1. Rob Gleasure R.Gleasure@ucc.ie www.robgleasure.com
IS3320 Developing and Using Management Information Systems Lecture 14: Data-Flow Diagrams 1 (Context-Level diagrams)
Rob Gleasure

2. IS3320
Today’s lecture
What are DFDs?
Exercise

3. What are DFDs? DFD stands for Data Flow Diagram
A DFD is a representation of a system that captures all of its important component pieces, as well as the interfaces between these components
DFDs focus on the movement of data between components

4. So… how is that different from flowcharts?
DFDs
Flow of control/activity
Flow of data
Flows are sequential
Flows are parallel
Flows are bound within a single time unit, e.g. over a minute, week, month, etc.
Flows can happen at any time, including on demand

5. Used during analysis and design!
Why use DFDs?
Use cases
Allows us to identify the key actors and uses of a system
Flowcharts
Allows us to break down uses into sequential algorithmic processes, including how these processes are effected by decisions and conditions
DFDs
Allows us to model the data and system resources available, as well as how these are required by different actors during different processes
Used during analysis and design!

6. How do we create a DFD? A DFD is made up of four components Component
Symbol (a la gane and Sarson)
Processes
External entities
Data flows
Data stores

7. Components: Processes
Work or actions performed on data (inside the system)
Receives input data and produces output
Labels should be verb phrases

8. Components: Processes
Rules
Can have more than one outgoing data flow or more than one incoming data flow
Can connect to any other symbol (including another process symbol)

9. Components: Processes
Rules
Needs to have input and output
Incorrect!

10. Components: Data Store
Is used in a DFD to represent data that the system stores
Labels should be noun phrases

11. Components: Data Store
Rule
Must have at least one incoming and one outgoing data flow
Incorrect!

12. Components: External Entity
External entity that is origin or destination of data (outside the system)
Is the singular form of a department, outside organisation, other IS, or person
Labels should be noun phrases
Note, sometimes the following terms are used:
Source – Entity that supplies data to the system
Sink – Entity that receives data from the system

13. Components: External Entity
Rule
Must be connected to a process by a data flow

14. Components: External Entity
Incorrect!

15. Components: Data Flow
A path for data to move from one part of the IS to another
Arrows depicting movement of data
Can represent flow between process and data store by two separate arrows

16. Components: Data Flows
Summary of rules for data flows
MAY connect a process to another process
MAY connect a process to an external entity
MAY connect a process to a data store
May NOT connect an external entity to another external entity
May NOT connect an external entity to a data store
May NOT connect a data store to another data store

17. Spot the errors

18. Levels of DFDs
DFDs are constructed separately across several layers, each of which serves a slightly different function
Context-level
Shows system boundaries, the data flows between external entities and the system
Level-1
Shows system’s major processes, data flows, and data stores at a high level of abstraction (not detailed)
Level-2, Level-3, …, Level n
Iteratively break down a system description down into finer and finer detail
New DFDs must be balanced at each stage to ensure they are complete and correct

19. Steps in Constructing Context Level DFD
Read the narrative/collection of narratives describing the system
Create your main system process
Identify the external entities from the text (usually these will be among the nouns in the narrative)
Identify and name the data flows to and from these external entities to your main system process

20. Example Context Level DFD

21. Exercise Consider an ATM interface system
Users enter their account details and password
Users view balance
Users retrieve cash
ATM checks account details and balance against bank system
ATM updates the bank database when cash is distributed
ATM checks quantity of each bank note available from cashbox
ATM retrieves bank notes from cashbox
ATM updates quantity of each bank note available from cashbox

22. Shazam
Started off at the turn of the millennium as a hotline number that mobile phone holders could call when music was playing
The call would last 30 seconds, after which time it would automatically disconnect and text callers the name of the song
As smartphones grew in popularity, this functionality moved to app format, launching on both iOS and Android in 2008
The iOS version provided links to purchase the track on iTunes, the Android version linked to Amazon’s MP3 Store
In January 2011, Shazam partnered with Spotify so that Spotify users could listen to tracks directly through that channel

23. Shazam
In 2009, several new features were added to increase the social nature of the app
Each time a song was ‘tagged’, the GPS coordinates were also logged
Tags could be sent to other users
Twitter alerts were integrated
In early 2011, Shazam was the 4th most commonly downloaded app on the iStore
In 2013, Techland voted it one of the 50 best Android apps

24. How Shazam Works
The sounds is recorded for 10 seconds and a spectogram is generated and sent from the user’s device to the central Shazam system
This is compared against records of songs on a dedicated Shazam server
If the song is not recognised, a message is sent to the user’s device informing them that no match was found
If the song is recognised, the name and details of that song are sent to the user’s device
A user may then browse lyrics or reviews of the album on the central Shazam system
They may also send a request via the central Shazam system to the Amazon MP3 Store or iTunes to buy the track or the ringtone
They may send a request via the central Shazam system to watch the music video on YouTube
They may send a request via the central Shazam system to share the track on Facebook or Twitter

25. How Shazam Works App review

26. Exercise
In groups of 2-3…
Draw a context-level DFD for Shazam

27. Want to read more?
Sullivan, L.H. (2006). +Just Enough Structured Analysis, Chapter 9: Dataflow Diagrams
Data Flow Diagrams: An Agile introduction