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1 DATA FLOW DIAGRAM

1 DATA FLOW DIAGRAM. 2 Outline Process decomposition diagrams Data flow diagram (DFD)

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Page 1: 1 DATA FLOW DIAGRAM. 2 Outline Process decomposition diagrams Data flow diagram (DFD)

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DATA FLOW DIAGRAM

Page 2: 1 DATA FLOW DIAGRAM. 2 Outline Process decomposition diagrams Data flow diagram (DFD)

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Outline

• Process decomposition diagrams• Data flow diagram (DFD)

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Process Model• A process is a business activity which when executed produces certain outputs from given inputs

• The function(s) performed by a process may be complex, with multiple inputs, outputs and users

• The entire application itself is a process

• We use successive decomposition into sub processes to reveal greater details of the processing

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Function Decomposition• Decomposition splits wok of a task into subtasks; subtasks together make-up the parent task; not like ‘calling’ a module

• Balanced decomposition: sub-tasks are roughly equal in complexity

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Function Decomposition …• Top-down decomposition gives hierarchical structure

• Decompose into 2 or more; not more than 5

• A high cohesion (high independence) and minimum coupling (minimum interdependence) are fundamental criteria

• Continue decomposition until elementary processes are identified

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Function Decomposition• Elementary process is a smallest unit of activity meaningful to end user (it sees and leaves data in consistent state)

• Process decomposition diagram– A tree structure– Elementary processes are leaf nodes– Data are not shown

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FD Diagrams: Examples

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FD Diagrams: Examples

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• Exercise: prepare FDDs for– Railway reservation system– Hospital patient management– Employee payroll

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Function Decomposition …• Use proper naming of processes• Business functions named as nouns (marketing, Inventory control, …)

• Process name consists of an active verb and an object (accept order, calculate interest, …)

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Naming …• Avoid long names (sentences containing and, if, then, etc. indicate non-cohesive complex tasks)

• Real world is a good reference for selecting proper names; organizational units are organized functionally and each unit has a well-defined task

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Data Flow Diagram (DFD)• Very popular tool for describing functions of a system in terms of processes and data used by them

– FDD may be done before DFD or we may prepare DFDs directly

– Have more contents than FDDs– Flow of data is shown, not flow of control

• DFDs are simple pictorial representations; easily understood by users and management.

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Data Flow Diagrams (DFD) …• DFDs are unambiguous and concise• They can describe processing at physical as well as logical levels

• DFDs facilitate top-down development

• They permit outlining of preferences and scope

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DFD Notation• Data Flow : labeled arrow

• Sources and sinks of information/data (also called external entity)

• Processn

or

(n : number for referencing• Available data (data store)

nor

or

n

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Example: Air line reservation

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Context Diagram• Shows the entire application as a single process

• Identifies its external interfaces• This is the starting point; also called Fundamental System Model, Level 0 DFD

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Context Diagram …

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Process Refinement• Decompose a process into sub processes

– May reveal more data stores, external interfaces

– Use decimal numbering system: process 1 is decomposed into 1.1, 1.2, etc.

• At each level, understand all data flows and processing; label processes, data stores and data flows (arrows) meaningfully.

• Continue decomposition and stop when control flow (conditional branches, loops) surfaces. Refine until processes are well understood.

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Refinement …• DFDs do not show control flow• DFDs do not show initializations (such as initial file creation), but show processes running in a steady state

• DFDs only show exceptions/errors having specific business rules to deal with them; handling of routine errors generally not shown

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Refinement …• Processes must be independent of each other (cohesion, coupling principles apply); work of a process depends only on its inputs and not on state of another process

• Only needed data should be input, and outputs should be based on data entering the process

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Refinement …• Ensure consistency among levels: inputs

and outputs at previous level should be present at next level

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Physical DFD• Shows implementation details

– Names and locations of places/people– Ways of storing data (like card indexes)– Tasks as performed today: e.g., 2

persons handling UG, PG students separately.

• Useful for describing existing system to validate it with users

• Needs to be converted into logical DFD after validation from users

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Showing Boundaries

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DFD Example 1: Payroll

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DFD Example 2: Old Car Mart• Buys and sells old cars; has large number in stock: different models, make, year,colors,…

• Does some repairs for adding value; records kept; has own garage

• Advertise in news papers• Salesmen hired on commission basis to handle customers, negotiate, etc.

• Needs to take stocks; prepare summary of sales, profits, etc; pay salesmen

Prepare DFD and ER diagrams

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Car Mart …

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Car Mart …• Define data stores and flow clearly to understand the DFD

• We can further refine some of the processes

• For process 5: Making a sale– Take buyer requirements and his/her

other details– List cars that match requirements– Show repair history, car history– Register sale and negotiated price– Compute commission

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E-R Diagram

Add importantattributes, givecardinalities

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Example 3: Book Supplier• Supplies books to customers; no stocks maintained; books sourced directly from publishers

• Prepare context diagrams

(all inputs/output not shown, such as invoices,…..

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Book-Supplier : Refinement 1

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Book Supplier: Exploding Process 2

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Book Supplier : homework• Refinement 1 needs to be extended for handling payments

• Extend for : Payments from customers

– Create ‘a/c receivable’ for books sent– Update receivable when payments

received– Credit rating will need to be adjusted

periodically• Extend for : Payments to publishers

– Create ‘a/c payable’ when invoices received

– Check invoices with books actually received

– Make payments periodically as per payment terms (within fixed days; incentives for early payment, etc.)

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Summary• Process modeling by process decomposition

• DFD shows data flows, stores and processes, but not control flows

• Proper naming of stores, processes and indicating data flowing among them very important for DFDs to be independently readable