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 won't find information about the processing timing (e.g. whether the processes happen in sequence or in parallel).
DFD is not a “flow chart” 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 .
TYPES OF DFDData flow diagrams (DFDs) are categorized as either logical or physical. 1) LOGICAL DFD:- A logical DFD focuses on the business and how the business operates. It describes the business events that take place and the data required and produced by each event. 2) PHYSICAL DFD:- A physical DFD shows how the system will be implemented.
DFD COMPONENTS Data Flow Diagrams are composed of the four basic symbols 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
Doctor ENTITIES Student Cashier Teacher Manager Customer User
2) Process:- The rounded cornered rectangles in a DFD indicate processes The Process symbol represents an activity that transforms or manipulates the data (combines, reorders, converts, etc.). Process
Processes Grading System Accounting System Reservation System Patient Administration System Marketing System
3) Data Store:- • 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 by an incoming arrow. • Two data stores cannot be connected by a data flow.
Data can be read from a data store. This is depicted by an outgoing 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
RULES OF DATA FLOW• Data can 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
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.
LEVELS OF DFD 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.
How to create Level 0 DFD Identify your main system Identify the external people who interact with the system Decide what data these entities will enter into the system Determine what these entities expect as output from the system
Context Level DFD for a Mail Order Business CUSTOMER ORDER PROCESSING ORDER DELIVERY
Level 1 DFD • 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.
How to create Level 1 DFD Focus on your process and break it into 2 or more sub-processes Identify what data flows between these processes and between the entities Identify What permanent data files are used in this system Note that no new entities can be introduced
Level 1 DFD for a Mail Order Business SALES PROCESSING CUSTOMER Order Delivery Credit Status Order Order CUSTOMER DATABASE Credit Status ORDERS ACCOUNTING SYSTEM Customer no.
2-Level DFD and other level of DFD Each process from level 1 is exploded even more into sub processes. This decomposition continues for each level. The number of levels possible depends on the complexity of the system
LEVELLING DFD A context diagram is expanded into a number of inter-related processes. Each process may be further expanded into a set of inter-connected sub processes. This procedure of expanding a DFD is known as leveling.
The value of a DFD 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 Data flow 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. The old system’s dataflow diagrams can also be drawn up and compared with the new system’s dataflow diagrams to draw comparisons in order to help implement a more efficient system.
1) In a DFD external entities are represented by a______ • a. Rectangle • b. Ellipse • c. diamond shaped box • d. Circle • A • 2) External Entities may be a_________ • a. source of input data only • b. source of input data or destination of results • c. destination of results only • d. repository of data • B
3) A data store in a DFD represents • a. a sequential file • b. a disk store • c. a repository of data • d. a random access memory • C • 4) A data cannot flow between a store and (i) a store (ii)a process (iii)an external entity a. i and iii b. i and iic. ii and iii d. ii • A
5) Data cannot flow from an external entity to an external entity because a. 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 • D • 6) A data flow can • a. only enter a data store • b. only leave a data store • c. enter or leave a data store • d. either enter or leave a data store but not both • C
7) The following portion of a DFD is not correct asa. there is no output data flow from the processb. there are three data flow inputs to the processc. there is no external entityd. there is no data store Quantity Quantity Cost/unit Billing Process Billing Process Discount Discount • A
8) The following portion of a DFD is not correct as (a) there are many data flows out of the process (b) there are no input data flows to the process (c) the output does not go to an external entity (d) there is no data store Order to vendor Out of stock Billing Proces Too much stock • B
9) A context diagram a. describes the context of a systemb. is a DFD which gives an overview of the systemc. is a detailed description of a systemd. is not used in drawing a detailed DFD B 10) By leveling a DFD we mean a. splitting it into different levels b. make its structure uniform c. expanding a process into one with more sub-processes giving more detail d. summarizing a DFD to specify only the essentials • C
11 ) A physical DFD specifiesa. what processes will be usedb. who generates data and who processes itc. what each person in an organization doesd. which data will be generated • B • 12) 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 confusing • a. i and iii • b. ii and iv • c. iii and iv • A
Your Assignment A general DFD for an airline reservation system
References http://en.wikipedia.org/wiki/Physical_Data_Flow http://www.visual-paradigm.com/product/bpva/tutorials/dfd.jsp http://www.civil.utm.my/staff/file/141/file/CIMLectureNotes2011.pdf http://spot.colorado.edu/~kozar/DFDtechnique.html http://www.fbk.eur.nl/PRJ/MEETEYEES/dfd.htm Http://ratandon.mysite.syr.edu/cis453/notes/DFD_over_Flowcharts.pdf http://www.slideshare.net/mohit4192/dfd-examples http://ecourses.vtu.ac.in/nptel/courses/Webcoursecontents/IIScBANG/System%20Analysis%20and%20Design/pdf/module5.pdf
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