week 23 - revision1 week 23 revision dsa. week 23 - revision2 agenda section a: multiple choice...

Week 23 - Revision 1

Week 23

Revision

DSA

http://dsa2006.blogspot.com/


Agenda

• Section A: Multiple choice

• Section B: Problem-oriented questions

• Topics for revision

• Review of issues arising from coursework

• Tutorial – work on section B questions


Section B

• Explaining and designing 3 tier applications based on a relational or an XML database

• Data modelling in ER models and XML structures– Location based data

• Understanding the languages used – PHP (good reading knowledge)– XML based languages (XSLT, XQuery) general understanding

• Tackling problems in design e.g.– Redundancy– Separation of concerns– State

• Alternatives in design – which technology to use – e.g. Relational v XML database, scripting languages


Topics

• Describing a web application

• XML technologies

• Redundancy

• Schema and data – separating the specific from the general

• Data- appearance separation


Describing web applications

• What the components are:– Developed resources

• html pages, scripts, data file, images

– Data sources outside the application• RSS feeds

– Infrastructure components needed• Web server, browser, plugins.

• How components interact to carry out a task ( a use case)– Use a sequence diagram– Use a narrative – tell a story


Meta-data for an active component (e.g. a PHP script)

• Admin– Who wrote it when– What work remains

• Interface– What inputs it has (parameters, data sources)– What sub-components it relies on– What it produces

• Function– An overview of what it does– Tricky parts


http://www.youtube.com/watch?v=quLDNFTJKU0


Sequence Diagrams

• Describe a specific sequence of actions of system components and interchanges between components

• Components represented as columns• Time goes down the page • Include only relevant components• Describes a specific ‘move’, not all possible

‘moves’• Examples

– Typical CW site

http://www.cems.uwe.ac.uk/xmldb/rest/db/chriswallace/sequence/makesd3.xql?name=track2


Diagrams

• Use an established convention

• Consider using a diagramming tool which supports the notation you want to use but get practice in hand drawing diagrams for the exam.

• Use different icons or symbols to distinguish different kinds of components


XML, XML Schema, XSLT, XQuery

• XML is a data representation format– e.g.track2.xml

• XML Schema is an (XML) language for defining the structure of a family of XML documents – e.g sd.xsd– Could we write a schema for XML schema?

• XSLT is an (XML) language for defining a transformation from one or more XML documents to an other

• XQuery is a (non XML) language for defining transformations, with an emphasis on quering an XML document or database of documents


SequenceDiagram name="track2"> <cast> <actor name="user" color="#CCEBD7"/> <actor name="browser" color="#CCC4D7"/> <actor name="GoogleEarth" color="#CCC477"/> <actor name="server" color="#FFEBFF"/> <actor name="showPlace.php" color="#DDFFAA"/> <actor name='weather.php' color='pink'/> <actor name="places.xml" color="red"/> <actor name='Yahoo' color='grey'/> </cast>


<trace> <action at="browser" do="render index page"/> <message from="user" to="browser" text="click on Racetracks"/> <message from="browser" to="server" text="get links.html"/> <action at="server" do="locate links.html"/> <message from="server" to="browser" text="links.html"/> <action at="browser" do="render links.html"/> <message from="user" to="browser" text="click on "Thruxton""/> <message from="browser" to="server" text="get showplace.php?name=Thruxton"/> <message from="server" to="showPlace.php" text="start (name=Thruxton)"/>…..


XSLT <xsl:variable name="nactors"> <xsl:value-of select="count(//cast/actor)"/> </xsl:variable> <xsl:variable name="width" select="100 div $nactors"/> <xsl:template match="/SequenceDiagram"> <table border="1"> <tr> <xsl:for-each select="//cast/actor"> <th width=“{$width%}" bgcolor=“{@color}"> <xsl:value-of select=“{@name}"/> </th> </xsl:for-each> </tr> <xsl:apply-templates select="trace/*"/> </table> </xsl:template>


XQuery<table border='1'><tr>{for $a in $sd/cast/actor return <th width='$width'>string($a/@name)</th>}</tr>for $x in $sd/trace/*

return <tr> { if (name($x)='action') then let $p := index-of($sd/cast/actor/@name,$x/@at ) return ( for $i in (1 to $p - 1) return <td/>, <td align='center'> if ($x/@url) then <a href='$x/@url' target='demo'>string($x/@do)</a> else string($x/@do) </td> ) else if (name($x)='message')


Redundancy

• The same text repeated a number of times causes problems in programming

• E.g. where the same code to access an RSS feed appears in 6 different but very similar scripts

• Why is this a problem?


Problems with redundancy

• If an fault was found in the RSS code, it would need fixing in many places, not just in one place

• If another place was added, the correct version of the code would need to be located

• If all place code was removed, the rss access code would be lost too.


Solution - Factoring• ‘Factoring’, normalisation..

– 42 = 2 * 3 * 7• Factor out the common text

– Into a module (function) for code• myFun( a, b)

– Into constant and variable parts• mylongurl?name=• XXX

– Into data and schema• Emp (name, number) – (‘fred,’3), (‘joe’,90)

• Recombine– At run-time, make the general script behave in a specific way by combining the constant part

with the variable part– Parameters on functions

• myFun( 10 , 20)– Concat($baseurl,$code,’.xml’)– SQL engine

• select * where name=‘joe’


Normalisation in Database

• Just the same problems

• Factor out the repeated data into a separate table and link with a common key

• To recreate the data, de-normalize using a join to get back to the combined data


Data and appearance3 Tier architecture

• Separating the base data – Data layer

• Relational tables, XML data

• from how it is used in an application– Business or application layer

• Server side scripting

• from how it appears to a user– Presentation layer

• CSS, Javascript

• Distinction is crucial but hard to make


Tutorial

• Work through a couple of the questions

• Answer pointers available next week

week 23 - revision1 week 23 revision dsa. week 23 - revision2 agenda section a: multiple choice...

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