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Ref: Prof Sarda

Process Modeling……

Lecture 08

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Outline

•Data flow diagram (DFD)

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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