Information Systems Development 3 Lecture 2

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Information Systems Development 3Lecture 2

• 3-tier application architecture
• A technical view

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Themes

• Architecture The large scale structure of a
  system, especially a computer system
• Communications The interaction between
  components
• Description Patterns and story-telling
• Choice
• Design decisions design decisions which face a
  developer
• Infrastructure choice selecting the most
  appropriate software from the available range.

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Displaying a Web page in a Browser

• lta href http//www.cems.uwe.ac.uk/cjwallac/UFIE
  8V-2004/l2_3tier.htmlgtLecture 2lt/agt
• URL is decoded protocol/host/file
• Page requested from host
• Page is returned
• HTML codes determine how text is rendered
• If page contains links e.g. ltimg srcred.gifgt,
  browser must repeat the same process on each

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Dynamic Web page needed when

• consistent look and feel on each page of a large
  site
• data is derived from a database
• depends on real time
• pages depend on user choice
• business transactions e.g. e-commerce

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3-tier architecture

• A Presentation layer using Browser technology
• An Application layer using a web application
  server platform application programs
• A Persistence layer using a relational database
  or other data store technology

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Presentation layer arch

• Issue request to remote server
• accept the returned HTML (or JPEG..) file
• render (i.e. create a 2-d image ) the HTML
• allow plug-ins to handle new file types
• execute client-side scripts in JavaScript
• support interaction between client-side scripts
  and the web page
• accept user input via a variety of controls on a
  form

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Persistence layer arch

• interaction with the database using standard
  languages e.g SQL queries and table returns
• define and modify the data structures (e.g.
  tables) themselves ( the Database Schema)
• insert, update and delete data
• maintain data persistently, with backup and
  recovery
• handle transactions to support concurrent access
  to the database via locking et
• optimise access by compilation of queries,
  indexing, replication of tables etc.

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Application Layer arch

• accepting requests from the Presentation layer
• interpreting the requests according to business
  rules and past transactions from this client
• requesting the appropriate data from the
  Persistence layer
• computing derived data
• creating the HTML (or GIF, MIDI..) for the page
• providing run-time support for
• thousands of concurrent users
• compilation to machine code
• multi-threading allow multiple processes to run
  concurrently
• caching holding results in a temporary store to
  reduce re-calculation

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Web Application Platforms

• Server side includes - files with a .shtml
  extension
• Cgi using Perl or other scripting language
• PHP like SSI, a scripting language which is
  embedded in an HTML page.
• Microsofts ASP (Active Server Pages )
• J2EE
• JSP for page scripting
• Java class library
• Multiple runtime vendors Tomcat, BEA
• .NET is the latest technology from MS.
• range languages (VB.NET, C, , C, Cobol... )
• compiled to a common intermediate code (MSIL)
• Extensive class library
• ASP.NET scripting page scripting

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3-tier Issues

• Advantages
• Re-use of appropriate software
• Software can be located on different machines for
  convenience and performance
• Resolves one-many relationships many clients
  one application, many applications one database
• Challenges
• well-defined, standardised interfaces between
  layers required.
• Software must be built to conform to interface
  standards
• The designer has to decide where to locate
  specific functions

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Communication

• The glue in this architecture is communication
  between software in the layers
• A single request from a user results in a complex
  flurry of communications and executions
• The flurry is composed of hundreds of simple
  interactions
• Sequence diagrams can be useful to provide a
  simplified description

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Simple Interaction

• Call/reply pattern

callee
caller
parameters
function
reply
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A function call in PHP

• lt?php print date(HI) ?gt in a php script
• the script is the caller
• the date procedure is the callee
• the string HI is the parameter
• the function is to get the current date and time
  
• the reply a string containing the time in the
  required format e.g. 1345

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A browser requests a page

• The user clicks on a link on a page. The link has
  an hrefhttp//host/dir/page.html
• The browser is the caller
• The web server program(Apache or IIS) on the
  machine host is the callee.
• The string dir/page.html is the parameter
• The function is to read that file in its file
  store
• The reply is the contents of the file

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A browser requests a script to execute

• The user clicks on a link on a page.
  hrefhttp//host/dir/getemp.php?empno5
• The browser is the caller
• The web server on the machine host is the callee.
• The string dir/getemp.php?empno5 is the
  parameter
• The function when the suffix is php is to find
  the script, call it with the parameter and get
  the output.
• The reply is the reply from getemp.php

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Server runs a PHP script

• The web server is the caller
• The getemp.php script is the callee
• The parameter is empid 5
• The function is to find the details of employee 5
  and format a report
• to perform this function, the script calls
  mysql_connect procedures which in turn calls the
  mySQL server .
• The reply is HTML text

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Simple Interaction Variations

• How does the caller find the callee?
• What does the caller do when is waiting for a
  reply?
• What happens if the callee doesnt reply?
• How long should the caller wait until it thinks
  the callee isnt going to reply?
• Is a reply always required?

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Sequence diagram of SMS bus times request
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A Pattern

• A pattern recurs in different guises
• have the same overall structure and purpose,
• with slight differences
• Patterns help us understand and describe complex
  systems
• Patterns normalise descriptions one thing in
  one place
• Same unit plays different roles
• webserver is callee with respect to (wrt)
  browser, caller wrt getemp.php
• Patterns are fractal

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Other layered architectures

• ANSI/SPARC Database model
• The external or user level, characterised by
  Views
• The Conceptual level, comprising the full
  corporate data model at a logical level
• The Physical level, where data is stored in files

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Channels between Layers

• Layered communications model (Tanenbaum)
• Application Layer
• Transport Layer
• Network Layer
• Data Link Layer
• Physical Layer
• Presentation ltgt Middleware
• HTTP (The protocol which supports GET and POST
  messages) carrying a limited number of types of
  content (MIME types)
• Middleware ltgt Data stores
• ODBC with SQL request and table returns (whole
  table or row at a time)

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Review Questions

• Place the following terms in the 3-tier
  architecture
• JavaScript, SSI, J2EE, MySQL, Oracle, CSS, PHP,
  ASP.NET, Flash, Java Applets, JSP, Dreamweaver,
  Visual .NET

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Straw Poll application

• Which parts belong to which tier of the 3-tier
  model script?
• What is the ER model of the database?
• What must be done to make selected improvements
  to this application?
• Describe the interaction when a user votes when
  the tally page is requested.

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Platform Comparision

• Enterprise solutions
• J2EE versus .NET
• Simple words such as better or faster are
  best used by simpletons. Life, especially when
  it involves critical enterprise applications, is
  more complicated.
• PHP versus .NET
• Language wars

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Comparison criteria

• Other considerations, such as the perception of
  the vendor as an enterprise company,
  conformance to standards, strength of the
  ecosystem around the product, the degree to which
  the product is a de facto standard amongst its
  class, stability of the product or technology,
  ease of use, IDE integration and other tool
  support, total cost of ownership (TCO),
  portability and lock-in (or lack thereof),
  scalability and many other such factors often
  outweigh Performance among our customers.