1. Webapp Design with System Sequence Diagrams
Webapp Design with System Sequence Diagrams

2. A Key Software Design Principle…
Also: Each of your objects should have only one reason to change.
From Head First Software Development Chapter 5

3. Design Questions: How many servlets should you have
Design Questions: How many servlets should you have? How to assign responsibilities to servlets?

4. Creating System Sequence Diagrams (SSDs) can help!
Model interactions between the system to be build and external actors
Capture one scenario of events
System is a black box
Emphasizes system events
Represented using UML sequence diagram notation

5. Consider a Use Case (UC) for a POS System
Point-of-Sale (POS) System
Use Case: Archetypal scenario of interaction w/ system

6. Process Sale Use Case (UC)
Main Success Scenario
Customer arrives at POS checkout with goods to purchase
Cashier starts a new sale
Cashier enters item identifier
System records sale line item and presents item description, price, and running total
Cashier repeats steps 3-4 until indicates done
System presents total, and asks for payment
Customer pays and System handles payment
System logs completed sale
System presents receipt

7. POS Example: Process Sale SSD
External actor
System as black box
Actor lifelines
Time progresses downward

8. Process Sale UC Main Success Scenario
Customer arrives at POS checkout with goods to purchase
Cashier starts a new sale
Cashier enters item identifier
System records sale line item and presents item description, price, and running total
Cashier repeats steps 3-4 until indicates done
System presents total, and asks for payment
Customer pays and System handles payment
System logs completed sale
System presents receipt

9. POS Example: Process Sale SSD
Message event

10. Process Sale UC Main Success Scenario
Customer arrives at POS checkout with goods to purchase
Cashier starts a new sale
Cashier enters item identifier
System records sale line item and presents item description, price, and running total
Cashier repeats steps 3-4 until indicates done
System presents total, and asks for payment
Customer pays and System handles payment
System logs completed sale
System presents receipt

11. POS Example: Process Sale SSD
Looping event
Loop guard
“Return” values from previous message

12. Process Sale UC Main Success Scenario
Customer arrives at POS checkout with goods to purchase
Cashier starts a new sale
Cashier enters item identifier
System records sale line item and presents item description, price, and running total
Cashier repeats steps 3-4 until indicates done
System presents total, and asks for payment
Customer pays and System handles payment
System logs completed sale
System presents receipt

13. POS Example: Process Sale SSD

14. Process Sale UC Main Success Scenario
Customer arrives at POS checkout with goods to purchase
Cashier starts a new sale
Cashier enters item identifier
System records sale line item and presents item description, price, and running total
Cashier repeats steps 3-4 until indicates done
System presents total, and asks for payment
Customer pays and System handles payment
System logs completed sale
System presents receipt

15. POS Example: Process Sale SSD

16. Why are SSDs useful?
Help you come up with a minimal set of system operations your system must support
Each system operation can be a servlet
System operations

17. System events discovered with the SSDs
reveal
System operations that handle the events
all of which form
The system interface

18. Translate this SSD into a servlet-based web app
New Sale
itemID:
Quant.:
Enter
Description: …
Total: $ ...
End
Total+tax: $ ...
Payment: $
Enter
Change: $...
Receipt: … …

19. Design Question: What standard actions should a servlet perform?
Validate input!
Perform appropriate interaction with model
Forward request to appropriate view

20. What types of bad input should you check for?
Hint:

21. What types of bad input should you check for?
Values that violate preconditions of model operations
SQL injection attacks
If input contains evil SQL snippet, and your program inserts that input into an SQL query, then ...
Cross-site scripting attacks
If input contains evil JavaScript snippet, and your program inserts that input into a web page, then …
Anything that might be a problem (hard!)

22. Recap Single-Responsibility Principle (SRP)
Create SSDs to reveal system operations
Each servlet has responsibility to handle one system operation
Make sure your servlets validate input!