1. Software Engineering
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2. 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
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3. Elements of data-flow diagrams
Information Flow/Data Flow
Process/Data Transform
External Entity/Information Sources
Data Store/Information Store
Output
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4. Information Flow/Data Flow
Information flows represent the information being passed into or out of a system. It is represented as a labeled arrow:
Taxable Income
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5. Data transforms/Process
Data transforms labeled circles with one or more incoming and outgoing information flows:
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6. Information sources/External Entity
Information sources and sinks are information that come into the system, or leave the system, and are represented by squares:
Any human or external software system which interact with the system is an external entity
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7. Information store/Data store
Information Stores represent locations where information can be store for the duration of the system activity:
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8. Output
This element shows the output of a system. This is represented as:
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9. Importance of DFDs in a good software design
DFD is a very simple formalism – it is simple to understand and use.
Starting with a set of high-level functions that a system performs, a DFD model hierarchically represents various sub-functions. In fact, any hierarchical model is simple to understand.
DFD is an elegant modeling technique that turns out to be useful not only to represent the results of structured analysis of a software problem
But also for several other applications such as showing the flow of documents or items in an organization.
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10. General Rules:- No internal Logic
To keep it uncluttered(input same then join)
No miracles/Blackholes(only i/p, no o/p)
Database cannot sink/source(no data storing, only reading)
No data stores should be there at level 0
No direct dataflow between two entities and two data stores.
Direct dataflows between two processes-yes
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11. Levelled DFDs
Even a small system could have many processes and DFD could be large and messy
use levelled DFDs - view system at different levels of detail
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12. Level 0 - Context Diagram
It is the most abstract data flow representation of a system.
It represents the entire system as a single bubble.
This bubble is labelled according to the main function of the system.
The various external entities with which the system interacts and the data flow occurring between the system and the external entities are also represented.
Only one process and all external entities identified at context level. No new entity can come in level 1 and level 2.
The name ‘context diagram’ is well justified because it represents the context in which the system is to exist, i.e. the external entities who would interact with the system and the specific data items they would be supplying the system and the data items they would be receiving from the system. The context diagram is also called as the level 0 DFD.
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13. 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
Process will be decomposed but external entity will remain same.
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14. Level 2 - Detailed Diagram
Level 1 process is expanded into more detail
Each process in level 1 is decomposed to show its constituent processes
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15. Numbering of Bubbles(Processes)
It is necessary to number the different bubbles occurring in the DFD.
These numbers help in uniquely identifying any bubble in the DFD by its bubble number.
The bubble at the context level is usually assigned the number 0 to indicate that it is the 0 level DFD.
Bubbles at level 1 are numbered 0.1, 0.2, 0.3, etc, etc. When a bubble numbered x is decomposed, its children bubble are numbered x.1, x.2, x.3, etc.
In this numbering scheme, by looking at the number of a bubble we can determine its level, its ancestors, and its successors.
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16. Commonly made errors while constructing a DFD model
Many beginners commit the mistake of drawing more than one bubble in the context diagram. A context diagram should depict the system as a single bubble.
Many beginners have external entities appearing at all levels of DFDs. All external entities interacting with the system should be represented only in the context diagram. The external entities should not appear at other levels of the DFD.
It is a common oversight to have either too less or too many bubbles in a DFD. Only 3 to 7 bubbles per diagram should be allowed, i.e. each bubble should be decomposed to between 3 and 7 bubbles.
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17. A common mistake committed by many beginners while developing a DFD model is attempting to represent control information in a DFD. It is important to realize that a DFD is the data flow representation of a system, and it does not represent control information. For an example mistake of this kind:
Consider the following example. A book can be searched in the library catalog by inputting its name. If the book is available in the library, then the details of the book are displayed. If the book is not listed in the catalog, then an error message is generated. While generating the DFD model for this simple problem, many beginners commit the mistake of drawing an arrow to indicate the error function is invoked after the search book. But, this is a control information and should not be shown on the DFD.
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18. Showing control information on a DFD - incorrect
Another error is trying to represent when or in what order different functions (processes) are invoked and not representing the conditions under which different functions are invoked.
If a bubble A invokes either the bubble B or the bubble C depending upon some conditions, we need only to represent the data that flows between bubbles A and B or bubbles A and C and not the conditions depending on which the two modules are invoked.
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19. Many beginners leave different levels of DFD unbalanced.
A data store should be connected only to bubbles through data arrows. A data store cannot be connected to another data store or to an external entity.
All the functionalities of the system must be captured by the DFD model. No function of the system specified in its SRS document should be overlooked.
Only those functions of the system specified in the SRS document should be represented, i.e. the designer should not assume functionality of the system not specified by the SRS document and then try to represent them in the DFD.
The data and function names must be intuitive. Some students and even practicing engineers use symbolic data names such a, b, c, etc. Such names hinder understanding the DFD model.
Many beginners leave different levels of DFD unbalanced.
Balancing: No. of Inputs and outputs in a process in context diagram should be equal to the number of inputs and outputs in processes in overview diagram and so on.
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20. Rules of Data Flow Data can flow from Data cannot flow from
External entity to process
Process to external entity
Process to store and back
Process to process
Data cannot flow from
External entity to external entity
External entity to store
Store to external entity
Store to store

21. Level 0 - Context Diagram
Customer
Sales Clerk
Cust. Deatils
Sales details CN
SuperMarket System
All Cust. details
External Entities identified: Customer, Sales Clerk and Manager
Manager
Winner lists
Winner Lists
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22. Level 1 – Overview Diagram
Reset
Cust. details
Register Customer
0.1
Register Sales
0.2
Customer details
Sales Details
Main processes and data stores identified
Generation of winner lists
0.3
Winner lists
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23. Level 2 – Detailed Diagram
Generation of Gold Coin winner list
0.3.2
Verification
0.3.1
Final Data
Winner list1
Generation of Surprise gift winner list
0.3.3
Winner list2
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