Ch 7 - Data Flow Diagrams

Data flow diagrams (DFDs)

graphical representation of data processes thoughout an organization

• Useful in agile and SDLC

• conceptualize how data move through the organization, the process or transformation that the data undergo, and what the outputs are

• Inputs (value creators), processes, outptus (value consumers)

Conventions Used in Data Flow Diagrams

Double Square (Entity)

Depict an external entity (another department, a business, a person, or a machine) that can send data to or receive data from the system

• Source or destination of data

Arrows

Shows movement of data from one point to another, head of arrow pointing toward the data’s destination

Double parallel arrows

• Shows simultaneous movement

Rectangle w/ rounded corners (Process)

Occurrence of a transforming process

• Denote a change in or transofrmation of data

• Unique identifying # that indiciates its level. inthe diagram

Naming convention:

Open-ended rectange (data store0

Data at rest

• filing cabinent or a computerized file or database

• where info is accessed

Developing Data Flow Diagrams Steps

1. List of business activities and use it to determine various

   1. External entities

   2. Data flows

   3. Processes

   4. Data stores

2. Context diagram

3. Draw Diagram 0

4. Create a child diagram for each of. the processes in Diagram 0

5. Check for errors and make sure the labels you assign to each process and data flow are meaningful

6. Develop a physical data flow diagram from the logical data flow diagram

7. Partition the physical data flow diagram by separating or grouping parts of the diagram in order to faciliate programming and implementation

Developing Data Flow Diagrams Step #2 Context Diagram

Data flow diagram that depicts the context of what’s going on in an org

• Con: doesn’t provide info of opportunities

Guidelines

• Must have at least one process

• Must receive at least one data flow

• Data store should be connected to at least one process

• External entities should not be connected to each other

Developing Data Flow Diagrams Step #1

• External entities

• Data flow

• Process

• Data store

Creating the Context Diagram: Top-down approach

Context diagram is the highest level and contains only one process, representing the entire system

DFDs move from general to specific

• does not show detailed processes or data stores

Developing Data Flow Diagrams Step #3: Drawing Diagram 0 (The Next Level)

Explod DFDs into subprocesses to fill. inthe details about data movement

• Each process is numbered w/ an integer, starting from upper left hand corner to right hand

Drawing Diagram 0; Decomposition

“Exploding the diagrams”

• Close-ups of 3. -9 processes and showing data stores and new lower-level data flows

Questions to ask:

1. What happens to the data entering the system?

2. Examine the output fields

3. Examine the data flow to or from a data store

4. Analyze well-defien process

5. Take not of any unclear areas

Developing Data Flow Diagrams Step #4: Creating child Diagrams (More Detailed Levels)

The processses on the child diagram are numbered using the parent process number, a decimal point, and a unique number for each child process

• process numbered in diagram 3 are 3.1,3.2,3.3,etc

Process may or may nont exploded depending on their elevl of complexity

Parent Process

Process on Diagram 0 that is exploded

Child diagram

Diagram that results from Diagram 0

• Entities must be consistent with its parents

Vertical balancing

dictates that a child diagram cannot produce output or receive input that the parent process does not also produce or receive

Interface data flow

Data flow that matches the parent flow

Functionally primiive or primitive process

When a process is not exploded

Developing Data Flow Diagrams Step #5: Checking Diagrams for Errors

Common errors:

1. Forgetting to include a data flow or pointing an arrow in the wrong direction

2. Connecting data stores and external entities directly to each other

3. Incorrectly labeling processes or data flow

4. Including more than nine processes on a DFD

5. Omitting data flow

6. Creating unbalanced decomposition (or explosion) in child diagrams

Features Common in Logical vs Physical Data flow Diagram

Features	Logical	Physical
Model	How the business operates	How the system will be implemented
Process	Essential sequence	Actual sequence
Data store	Collections of data	Physical files and databases, manual files
Type of data store	Permanent data collections	Master files, transaction files
System controls	Business controls	Controls for data validation, record status, system security

Logical data flow diagrams

focus on business and how it operates. This conceptual diagram describes the business events that take place and the data required and produced by each event

• processes represent the business activities

• data stores represent the collection of data regardless of how the data are stored

Physical data flow diagram

how the system will be implemented, including hardware, software, files, and people

• programs, program modules, manual procedures

• data stores represent the physical files and databases

• show controls for validating input data, for obtaining a record

Contents:

• Manual process

• Proceses for adding, deleting, changing and updating

• Data entry and verifying processes

• Validation processes

• Sequencing processes

• Process to produce every unique system output

• Intermediate data stores

• Actual file names used to store data

• Controls to signify completion of tasks or error conditions

The progression of Models from Logical to Physical

Logial vs Physical DFD

Advantages of Logical DFD

1. Better communication w/ users

2. More stable systems

3. Better understanding of the business by analysts

4. Flexibility and maintenance

5. Elimination of redundancies and easier creation of the physical model

Advantages of Physical DFD

1. Clarifying which processes are performed by humans and which are automated

2. Describing processes in more detail than logical DFDs

3. Sequencing processes that must be done in a particular order

4. Identifying temporary data stores

5. Specificying actual names of files, database tables, and printout

6. Adding controls to ensure th processes are done properly

CRUD matrix (create, read, update, and delete)

Tool to represent where each of these processes occurs in a system

transaction data store

• required to link each tasks in Physical DFD

• Activties would be represented by proces on a physical DFD

base elements

data fields or elements that need to be keyed

Data Dictionaries

• Reference guide about data (metadata)

• Collects and coordinates data terms, and confirms what each term means to different people in the organization

derived elements

elements that are not keyed but are rather the result of a calculation or logical operation

Event Modeling and Data Flow Diagrams

The process of creating a simple data flow diagram fragment for each unique system trigger

Triggers

Start activities and processes, which in turn use data or produce outputs

Event Response Table

A table that summarizes events.

Characteristics:

• DFD fragment is represented by row in the table

• Rows are combined to form Diagram 0

• Trigger and Response become inoput and output data flows and activity becomes the process

• Must determine data stores

Data Flow Diagram for First Three Rows

Use Case

Summarizes an event and has a similar format to process specifications

• Defines one activity and its trigger, input, and output

Partitioning

The process of examining a DFD and determining how it should be divided into collections of manual procedures and collections of computer applications

Six reasons for partitioning DFDs:

1. Different user groups

2. Timing

3. Similar tasks

4. Efficiency

5. Consistency of data

6. Security

Step for Data Flow Diagram

1. Develop the list and summary of business activities

2. Crete a context-level DFD as shown

3. Drawing Diagram 0

   1. Add as many processes and data stores

   2. Draw data flows between them and the external entities

   3. number processes and data stores

   4. check errors

4. Creating a Child Diagram

   1. Level 1 diagram

   2. Make a list of subprocesses

   3. Does not have to be connected to external entities

   4. Label them 1.1, 1.2, 1.3

5. Create a physical data flow diagram from the logical DFD

6. Partitioning the Physical DFD

Partitioning Websites

Web designers may divide a website into a series of web pages

Ajax

Combination of web application development technologies that make web applications more responsive to user interaction

Communicating Using Data Flow Diagrams

DFDs are used to document the system