1. Data Flow Diagrams

2. Recap Collaboration Diagram Semantics – objects and messages
– static representation of messages which could be sent
– sequence of messages indicated
Collaboration Diagram Notation
– rectangles for objects
Object signature: name : Class
– messages with arrows
Message signature
– [ guard] sequence number : result := messageName(int)
Collaboration Diagram Issues
– multiplicity
– when to use

3. Contents Data Flow Diagrams (DFDs) When Where Why Notations/Elements
Examples

4. What is a Data Flow Diagram?
Known as DFDs
A way to model a real world situation
They are the interface between the real world activities and an understanding of how this can be converted into a computer system.

5. Why do we use DFDs?
It is a way of taking the physical view and converting it into a logical view.
The physical view - all documents involved
The logical view - the data they contain
Their main purpose is to communicate with the user, the analyst’s understanding of the scope of the required system

6. Where do they fit in? Life Cycle Phases Planning Feasibility Study
Analysis
Design
Code and Unit test
Analysis (What do we do?)
Fact finding
investigate business process and the current system
modelling the current and required systems
deliverables -
requirements specification
logical models of the required system

7. Data Flow Diagrams (DFD)
DFDs describe the flow of data or information into and out of a system
what does the system do to the data?
A DFD is a graphic representation of the flow of data or information through a system
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system 3
BCS - GP1 3

8. Decomposition and Abstraction
Decomposition - Divide and subdivide into manageable size problems
Abstraction - Concentrate on the important issues and ignore the irrelevant

9. 4 Main Elements
external entity - people or organisations that send data into the system or receive data from the system
process - models what happens to the data i.e. transforms incoming data into outgoing data
data store - represents permanent data that is used by the system
data flow - models the actual flow of the data between the other elements

10. Outside Entity
Is anything outside the system that is of interest to the system. Can be a person, a company or another system.
Customer a
Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it.
Customer a
If an outside entity is repeated for the purpose of neat layout a line is added across the top.

11. Data Flow
Is shown by a line with an arrowhead, indicating the direction of the flow of data. Each data flow should be named to indicate what data is being passed. Nouns or adjectives only no verbs are permitted.

12. The Levels Context - Overview - contains only one process
Level 1 - Utilises all four elements
Level 2 - A breakdown of a level 1 process
Level 3 - A breakdown of a level 2 process
There is no rule as to how many levels of DFD that can be used.

13. Rules
Sequence not important - getting the Process correct is
Context or Level 0 - Identifies the system/ boundary/External Links
Level 1 - Overview of function
Level 2 - Breakdown to Understand
Hard to know where to stop
Rule of Thumb
If there are more than 8 data flows break it
Process of Identifying major Processes

14. The Procedure for Constructing DFD’s
Draw a document flow diagram of the current situation
Draw a systems boundary around the agencies that are part of the system
Draw a Context Diagram
Identify processes in the system
Complete the level 1 Current Physical DFD

15. Notation Data Flow Process External Entity Data Store Data Flow
Process box
External
Entity D
Data Store

16. Level 0 - Context Diagram
models system as one process box which represents scope of the system
identifies external entities and related inputs and outputs
Additional notation - system box

17. Level 1 - overview diagram
gives overview of full system
identifies major processes and data flows between them
identifies data stores that are used by the major processes
boundary of level 1 is the context diagram

18. Level 2 - detailed diagram
level 1 process is expanded into more detail
each process in level 1 is decomposed to show its constituent processes
boundary of level 2 is the level 1 process

19. Other Notation Duplicates marked by diagonal line in corner
System Boundary
Elementary Processes - star in corner
Process that is levelled - dots on top

20. Rules for DFDs Numbering Labelling Balancing
Balancing - Give example of Level 1 with 3 processes, 1,2,3 and inputs between them. In level 2 add in extra flow and leave out one of the flows. Also at level 2 introduce new flows that are internal to level 2 to show that that is OK
e.g. for combining dataflows - use level 1 diagram with customer inputting deposit to accept booking process and cutomer inputting balance to issue ticket process. Context diagram then has one input flow of payment = deposit +balance. 5
BCS - GP1 5

21. Numbering On level 1 processes are numbered 1,2,3…
On level 2 processes are numbered x.1, x.2, x.3… where x is the number of the parent level 1 process
Number is used to uniquely identify process not to represent any order of processing
Data store numbers usually D1, D2, D3...

22. Labelling
Process label - short description of what the process does, e.G. Price order
Data flow label - noun representing the data flowing through it e.G. Customer payment
Data store label - describes the type of data stored
Make labels as meaningful as possible

23. Balancing and data stores
any data flows entering or leaving a parent level must by equivalent to those on the child level
Data stores
data stores that are local to a process need not be included until the process is expanded

24. Conventions Balancing
Process at lower level should have identical data flows if they flow out of a process
Modelling Data Stores
Only use DATA STORES used within this process on the diagram
Numbering
Labels
Should carry as much meaning as possible

25. Data Flows
Allowed to combine several data flows from lower level diagrams at a higher level under one data flow to reduce clutter
Flows should be labelled except when data to or from a data store consists of all items in the data store

26. Identifying operations
The SafeHome security function enables the homeowner to configure the security system when it is installed, monitors all sensors connected to the security system, and interacts with the homeowner through the Internet, a PC, or a control panel.
During installation, the SafeHome PC is used to program and configure the system. Each sensor is assigned a number and type, a master password is programmed for arming and disarming the system, and telephone number(s) are input for dialing when a sensor event occurs.
When a sensor event is recognized, the software invokes an audible alarm attached to the system. After a delay time that is specified by the homeowner during system configuration activities, the software dials a telephone number of a monitoring service, provides information about the location, reporting the nature of the event that has been detected. The telephone number will be redialed every 20 seconds until a telephone connection is obtained.
The homeowner receives security information via a control panel, the PC, or a browser, collectively called an interface. The interface displays prompting messages and system status information on the control panel, the PC, or the browser window. Homeowner interaction takes the following form…

27. Context Diagram Find the people who send data into the system
Often data is part of a PHYSICAL transaction
When handing a bar of chocolate to a shopkeeper, you are handing him/her a barcode.
Find the people who get data out of the system.
The only data you need is data that is transformed or sent completely out of the system – not data that is handled by an operator within the system.

28. Data Flow Diagram
Context-level DFD for SafeHome security function

29. Summary Data Flow Diagrams (DFDs) When Where Why Notations/Elements
Safe Home example (Context DFD)

30. Identifying operations
The SafeHome security function enables the homeowner to configure the security system when it is installed, monitors all sensors connected to the security system, and interacts with the homeowner through the Internet, a PC, or a control panel.
During installation, the SafeHome PC is used to program and configure the system. Each sensor is assigned a number and type, a master password is programmed for arming and disarming the system, and telephone number(s) are input for dialing when a sensor event occurs.
When a sensor event is recognized, the software invokes an audible alarm attached to the system. After a delay time that is specified by the homeowner during system configuration activities, the software dials a telephone number of a monitoring service, provides information about the location, reporting the nature of the event that has been detected. The telephone number will be redialed every 20 seconds until a telephone connection is obtained.
The homeowner receives security information via a control panel, the PC, or a browser, collectively called an interface. The interface displays prompting messages and system status information on the control panel, the PC, or the browser window. Homeowner interaction takes the following form…

31. Verbs from the description
The SafeHome security function enables the homeowner to configure the security system when it is installed, monitors all sensors connected to the security system, and interacts with the homeowner through the Internet, a PC, or a control panel.
During installation, the SafeHome PC is used to program and configure the system. Each sensor is assigned a number and type, a master password is programmed for arming and disarming the system, and telephone number(s) are input for dialing when a sensor event occurs.
When a sensor event is recognized, the software invokes an audible alarm attached to the system. After a delay time that is specified by the homeowner during system configuration activities, the software dials a telephone number of a monitoring service, provides information about the location, reporting the nature of the event that has been detected. The telephone number will be redialed every 20 seconds until a telephone connection is obtained.
The homeowner receives security information via a control panel, the PC, or a browser, collectively called an interface. The interface displays prompting messages and system status information on the control panel, the PC, or the browser window. Homeowner interaction takes the following form…

33. Level 2 DFD that refines the monitor sensors process