data flow diagrams

data flow diagrams shows how data moves through an information system but does not show program logic or processing steps provides a logical model that shows what the system does, not how it does it

data flow diagrams notation use four basic symbols that represent processes, data flows, data stores, and external entities Gane and Sarson symbol set Yourdon symbol set referenced by using all capital letters for the symbol name

DFD notation Gane & Sarson Yourdon EXTERNAL EXTERNAL ENTITY ENTITY DATA FLOW DATA FLOW Gane & Sarson PROCESS Yourdon PROCESS DATA FLOW DATA FLOW DATA STORE DATA STORE

DFD notation process symbol data flow symbol receives input data and produces output that has a different content, form, or both contain the business logic, also called business rules referred to as a black box data flow symbol represents one or more data items

DFD notation data store symbol external entity symbol represent data that the system stores logical representation (physical characteristics of data store are unimportant) external entity symbol represents the source or sink of data

modeling with DFDs: basic steps step 1: draw a context diagram context diagram – represents the system as a single process (process 0) step 2: draw a diagram 0 DFD diagram 0 - exploded view of process 0; zooms in on the context diagram and shows major processes, data flows, and data stores; must retain all the connections that flow into and out of process 0

modeling with a DFD: basic steps step 3: draw the lower-level diagrams functional decomposition of diagram 0 observe leveling – use a series of increasingly detailed DFDs to describe the information system observe balancing – maintain the input and output data flows of the parent DFD on the child DFD

modeling with DFDs guidelines for drawing DFDs context diagram should fit on one page use the name of the information system as the process name in the context diagram use unique names within each set of symbols do not cross lines provide a unique name and reference number for each process obtain as much user input and feedback as possible

modeling with DFDs other syntactic rules avoid the following combinations of data flow and process spontaneous generation black hole gray hole an external entity should be connected by a data flow to a process only data stores cannot be connected to each other

modeling with DFDs other concepts naming conventions conservation of data diverging data flows converging data flows composite data flow control flow (e.g., time)

Example: Context Diagram GRADING SYSTEM GRADED WORK STUDENT RECORDS SYSTEM STUDENT CLASS LIST SUBMITTED WORK FINAL GRADE GRADE REPORT GRADE PARAMETERS INSTRUCTOR

Example: Diagram 0 STUDENT RECORDS ASSIGN GRADE SYSTEM FINAL STUDENT 2 GRADE STUDENT WORK 3 STUDENT RECORDS SYSTEM STUDENT GRADE FINAL GRADE CLASS LIST GRADING DETAIL SUBMITTED WORK GRADED WORK ESTABLISH GRADEBOOK 1 CLASS GRADEBOOK GRADEBOOK D1 STUDENT CLASS DETAIL GRADE PARAMETERS 4 INSTRUCTOR PRODUCE GRADE REPORT GRADE REPORT

DFD levels Context diagram Level 0 diagram Level 1 diagram

4-model approach (classical) Current Proposed Logical physical aspects of system are removed as much as possible; current system is reduced to data and processes that transform them includes additional functions; obsolete functions are removed; inefficient data flows are reorganized Physical processes labels include the technology (people or systems) used to process the data; data flows and data stores are labeled with the actual name of the physical media on which data flow or in which data are stored represents the physical implementation of the new system

data dictionary data elements must be documented in a data dictionary so that clear, comprehensive information about the data and processes that make up the system are stored

data dictionary data flow attributes name or label description alternate name(s) origin destination data structure volume and frequency

data dictionary data store characteristics data store name or label description alternate name(s) data structure volume and frequency

data dictionary process characteristics process name or label description process number process description

data dictionary external entity characteristics entity name description alternate name(s) input data flows output data flows

data dictionary to document attributes, a data structure is used data structure – specific arrangement of data attributes that define a data store or a single instance of a data flow notation = + { } [ ] ()

data dictionary: example ORDER = ORDER NUMBER + ORDER DATE + [CUSTOMER NUMBER|CORPORATE ACCOUNT] + SHIP ADDRESS + BILL ADDRESS + {PRODUCT} + AMOUNT PRODUCT = PRODUCT NUMBER + DESCRIPTION + QUANITY ORDERED + PRICE + EXTENDED PRICE SHIP ADDRESS = ADDRESS BILL ADDRESS = ADDRESS ADDRESS = (POBOX NUMBER) + STREET + CITY + [STATE|PROVINCE] + (COUNTRY) + ZIPCODE ORDERS = {ORDER}

decomposition diagram depicts the partitioning of the system into logical subsystems or processes shows the top-down functional decomposition of a system used as a planning tool for the DFD also known as a hierarchy chart

decomposition diagram

process description describes the logic of primitive processes may be represented in the following forms: structured English decision table decision tree

process description structured English use the three logical building blocks sequence selection iteration use indentation for readability use standard terms used in the data dictionary and specific words that describe the processing rules

process description structured English examples Profits = Revenues - Expenses Generate Inventory Report Add Product record to Products IF Customer Not in Customers THEN Add Customer to Customers ELSE Add Current Sale to Customer’s Total Sales Add Total Sales to Customer balance in Customers FOR EACH Customer in Customers Delete Customer with matching Customer ID

process description decision tables alternative way to describe business rules or processing that involve a complex set of conditions shows a logical structure, with all possible combinations of conditions and resulting actions rows represent conditions and possible outcomes; columns indicate which combinations of conditions result in specific actions combination of conditions may result in more than one outcome

process description decision tables example Conditions and Actions Rule 1 Rule 2 Rule 3 Rule 4 type of check personal payroll check amount > 1000 yes - no company accredited by X cash the check X do not cash the check

process description decision trees Graphical representation of the conditions, actions, and rules found in a decision table choice between a decision table or decision tree is a matter of personal preference

process description decision trees example

other process analysis tools event-driven approach

use case analysis tool used to describe a business task and how the user (actor) interacts with the system to accomplish the task may be represented in narrative or graphical form no standard template for documenting detailed use cases analysts are encouraged to use templates that work for them or the project they are on

use case example Name: Place New Order Use-Case ID: UC123 Source: Requirement 1.00 Description: A club member submits a new order for products. The member’s account standing is validated. When the availability of products is verified, a packing order is sent to the warehouse for it to prepare the shipment. Back orders are created for products not in stock. On completion, the member is sent an order confirmation.

use case example Precondition: The party making the order must be a member Trigger: A new order is submitted Basic flow: Member provides order and payment information. All information are verified …. Result: An order confirmation is generated and sent to the club member

use case sections Use Case Name Iteration Description Preconditions Triggers Basic course of events Alternative paths Postconditions Business rules Notes Author and date Additional sections such as assumptions, exceptions, recommendations, technical requirements

event-response list depicts the business events that the system must respond to employ use case analysis contains information on: actor that initiates the event event trigger or input response or output

event decomposition diagram map the events to the decomposition serves as an outline for the lower-level data flow diagrams

system diagram merged events depicted in the data flow diagram a single diagram may represent the entire system may be a collection of DFDs representing a set of events for a major component of the system (e.g., a process in diagram 0)

modern structured analysis draw the context DFD of the proposed system make the corresponding functional decomposition diagram and diagram 0 create an event-response or use-case list draw an event DFD for each event * merge event DFDs into a system diagram draw detailed, primitive DFDs for the more complex events update the data dictionary make the process descriptions