1. Web Software Model CS 4640 Programming Languages for Web Applications
[Robert W. Sebesta, “Programming the World Wide Web]

2. Web Applications
Are user interactive software programs, deployed on a web server, accessed via a web browser
Use enabling technologies to
make web site contents dynamic
allow users of the system to implement business logic on the server
Let users affect state on the server
Constructed from diverse, distributed, and dynamically generated web components
Web components are software modules that implement different parts of the application’s functionality
An enabling technology makes web pages interactive and responsive to user input
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3. How User Interact With Web Apps
Web server
Web application
HTTP
Request
PHPs
Java servlets
JSPs
Usability
Web component1
Web component2
Web componentn
Request
HTML JS
CSS
Web client DB
Response
HTTP
Response
HTTP request / response cycle
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4. How the Web Software Works
Browser
To server
Java servlet
(idProgram)
Browser
To client
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5. Server Side Processing
data
HTTP Request
XAMPP and
Tomcat
Client
UI implemented in a browser
Server
Web server
Container engine
Program components
HTTP Response
HTML
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6. Execution Overview Response back to requestor Incoming request Server8 1
HTTP Request
HTTP Response 2 7
Web server
Request / Response Objects 3
Modified Response Object 6
Container engine
Create thread / call method 4
Return 5
Program component
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7. Page Centric and Dispatcher Designs
Page-centric (client-server)
Requests are made to a main component, and the main component response to clients
Dispatcher (N-tier)
Requests are sent to a dispatcher that then forward the requests to another component
The goal is to separate logic from presentation and
to separate as many concerns in the logic as possible
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8. (Client requests are intercepted here)
Page-centric Design
Server
Web client
Main component
(Client requests are intercepted here)
Component
(data)
Uses or instantiates
Request
Response
Web client
Database
Web client
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9. Dispatcher Design Server Request Response Dispatcher Database
Web client
Web client
Request
Response
Dispatcher
Database
Component
presentation
Data
Business processing
Web client
Usually use forward
Component presentation
Data
Component presentation
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10. Model View Controller An abstraction frequently used in web app design
Provide a way to divide the responsibilities of objects
Decreases coupling between objects and layers (supports easier maintenance)
Help divide the work (supports development expertise areas)
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11. Model View Controller (2)
data structures
HTML / JSP / frontend
Comppnents
Model
method calls
Encapsulates application state
Responds to state queries
Exposes application functionality
Notifies views of changes
events
state change
state query
change notification
View
Controller
view select
Renders the model
Requests updates from the model
Sends user inputs to controller
Allows controller to select view
Defines application behavior
Maps user actions to model updates
Selects a view for response
One view for each function
user input
* Graphic from Designing Enterprise Applications with the Java 2 Platform, Enterprise Edition, Nicholas Kassem et al., October 2000
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12. Session Management HTTP client-server communication is connnectionless
As soon as the request is made and fulfilled, the connection is terminated
Communication is simple and resistant to network problems
Stateless property
Server
send a request
response a request
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13. We will come back to this later …
Session Management (2)
How can servers keep track of state of different clients?
Session : A single coherent use of the system by the same user
Example : shopping carts
Cookies : A string of characters that a web server places on a browser’s client to keep track of a session
Usually used as an index into a table (dictionary) on the server
Most dictionaries expire after a period of time (15 to 30 minutes)
Additional mechanisms
URL rewriting
Hidden form control
We will come back to this later …
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14. Additional Web Features
User’s ability to control web application via web browser features
index
login
browse
record_add
submit a form
post (userid , password)
redirect
record_insert
post (name, category, content)
back
component
simple link transition
form link transition
redirect transition
operational transition
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15. Additional Web Features (2)
Identifying web application resources
with URLs
index
login
browse
record_add
submit a form
post (userid , password)
redirect
record_insert
post (name, category, content)
back
operational transition
component
simple link transition
form link transition
redirect transition
userlogin.jsp
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16. Additional Web Features (3)
Communication among web components depending on requests through the HTTP
index
login
browse
record_add
submit a form
post (userid , password)
redirect
record_insert
post (name, category, content)
back
component
simple link transition
form link transition
redirect transition
operational transition
get
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17. Additional Web Features (4)
Control connections: forward, include, redirect
index
login
browse
record_add
submit a form
post (userid , password)
redirect
record_insert
post (name, category, content)
back
component
simple link transition
form link transition
redirect transition
operational transition
forward
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18. Additional Web Features (5)
State scope of web components
application
session
request
forward
page
request
Client 1
page
request
forward
request
Client 2
request
page
[Jeff Offutt, Scope and Handling State]
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19. Summary Web Applications
The major difference is deployment
Software is deployed across the Web using HTTP
Other deployment methods include bundling, shrink-wrapping, embedding, and contracting
New software technologies
New conceptual language constructs for programming
Integration
Data management
Control connections
These differences affect every aspect of
how to engineer high quality software
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