DEFINITION A data flow diagram (DFD) is a graphical representation of the "flow" of data through a computer system. OR A data flow diagram looks at how data flows through a system. It concerns things like where the data will come from and go to as well as where it will be stored. But you wont find information about the processing timing (e.g. whether the processes happen in sequence or in parallel).
Flow chart shows “flow of Control “ . DFD shows “ flow of Data The flowchart describes boxes that describe computations, decisions, interactions & loops. It is important to keep in mind that data flow diagrams are not flowcharts and should not include control elements .
DFD COMPONENTSData Flow Diagrams are composed of the four basicsymbols shown below:-
1)External Entity:- The sharp cornered rectangles(or simply boxes) in a DFD indicates entities. The External Entity symbol represents sources of data to the system or destinations of data from the system. Entities are people things, organizations etc Entity
DoctorStudent CashierTeacher Manager Customer User
The rounded cornered rectangles in a DFD indicateprocesses The Process symbol represents an activity thattransforms or manipulates the data (combines, reorders,converts, etc.). Process
Accounting System Grading System Reservation System Patient Marketing System Administration System
Opened sided rectangles in DFD indicates data store. The Data Store symbol represents data that is not moving(delayed data at rest). A Data Store is a repository of data. Data can be written into the data store. This is depicted byan incoming arrow. Two data stores cannot be connected by a data flow.
Data can be read from a data store. This is depicted by anoutgoing arrow.External entity cannot read or write to the data store.
4) Data Flow:- Arrow symbol in DFD indicate data flow The Data Flow symbol represents movement of data
EXAMPLE 1 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.
Level 0 DFD• The level 0 DFD (also known as the context level DFD ) is the simplest DFD.• The outermost level (Level 0) is concerned with how the system interacts with the outside world.• This level basically represents the input and output of the entire system.
1. Identify your main system2. Identify the external people who interact with the system3. Decide what data these entities will enter into the system4. Determine what these entities expect as output from the system
ORDERCUSTOMER ORDER PROCESSING DELIVERY
The basic module of the system are represented in this phase and how data moves through different module is shown. The level 1 DFD provides a high –level view of the system that identifies the major processes and data stores.
1. Focus on your process and break it into 2 or more sub- processes2. Identify what data flows between these processes and between the entities3. Identify What permanent data files are used in this system4. Note that no new entities can be introduced
Order CUSTOMER SALES PROCESSING Delivery Credit Order Status OrderCUSTOMERDATABASE Credit ORDERS Status ACCOUNTING Customer SYSTEM no.
Each process from level 1 is exploded even more into subprocesses. This decomposition continues for each level.The number of levels possible depends on the complexityof the system
1. With a dataf low diagram, users are able to visualize how the system will operate, what the system will accomplish, and how the system will be implemented2. Data f low diagrams can be used to provide the end user with physical idea of how the data they input ultimately has an effect upon the structure of the whole system.3. The old system’s dataf low diagrams can also be drawn up and compared with the new system’s dataf low diagrams to draw comparisons in order to help implement a more efficient system.
1) In a DFD external entities are represented bya______a. Rectangleb. Ellipsec. diamond shaped boxd. CircleA2) External Entities may be a_________a. source of input data onlyb. source of input data or destination of resultsc. destination of results onlyd. repository of dataB
3) A data store in a DFD representsa. a sequential fileb. a disk storec. a repository of datad. a random access memoryC4) A data cannot flow between a store and (i) a store (ii)a process (iii)an external entitya. i and iiib. i and iic. ii and iiid. iiA
5) Data cannot flow from an external entity to anexternal entity becausea. it will get corruptedb. it is not allowed in DFDc. an external entity has no mechanism to read or writed. both are outside the context of the system D6) A data flow cana. only enter a data storeb. only leave a data storec. enter or leave a data stored. either enter or leave a data store but not bothC
Quantity Quantity Cost/unit Billing Billing Process Process Discount Discount A
Order to Out of vendor stock Billing Proces Too much stock B
B10) By leveling a DFD we meana. splitting it into different levelsb. make its structure uniformc. expanding a process into one with more sub -processes giving moredetaild. summarizing a DFD to specify only the essentials C
B12) Data flow in a DFD must have (i) an arrow showing direction of flow of data (ii)a meaningful name (iii)a label such as: xyz (iv)no arrows as they are confusinga. i and iiib. ii and ivc. iii and iv A