← VCE Software Development U3O2

Drawing a Level 1 data flow diagram from a scenario

Breaks a system into processes, data stores, entities and labelled flows found in a scenario, then checks the rules, e.g. every process has an input and an output and no flow is missing.

Part of: VCE Software Development - U3O2 - Analysis and Design — Module 46, QCE Digital Solutions Unit 2 — Data-Driven Problems and Solution Requirements — Module 90, QCE Digital Solutions Unit 3 — Interactions Between Users, Data and Digital Systems — Module 28, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 131, QCE Digital Solutions Unit 4 — Complex Digital Data Exchange Problems and Solution Requirements — Module 93

Understanding Drawing a Level 1 data flow diagram from a scenario in VCE Software Development U3O2

What it does: Data kept for later is drawn as a data store. It is labelled with a noun, and it is found by looking for records or files the system keeps. One rule check is that every flow touching a data store must connect to a process. The process Update Stock sits between the data and the store. Processes are actions named with a verb and a noun. Librarian is a person, so it would be an external entity, not a process. Every process needs at least one input and at least one output. A process with no output is called a black hole.

Common mistakes: A data store cannot send data straight to an entity. The data must pass through a process, so the flow should be P1 Loan -> Member: receipt. Data cannot flow directly between two external entities. The invoice must come from a process, so it should be P1 Order -> Customer: invoice.

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This module's key terms are also in a printable crossword (Modules 42–51) and word search (Modules 26–51).

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Learning
Duo
A scenario says: 'The librarian checks a book out to a student.' Which is the best process name?
LibrarianCheck out book
Matches
Customer -> D1 Orders (flow goes direct from entity to store)
↓
An entity cannot send data straight to a data store. The flow must pass through a process such as Record Order.
Applying
Meanings
Term
Level 1 DFD
Meaning
Breaks a system into processes, data stores, entities and labelled flows found in a scenario, then checks the rules, e.g. every process has an input and an output and no flow is missing.
Practice
Entity: Student
Testing
Gaps
1.0: Validate Order
Check Answer
Spots
3D1 Loans -> P1 Loan: due date
4D1 Loans -> Member: receipt

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Quick Questions

1. A scenario says: 'The librarian checks a book out to a student.' Which is the best process name?
A. Check out book ✓
B. Librarian
Processes are actions named with a verb and a noun. Librarian is a person, so it would be an external entity, not a process.
2. A scenario says: 'Member details are saved so they can be looked up later.' What does this tell you to draw?
A. A data store holding member details ✓
B. A new external entity called Member Details
Data that is saved and retrieved later belongs in a data store. Entities are people or systems outside the system.
3. When reading a scenario to find processes, what should you look for?
A. Verbs that show the system doing something to data ✓
B. Nouns that name the people involved
Processes are actions, so look for verbs. People and organisations are usually external entities.
4. A process called 'Calculate total' has two data flows going in but none coming out. What is the problem?
A. Nothing is wrong, because it has inputs
B. The process is missing an output flow ✓
Every process needs at least one input and at least one output. A process with no output is called a black hole.
5. A flow is drawn directly from the Customer entity to the Orders data store. Is this correct?
A. Yes, entities can write straight to stores
B. No, the flow must pass through a process ✓
Data cannot move between an entity and a store without a process acting on it. Add a process such as Record order in the middle.

Match the Pairs

Verb phrase in a circle: Validate Payment → A process found in the scenario by spotting an action the system performs on data, named with a verb and a noun
Process with only an arrow going in: Check Stock → A missing flow error, because the process needs an output as well as an input
A noun in the scenario that holds data for later use, such as a record of members, becomes a data store → D1 Member Records
Data must move between a store and a process, never straight from one store to another → Process: Update Stock -> D2 Inventory
Customer -> D1 Orders (flow goes direct from entity to store) → An entity cannot send data straight to a data store. The flow must pass through a process such as Record Order.
Process: Calculate Total (input: Item Prices, no output) → The process has no output flow, so the calculated total goes nowhere. It needs an output flow such as Order Total to the Customer or a store.

Key Term

Level 1 DFD
Breaks a system into processes, data stores, entities and labelled flows found in a scenario, then checks the rules, e.g. every process has an input and an output and no flow is missing.

Fill in the Blanks

1. Process 1.0: Validate Order
Hint: Type the DFD element name for a numbered step that changes data, named with a verb and a noun.
2. Entity: Customer (outside the system)
Hint: Type the element name for an external person or organisation that sends or receives data.
3. Data store D1: Orders Flow: 1.0 -> D1 (valid order)
Hint: Type the symbol that shows which way the data flows from the process to the store.
4. Error: Customer -> D1 skips the process
Hint: Type the element that must sit between an external entity and a data store.
5. Check: 2.0 Process Payment has an input Error: no output flow leaves 2.0
Hint: Type the word for the flow that must leave every process, which is missing here.

Spot the Error

Question 1
A Member -> P1 Loan: request
B P1 Loan -> D1 Loans: record
C D1 Loans -> P1 Loan: due date
D D1 Loans -> Member: receipt
Line D — A data store cannot send data straight to an entity. The data must pass through a process, so the flow should be P1 Loan -> Member: receipt.
Question 2
A Customer -> P1 Order: details
B P1 Order -> D1 Stock: update
C Customer -> Courier: invoice
D P1 Order -> Courier: label
Line C — Data cannot flow directly between two external entities. The invoice must come from a process, so it should be P1 Order -> Customer: invoice.
Question 3
A Student -> P1 Enrol: form
B P1 Enrol -> D1 Classes: place
C P1 Enrol -> Student
Line C — Every data flow must be labelled with the data it carries. The flow should be P1 Enrol -> Student: timetable.
Question 4
A Patient -> D1 Bookings: slot
B P1 Book -> D1 Bookings: slot
C D1 Bookings -> P2 Remind: slot
D P2 Remind -> Patient: SMS
Line A — An entity cannot write directly into a data store. The data must go through a process first, so the flow should be Patient -> P1 Book: request.
Question 5
A Teacher -> P1 Mark: results
B D1 Grades -> P1 Mark: marks
C D1 Grades -> P2 Report: marks
D P2 Report -> Parent: report
Line B — The marks are being saved, so the flow must go from the process into the store. As written, D1 Grades has no input flow, so nothing is ever stored. It should be P1 Mark -> D1 Grades: marks.

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