1. 3-tier application architecture A technical view
Database Management
3-tier application architecture
A technical view

2. Themes
Architecture : The large scale structure of a system, especially a computer system
Communications : The interaction between components
Database: enabling applications with database driven applications

3. Displaying a Web page in a Browser
Click on a link on a page
<a href=
URL is decoded: protocol:://host/file
Host is decoded using DNS to get the host IP address ….
Page requested from HTTP server on the host (apache)
Page is returned
HTML codes determine how text is rendered
If page contains links e.g. <img src=red.gif>, browser must repeat the same process on each

4. 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…

5. 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

6. Bus timetable BUS (Level 1 Diagram) users a information need
presentation
layer 1
application 2
HTTP requests
persistance 3
SQL requests
tables
HTML files
information
service
providers b
timetables
Browser such as
IE6, FoxFire
Apache or MS IIS
server +
PHP
MySQL RDBMS
Bus timetable

7. 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

8. Persistence layer arch
support application 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.

9. 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 using SQL
Handling the returned result codes and tables
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]

10. 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.
Microsoft’s 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

11. 3-tier Issues Advantages: Challenges: 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

12. 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

13. Sequence diagram of SMS bus times request

14. 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

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

16. SMS straw poll A simple application to tally votes on an issue.
A simple database to hold details of candidates and votes.
Votes are texted into the server from a mobile phone, and are processed by cast.php
At any time, the current status of the poll can be displayed using tally.php

17. Casting a vote

18. The simple database

19. The generated SQL CREATE TABLE tcandidate( candid CHAR(1) NOT NULL,
candname VARCHAR(40) NOT NULL,
CONSTRAINT pk_tcandidate PRIMARY KEY (candid)
) TYPE=INNODB;
CREATE TABLE tvoter(
voteid CHAR(10) NOT NULL,
CONSTRAINT pk_tvoter PRIMARY KEY (voteid)
ALTER TABLE tvoter
ADD INDEX (candid),
ADD CONSTRAINT fk1_tvoter_to_tcandidate
FOREIGN KEY(candid) REFERENCES tcandidate(candid)
ON DELETE RESTRICT
ON UPDATE RESTRICT;

20. Initialising the database
delete from tcandidate;
insert into tcandidate values('B', 'George Bush');
insert into tcandidate values('K', 'John Kenney');

21. The cast script (simplified)
<?php /*
input
text the vote
from the originating mobile number */
// map text to candidate id
$query = "select * from tcandidate where '$text' = candid";
$dbresult = mysql_db_query($db,$query,$dblink);
$cand = mysql_fetch_object($dbresult);
$cid = $cand->candid;
// insert the vote for this mobile
$query = "insert into tvote values('$from', '$cid')";
print "Reply: you cast your vote for $cand->candname";
?>

22. The tally script (simplified)
<?php
// get the total number of votes cast
$query = "select count(*) as total from tvote";
$dbresult = mysql_db_query($db,$query,$dblink);
$total_tally = mysql_fetch_object($dbresult);
$total = $total_tally->total;
// get the total for each candidate
$query = "select *, count(*) as tally
from tvote natural join tcandidate
group by tcandidate.candid";
//generate the results table
while($cand = mysql_fetch_object($dbresult)) {
$tally = $cand->tally;
$tpc= round(( $tally/ $total ) * 100,2);
$tpcr = round($tpc,0);
print "$cand->candname has $tpc % <br/>"; }
print "$total votes cast";
?>

23. Exam application Input is a spreadsheet
Design a suitable normalised data model