A data-flow diagram (DFD) represents where data comes from, how a system transforms it, where it is stored and where it goes. It focuses on logical data relationships rather than screen layout, program-control order or network hardware.
Ask four questions: Who supplies or receives data? What process transforms it? Where is it stored? What data crosses each connection?
A DFD can contain physical objects and manual storage as well as digital data. For example, a library process may receive a book identifier, update a digital record and release a physical book from a shelf.
Determine which representation best suits each purpose.
QCAA Digital Solutions uses Gane–Sarson notation. Copy the shapes accurately enough that their different functions are unambiguous.
| Element | Job | Naming convention |
|---|---|---|
| External entity | A source or destination outside the area being studied. | Noun, such as Student or Payment service. |
| Process | Transforms incoming data into outgoing data. | Verb or verb–object name, such as Check booking. |
| Data store | A repository holding data for later use. | Numbered noun, such as D1 Bookings. |
| Data flow | Shows data moving in the arrow's direction. | Noun describing the actual data, such as booking request. |
A canteen ordering system includes a student, a process that calculates the order total, a collection of menu records and the order details sent into that process.
Recognise each DFD element and give it a suitable label.
Good labels allow a reader to understand the system without guessing.
| Prefix | Store type | Example |
|---|---|---|
| D | Data on disk, retained when power is off | D1 Accounts database |
| M | Manual data: physical files, paper records or manually processed digital data | M1 Consent forms |
| T | Temporary or transient data, not retained long-term | T1 Session cache |
Avoid vague labels: data, information, do booking and system hide the relationship. Name the actual data and transformation.
A draft DFD uses Member for an entity, Bookings for a process, Save for a data store and data for a flow containing a requested date and room identifier.
Analyse the labels and rewrite the three unsuitable ones.
A process is the active element that transforms, validates, records or retrieves data. Therefore an external entity cannot exchange data directly with another external entity or a data store.
| Connection | Valid? | Reason |
|---|---|---|
| Entity → process | Yes | The process receives data to transform. |
| Process → store | Yes | The process writes data to storage. |
| Store → process | Yes | The process retrieves stored data. |
| Entity → store | No | Data must be processed first. |
| Entity → entity | No | That exchange is outside the represented system unless a process handles it. |
Use a separate arrow for each direction. A request and reply are different flows and should be labelled separately.
A draft diagram shows a Customer entity sending details directly to D1 Orders. The store sends stuff directly to a Kitchen entity. A process between them is named Order.
Analyse the draft and describe three corrections.
A student submits a student ID and equipment ID. The system checks the equipment record, stores an approved loan and returns either a collection notice or an unavailable notice.
Do not merely list connected shapes. Explain what moves, in which direction, and what each process does with it.
Explain the relationship between the Student entity and D2 Loans. Include any process between them.
The Student supplies a student ID and equipment ID to Check availability. If D1 Equipment returns an available status, approved loan details pass to Record loan. Record loan then writes a loan record to D2 Loans and returns a collection notice to the Student. The Student does not access D2 Loans directly.
An explanation should connect evidence from the complete path.
[Source] sends [named data] to [process]. The process [transformation] using [data read from a store, if relevant], then sends or stores [named output] in [destination].
If the diagram mixes manual, physical and digital flows, state that distinction. A paper consent form can be an M store; a physical item can flow from a manual store through a delivery process; a digital identifier can trigger that action.
A volunteer sends a ticket code to 1.0 Validate ticket. The process reads valid-code data from D1 Tickets and returns an entry decision to the volunteer.
Explain the relationship between the Volunteer and D1 Tickets in two or three sentences.
The Volunteer sends a ticket code to Validate ticket. That process uses the code to retrieve valid-code data from D1 Tickets, compares the records and returns an entry decision to the Volunteer. The process mediates the relationship, so the Volunteer does not read the store directly.
A digital solution may contain a front end, back end and database while also exchanging data with external services.
Whether something is an external entity depends on the chosen system boundary. If the diagram studies a booking app, a payment service may be external. If it studies the payment provider itself, that provider contains processes and stores.
Do not draw technology for its own sake: a phone, Wi-Fi router or API is not automatically a DFD element. Represent a source, transformation, store or flow only when it helps explain the studied data process.
A beach-safety app receives a location from a Beachgoer. Its back end sends the location to an external Weather service, receives current conditions, combines them with alert rules from a database and returns a safety message.
Before drawing, extract the elements:
Notice that conditions request and current conditions are separate arrows because they travel in opposite directions.
Symbolise this scenario as a DFD on paper. Then check that every entity and store connects only through the process.
A correct diagram shows Beachgoer → location → 1.0 Generate safety message; the process → conditions request → Weather service; Weather service → current conditions → the process; D1 Alert rules → alert rules → the process; and the process → safety message → Beachgoer.
A community garden lends tools through secure lockers. A Member enters a membership code and tool ID at a terminal. The system must:
Use these required components:
On paper, develop a complete Gane–Sarson DFD. Number all processes and stores, identify each store type, and label every flow with the data it carries. Then explain the relationship between the Member and Loans store.
D1, D2 and D3 are disk stores because the scenario describes retained records.
The Member sends a membership code and tool ID to Validate request. That process checks member status from D1 Members and tool availability from D2 Tools. When both checks pass, approved loan details flow to Record loan, which writes a loan record to D3 Loans and returns a collection message. The Member therefore affects and receives confirmation of stored loan data through two processes rather than accessing the store directly.