishing somecontent. A group of users might want a private discussion forum. Agroup of users might be the only people authorized to perform certainactions or view certain files. The bottom line is that you'll want to beable to refer to groups of users from other objects in your database.When building user groups you might want to think about on-the-flygroups. You definitely want to have a user group where eachmember is represented by a row in a table: "user #37 is part of usergroup #421". With this kind of data model people can explicitly joinand separate from user groups. It is also useful, however, to havegroups generated on-the-fly from queried properties. For example, itmight be nice to be able to say "this discussion forum is limited tothose users who live in France" without having to install databasetriggers to insert rows in a user group map table every time someoneregisters a French address. Rather than denormalizing the data, itwill be much cleaner to query for "users who live in France" everytime group membership is needed.A typical data model will include a USERS table and aUSER_GROUPS table. This leads to a bit of ugliness in that many ofthe other tables in the system must include two columns, one foruser_id and one for user_group_id. If the user_id column is notNULL, the row belongs to a user. If the user_group_id is not NULL,the row references a user group. Integrity constraints ensure thatonly one of the columns will be non-NULL.5.4 Representing Membership in a Group (FirstNormal Form)Suppose that you have a USERS table and a USER_GROUPS table.How do you represent the fact that User 234 is a member of Groups17 and 18? It might seem that the simplest way to do this is with adata structure stored in a column within the USERS table:create table users (user_id integer primary key,...-- a space-separated list of group IDsgroup_memberships varchar(4000),...);81
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SoftwareEngineering forInternetAppl
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Signature: ________________________
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end-users. We use every opportunity
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• availability of magnet content
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• we want to see if a student is
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you supply English-language queries
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What to do during lecturesWe try to
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The one-term cram courseWhen teachi
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332• spend a term learning how to
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Once we've taught students how to b
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has permission to perform each task
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UDDIUnixcustomer's credit card. If
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thousands of concurrent users. This
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OraclePerlnamed XYZ" without the pr
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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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