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Overview: Data Flow Diagrams (DFDs)
Used to:
• Specify the flow of data
•
Specify the functions
Features:
• Divide and concur
Dividing the system into a number of small separate tasks and then link them
together.
•
Levels of abstraction
Use different levels starting from a simple level to a more abstract level.
•
Information hiding
The structure, the statements that access the structure, and the statements that modify
the structure should be part of just a single module. This is an approach to structuring
software in a highly modular fashion.
•
DFDs are constructed using different levels
o Starting from Level 0 (Context Level) the includes only the terminals and one process
representing the system.
o Then Level 1 , Level 2, ... Level n. (Levels 2 to n cannot have terminals).
•
A DFD should end with PSPECs and CSPECs.
•
PSPEC (Process Specification)
A description of what is happening inside each bottom-level, primitive bubble in a DFD.
PSPECs are either described using pseudo code, or graphically using flowcharts, decision
tables, etc...
•
CPSEC (Control Specification)
Used to indicate how the system behaves when an event or control signal is sensed.
DFD Notations
Two main notations:
• Gane/Sarson
• Yourdon/DeMarco
Symbols Gane/Sarson
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Symbols Yourdon/DeMarco
Control and Event Flows
•
In conventional data flow diagrams, control or event flows are not represented explicitly.
•
A specialised notation for representing event flows and control processing has been
developed.
•
This notation can be inputted directly to a (conventional) process or into a control process.
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Demonstration: A Basic DFD for a University Course Registration
System
Level 0 (Context Diagram)
********************
Level 1
********************
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Level 2
Level 2 for Validate Inputted Details
Level 2 for Process Application
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Super imposed
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Example: Maltese number plate registration system
(Taken from CSA1011 past paper [February 2005])
Draw up a DFD of a very simple Maltese car number plate registration system storing registered
number plates on a database. Your system should offer the following functionality:
•
•
•
•
•
Register (add) a number
Retire (delete) a previously registered number
Check number for validity
Check numbers for uniqueness against a database of assigned numbers
Confirm valid registration
You should decompose your DFDs to whatever level you deem necessary to produce PSPECs in any
form of pseudo code you wish. All PSPECs must be shown.
Level 0 (Context Diagram)
********************
Level 1
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Level 2
Register Car
Delete Car
C
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Some DFDs Exercises
Exercise 1 (Taken from CSA 1022 past paper 2009)
Construct a basic DFD to model the possible behaviour of an on-line shopping website. The website
should allow visitors to register, edit their profile, browse, share a common forum for registered
visitors, and of course purchase products. You should propose a DFD model decomposed not
further than level one.
Exercise 2 (Taken from CSA 1022 past paper 2010)
Propose DFD (i.e. excluding control) to describe the behaviour of a hypothetical and trivial on-line
social interaction environment. The solution should support the following eleven features:
1. Guest login;
2. User registration;
3. User login;
4. Profile creation and editing;
5. Upload of images and PDF data up to a limit of 2 Gb;
6. Removal of images and PDF data;
7. Browsing of other registered users’ data;
8. Designation and separation of “friends-only” and “public” data;
9. Designation of registered users as friends, and vice-versa;
10. System administration features (treat this as one general feature);
11.User de-registration.
The DFD should be described up to level one. In level one, select the sub-system describing features
number 2 and number 5 in the list above and decompose them to level two including any WardMellor extensions (i.e. using both DFD & CFD notation) deemed necessary. No FSMs or DSDs are
required.
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Exercise 3 (Taken from CSA 1022 past paper 2011)
A very basic patient online appointment management solution allows a patient to do a number of
things, namely to (not necessarily in this order):
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Register/Unregister from/on the system;
Log in/out;
Enter/edit their personal details;
Enter/edit the details f their ailment and other comments;
View doctor availability;
Book/edit/cancel appointments;
View one’s personal history;
Send e-Mail messages to clinic reception staff;
Print appointment slip(s);
Manage payments and billing.
Propose a DFD (excluding control) to describe the functional behaviour of this system. The solution
should be described up t and including level one. In level one, select the sub-system describing
feature number 3 and decompose it to level two. In this case, use any Ward-Mellor extended
notation deemed necessary. No FSMs or DSDs are required. Make sure all notation used in your
diagrams is clearly labelled.
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