DOCUMENTATION OF INFORMATION SYSTEMS
Several tools are available for documenting the flow of information within an organization, from a simple descriptive method to a
diagrammatic model. For example, using short statements to list the set of activities to be performed within a department of an
organization is an example of a simple descriptive method.
Data Flow Diagrams (DFDs)
A data flow diagram, also known as DFD, is a graphical representation of the information flow within a system. It is an example of a
diagrammatic model. A data flow diagram can also be used for the visualization of data processing (structured design). It is common
practice for a designer to draw a context-level DFD first which shows the interaction between the system and outside entities. This
context-level DFD is then "exploded" to show more detail of the system being modeled. With a dataflow diagram, users are able to
visualize how the system will operate, what the system will accomplish and how the system will be implemented. Old system dataflow
diagrams can be drawn up and compared with the new systems dataflow diagrams to draw comparisons to implement a more efficient
system. Dataflow diagrams can be used to provide the end user with a physical idea of where the data they input, ultimately has an
effect upon the structure of the whole system from order to dispatch to restock how any system is developed can be determined
through a dataflow diagram.
Types of DFD
In analyzing a business, several sets of DFDs are drawn. Initial DFDs might model the existing system (flaws and all), while later
DFDs may model a solution to the problem being analyzed. For these solution DFDs a logical and physical DFD is drawn. Physical
DFDs represent physical files and transactions, while logical or conceptual DFDs can be used to represent business functions or
processes.
Components of a DFD
A data flow diagram illustrates the processes, data stores, and external entities in a business or other system and the connecting
data/resource flows.
The four components of a data flow diagram (DFD) are:
 External Entities (represented by a square or oval)
 Processes (represented by a rounded rectangle)
 Data Stores (represented by an open ended narrow rectangle)
 Data Flows (represented by an arrow)
 Resource Flows
Data Flow Diagrams – Diagram Notation
There are only five symbols that are used in the drawing of business process diagrams (data flow diagrams). These are now explained,
together with the rules that apply to them.
This diagram represents a banking process, which maintains customer accounts. In this example, customers can withdraw or deposit
cash, request information about their account or update their account details. The five different symbols used in this example represent
the full set of symbols required to draw any business process diagram.
External Entities
An external entity is a source (where the data comes from) or destination (where the data goes) of a data flow which is outside the
system but interacts with the system (e.g., person, department or organization). When modeling a DFD, the designer is not interested
in the inner workings of the external entity, but only what data is produced/needed by the entity. The symbol used to represent an
external entity is a square or oval.
OR
Student
Customer
Diagrams used to represent external entities
Process
A process shows a transformation or manipulation of data flows within the system. A process is a manual or computerized routine that
changes data by performing a calculation, updating information, sorting a list, and so on. There are two ways in which a process is
represented, one with a rounded rectangle, which is numbered and labeled with a verb phase (except for the process numbered 0,
which gives the system’s name). Two, with a rectangle that has three parts an identification number, a location that appears to the right
of the identifier which describes where in the system the process takes place and a descriptive title placed in the centre of the box. The
process shows a part of the system that transforms inputs into outputs; that is, it shows how one or more inputs are changed into
outputs.
2
Enroll
student
OR
Diagrams used to represent a process
Data Store
A data store is a holding place for information within the system. It is represented by an open ended narrow rectangle. Data stores may
be long-term files such as sales ledgers, or may be short-term accumulations: for example batches of documents that are waiting to be
processed. Each data store should be given a reference followed by an arbitrary number. It could represent the data being held
manually or on a computer. Basically, a data store is a logical collection of data.
D1
Student’s Records
OR
Diagram used to represent data store
Data flow
A data flow shows the flow of information from its source to its destination. A data flow is represented by a line, with arrowheads
showing the direction of flow. Information always flows to or from a process or a data store and may be written, verbal or electronic.
Diagram used to represent data flow
Resource Flow
A resource flow shows the flow of any physical material from its source to its destination. For this reason they are sometimes referred
to as physical flows. The physical material in question should be given a meaningful name. Resource flows are usually restricted to
early, high-level diagrams and are used when a description of the physical flow of materials is considered to be important to help the
analysis.
Diagram used to represent resource flow
Rules governing how the diagram components relate to each other
Data Flows
For data flows the rules are as follows:
Data flows and resource flows are allowed between external entities and processes. Data flows are also allowed between different
external entities. However, data flows and resource flows are not allowed between external entities and data stores.
Processes
For processes the data flow rules are as follows:
Data flows and resource flows are allowed between processes and external entities and between processes and data stores. They are
also allowed between different processes. In other words processes can communicate with all other areas of the business process
diagram.
Data Stores
For data stores the data flow rules are as follows:
Data flows and resource flows are allowed between data stores and processes. However, these flows are not allowed between data
stores and external entities or between one data store and another. In practice this means that data stores cannot initiate a
communication of information, they require a process to do this.
Data Flow Diagrams – Context Diagrams (Level 0 Diagram)
The first step in constructing DFDs is to draw a context diagram. A context diagram is a top-level view of an information system that
shows the system’s boundaries and scope. To draw a context diagram, you start by placing a single process symbol in the centre of the
page. The symbol represents the entire system, and you identify it as process 0. Then you place the entities around the perimeter of the
page and use data flows to connect the entities to the central process. You do not show any data stores in a context diagram because
data stores are internal to the system. In drawing the context diagram you should only be concerned with the most important
information flows. Remember that no data flow process diagram is the definitive solution - there is no absolute right or wrong.
Context Diagram Example
The context diagram shown above represents a book lending library. The library receives details of books, and orders books from one
or more book suppliers. Books may be reserved and borrowed by members of the public, who are required to give a borrower number.
The library will notify borrowers when a reserved book becomes available or when a borrowed book becomes overdue. In addition to
supplying books, a book supplier will furnish details of specific books in response to library enquiries.
Note, that communications involving external entities are only included where they involve the 'system' process. Whilst a book
supplier would communicate with various agencies, for example, publishers and other suppliers - these data flow are remote from the
'system' process and so this is not included on the context diagram.
Data Flow Diagrams – Level 1 Diagrams
The level 1 diagram shows the main functional areas of the system under investigation. As with the context diagram, any system under
investigation should be represented by only one level 1 diagram.
There is no formula that can be applied in deciding what is, and what is not, a level 1 process. Level 1 processes should describe only
the main functional areas of the system, and you should avoid the temptation of including lower level processes on this diagram. As a
general rule no data flow diagram should contain more than 12 process boxes.
The level 1 diagram is surrounded by the outline of a process box that represents the boundaries of the system. Because the level 1
diagram depicts the whole of the system under investigation, it can be difficult to know where to start.
Some additional guidelines for creating DFDs:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
All processes must have at least one data flow in and one data flow out.
All processes should modify the incoming data, producing new forms of outgoing data.
Each data store must be involved with at least one data flow.
Each external entity must be involved with at least one data flow.
A data flow must be attached to at least one process.
Two entities, or two data stores, or an entity and a data store or vice versa, cannot be connected together with a data flow.
Choose meaningful names for processes, flows, stores, and external entities.
Number the processes.
Avoid overly complex DFDs, do not cross lines.
Make sure the DFD is internally consistent and consistent with any associated DFDs.
The general principle in Data Flow Diagramming is that a system can be decomposed into subsystems, and subsystems can
be decomposed into lower level subsystems, and so on.
12. Each subsystem represents a process or activity in which data is processed. At the lowest level, processes can no longer be
decomposed.
13. Each process in a DFD has the characteristics of a system.
14. Data enters the system from the environment; data flows between processes within the system; and data is produced as output
from the system.
Dos and Don’ts
1. An Entity must be a noun (e.g. Member, Student, Customer etc.)
2. Two Data Stores, two Entities, or an Entity and a Data Store or vice versa, must not be directly connected.
D1
D1
Video File
Video File
Member
Student
D1
Member
Video File
N.B.
Always use one or more processes
3. A Process must have at least one Data Flow entering it and one leaving it.
Video Number
1
Get Video
Details
Video Details
4.
1
Print airline
tickets
Ticket
2
Sort Serial
numbers
A Process that has no input is called a Miracle. It is impossible for an information
system to process data it doesn’t have.
5.
Serial numbers
A Process that has no output is called a Black hole. A process that just does
something to data, but doesn’t put that data anywhere is useless.
6.
A Data Flow must not be split into two or more Data Flows
7.
All Data Flows, Entities, Processes or Data Store must be labelled.
8.
The Data Flow label leading to a Process must be different from the Data Flow label being outputted from that same Process.
(If you put grapes into a blender and blend you will not get grapes when finished but grape juice)
Example of a Data Flow Diagram
This data flow diagram represents a system used in a video rental store.
D1
Video File
Video Record
1
Video
Video
Number
1
Video
Details
Get Video
Details
Member
Details
Member
Member
Number
Bill Total
1
1
Get
Member
Details
Get
Payment
Member
Record
D1
Compute
Bill Total
Payment
Details
Payment
Member
Member File
1
Member
Record
Print
Receipt
Class Activity
Use the Data Flow Diagram below to answer the following questions.
1.
How many processes are in the diagram?
2.
Give the names of two of the processes.
3.
What action is done to bill amount?
4.
What is bill amount?
5.
How many data stores are in the diagram
6.
Name an external entity.
Data Flow Diagrams – Practices Exercises
Question 1
A student completes an application to become a member of a library. The student takes the application to the librarian who checks the
form and then issue a membership card. The completed application is stored as Member Record in file, named Member Folder.
To borrow books, a member takes the books and his membership card to the librarian. She updates her cards in the Book Folder.
When a member returns the borrowed books to the library, the librarian updates her records.
New books come into the library periodically. The librarian records the relevant details on cards, which are stored in the Book Folder.
Required: Construct a Context Diagram (Level 0).
Question 2
You have an address book in which you keep details of your friends. From time to time, you will make amendments such as change of
address or telephone number for ‘old’ friend or add details for ‘new’ friends. In addition, a friend may request that you get the address
or telephone number (from the address book) of another contact.
Required: Construct a Context Diagram (Level 0.