data flow diagram

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dfd

Published on March 6, 2011

Author: Saurabh6

Source: authorstream.com

Data Flow Diagrams: Data Flow Diagrams BCA Sem IV KIRAS What is A DFD?: What is A DFD? Data flow diagrams (DFDs) are used to depict the flow and transformation of data in an information processing system. DFDs give an overview to an analyst specifying where data originates, how it is processed and where the results go. DFDs act as a graphical communication aid between a user and an analyst. It is also useful as a communication aid between an analyst and a system designer. The procedure to develop a DFD starts with one DFD giving an overview of the system to be designed. This is called a context diagram. The context diagram is expanded into a series of DFDs, each describing a specific function. This method of top down analysis and breaking down DFDs to give more and more detail is known as levelling. Saurabh What is A DFD?: What is A DFD? The main merit of DFD is that it provides an overview of what data flows in a system, what transformations are done on the data, what files are used and where results flow. It is a good documentation aid which is understood by both programmers and non-programmers (i.e., laypersons). As DFD specifies only what processes are performed and not how they are performed it is easily understood by a non-programming user. A DFD allows you to identify the transformations that take place on data as it moves from input to output in the system. Saurabh Types of Diagrams: Types of Diagrams Context Diagram A data flow diagram (DFD) of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system Level-O Diagram A data flow diagram (DFD) that represents a system’s major processes, data flows and data stores at a high level of detail Saurabh Context diagram: Context diagram A diagram giving an entire system’s data flows and processing with a single Process (circle) is called a context diagram. 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 levelling. Saurabh Elements of data-flow diagrams: Elements of data-flow diagrams or Saurabh Slide 7: Show all processes data flows must start or end in a process Saurabh Elements of data-flow diagrams: Elements of data-flow diagrams There are four basic elements of data flow diagrams: Data Transforms : where data is filtered, formatted or synthesized. Information sources and sinks : essentially inputs and outputs to the system. Information/Data stores : where information is kept for the duration of system activity. Information Flows : data that is passed into and out of transforms, information stores. Saurabh Source: Source Source represent persons, processes which produce data to be used by the system or receive data that is output by the system Reside outside the system, but interact with system Either a) receive info from system, b) trigger system into motion, c) provide new information to system Examples: Student, Customer, Client Saurabh Data Stores: Data Stores Data stores are temporary or permanent repositories of information that are inputs to or outputs of processes Internal to the system Information Stores represent locations where information can be store for the duration of the system activity Examples: Student Master, Client List Total Payroll Expense Saurabh Data Flows: Data Flows Data flows represent the transfer of data over time from one “place” (entity, process, data store) to another Information flows represent the information being passed into or out of a transform, source, sink or store. It is represented as a labeled arrow: Data in motion, moving from one place to another in the system From external entity (source) to system From system to external entity (sink) From internal symbol to internal symbol, but always either start or end at a process Examples: New Student Information (from Student, to Student Master) Taxable Income Saurabh Data transforms: Data transforms Data transforms labeled circles with one or more incoming and outgoing information flows: Calculate Tax Taxable Income Tax Tables Tax owed Saurabh Processes: Processes Processes are discrete actions that transform input data to output data Examples: Create Student Record, Calculate Purchase Cost, Register Client Always internal to system Law of conservation of data: #1: Data stays at rest unless moved by a process. #2: Processes cannot consume or create data Must have at least 1 input data flow Must have at least 1 output data flow (to avoid black holes) Should have sufficient inputs to create outputs Saurabh Slide 14: Define the System A system is the collection of all business processes which perform tasks or produce outputs we care about. It is “what happens.” The system is a single process, connected to external entities Represented in a “Context Diagram” Define Subsystems A subsystem gives a more detailed view individual processes contained in the context diagram Includes data stores, more elementary processes Saurabh Creating Data Flow Diagrams: Creating Data Flow Diagrams Creating DFDs is a highly iterative process of gradual refinement. General steps: 1. Create a preliminary Context Diagram 2. Identify the ways in which users most commonly use the system 3. Create a Level 0 diagram from fragments 4. Decompose to Level 1,2,… 5. Go to step 1 and revise as necessary 6. Validate DFDs with users. Saurabh DFD Rules: DFD Rules Data Store Data cannot be moved directly from one store to another Data cannot move directly from an outside source to a data store Data cannot move directly from a data store to a data sink Data Flow A data flow has only one direction of flow between symbols Saurabh DFD Rules—Context Diagram: DFD Rules—Context Diagram One process, numbered 0. Sources (external entities) as squares Main data flows depicted No internal data stores are shown They are inside the system Saurabh DFD Example: DFD Example A mail-order company distributes CDs and tapes at discount price to record-club members. When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file. If the sender is not a member, the clerk returns the order along with a membership application form. If the customer is a member, the clerk verifies the order item data by checking the Item file. Then the clerk enters the order data and saves it to the Daily Orders file. The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment. Saurabh DFD Example - Step 1: DFD Example - Step 1 SECOND SENTENCE : When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file. RESULT : Order Processing System Record Club Member Customer (Non Member) order Verify Member status Member data Members Saurabh DFD Example - Step 2 continued: DFD Example - Step 2 continued SECOND SENTENCE : When an order processing clerk receives an order form, he or she verifies that the sender is a club member by checking the Member file. RULES FOLLOWED : data flows start or end in a process Saurabh DFD Example - Step 3: DFD Example - Step 3 THIRD SENTENCE : If the sender is not a member, the clerk returns the order along with a membership application form. RESULT : Order Processing System Record Club Member Customer (Non Member) Verify Member status order application form Member data Members Saurabh DFD Example - Step 3 continued: DFD Example - Step 3 continued THIRD SENTENCE : If the sender is not a member, the clerk returns the order along with a membership application form. RULES FOLLOWED : shows all processes without conditions Saurabh DFD Example - Step 4: DFD Example - Step 4 FOURTH SENTENCE : If the customer is a member, the clerk verifies the order item data by checking the Item file. RESULT : Order Processing System Record Club Member Customer (Non Member) order application form Verify Member status Verify Order item data Member data Members Items Item details Saurabh DFD Example - Step 4 continued: DFD Example - Step 4 continued FOURTH SENTENCE : If the customer is a member, the clerk verifies the order item data by checking the Item file. RULES FOLLOWED : data flows start or end in a process shows all processes without conditions Saurabh DFD Example - Step 5: DFD Example - Step 5 FIFTH SENTENCE : Then the clerk enters the order data and saves it to the Daily Orders file. Record Club Member Customer (Non Member) Verify Member status Verify Order item data Daily Orders application form order Member data Members Item details Items Verified member order order Daily Orders Member order Saurabh DFD Example - Step 5 continued: DFD Example - Step 5 continued FIFTH SENTENCE : Then the clerk enters the order data and saves it to the Daily Orders file. RULES FOLLOWED: data flows start or end in a process Saurabh DFD Example - Step 6: DFD Example - Step 6 SIXTH SENTENCE : The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment. Saurabh DFD Example - Step 6: DFD Example - Step 6 Record Club Member Customer (Non Member) Verify Member status application form Verify Order item data Member order Member data Members Item details Items Verified member order Daily Orders order order Daily Orders order Print Invoice & shipping list Order data Order Fulfillment shipping list Saurabh DFD Example - Step 6 continued: DFD Example - Step 6 continued SIXTH SENTENCE : The clerk also prints an invoice and shipping list for each order, which are forwarded to Order Fulfillment. RULES FOLLOWED : data flows start or end in a process shows all processes without conditions beware of black holes - every process and data store must produce data Saurabh University Course Registration System: University Course Registration System University Registration Student Course details Student details, marks details University Administration Course details Student details Level 0 (Context Model) Acceptance / Denial note Saurabh University Course Registration: University Course Registration Validate inputted details Student details, marks details Course details Denial note Course details Student details Process Application University Administration Student Level 1 Student database Acceptance note Saurabh Publisher's present ordering system: Publisher's present ordering system Process Orders Saurabh Publisher's present ordering system: Publisher's present ordering system Level 1 Saurabh Level 2: Showing Order Verification & credit check: Level 2: Showing Order Verification & credit check Saurabh ATM: ATM ATM System User PIN Transaction amount Invalid PIN Invalid Card Receipt Card details Saurabh ATM: ATM User Verify Card PIN Card details Invalid Card Valid Card Details Verify PIN Cards Card Num, PIN Card status Validate Amount Card status Accounts Account balance Daily Balance Invalid PIN amount Overdraw Transact Withdrawal Transaction details Receipt Validated Transaction details Saurabh Level 2 Validate Amount: Level 2 Validate Amount Accept Amount Transaction details Check Daily limit Accounts Daily Balance Daily limit exceeded Notify Invalid amount User Card status amount Overdraw Check Weekly limit Daily O.K Weekly limit exceeded Weekly O.K Check Account Balance Insufficient funds Validated Transaction details Weekly Balance Account balance Saurabh Payroll Management : Level 0 : Payroll Management : Level 0 Employee Employee No, Hours worked Payroll Management cheque & payslip data Accounts Payroll summary data Saurabh Payroll Management : Level 1: Payroll Management : Level 1 Employee Employee No, hours worked Validate Data Employee Table Employee No, hours worked Invalid employee data Prepare Payroll Name, pay rate, tax code, etc. Payroll Details Print Payroll Accounts Employee no, pay, tax, etc. cheque & payslips Saurabh Decomposition of DFDs: Decomposition of DFDs Functional decomposition Act of going from one single system to many component processes This is a repetitive procedure allowing us to provide more and more detail as necessary The lowest level is called a primitive DFD Level-N Diagrams A DFD that is the result of n nested decompositions of a series of sub-processes from a process on a level-0 diagram Saurabh Uses of DFD: Uses of DFD Used to analyze the system to ensure that the design is complete Used to partition the system into programs Used for system documentation Better communication with users Better understanding of the business by analysts Saurabh

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