DRAFT — under teacher review.
What Is an Entity? (It's Not Just a Person)
Hamilton College · Year 12 · 2026
On context diagrams and data flow diagrams, students frequently draw only the humans they can think of — "Student", "Teacher" — and miss every external system their software talks to. Missing an entity is a rubric error at every level above 3–4, because the rubric rewards "relationships between the existing system, entities and data flows".
If your context diagram only has people as entities, you've almost certainly missed half of them. Every login, every email, every external API your project touches is its own entity.
The VCAA definition
Entity — the users or external systems that interact with the system being created.
Two words matter: users (human roles) and external systems (non-human). Both go on your context diagram as rectangles.
Types of entity — with examples
| Type | What it is | VCE SAT examples |
|---|---|---|
| Human user (primary) | The person the software is primarily built for | Student, Teacher, Administrator |
| Human user (secondary) | A person affected by the system's outputs | Parent, School Principal |
| External software system | An API, platform, or service your system sends data to or receives data from | Google Calendar API, Payment Gateway, Email Server |
| External data source | A data feed or file system outside your application boundary | School database, CSV import, Government open data |
A quick test: is it an entity?
Ask three questions:
- Does it send data into my system, or receive data out of it?
- Is it outside the boundary of the software I am building?
- Is it a distinct role or system (not an internal component of my software)?
If all three answers are "yes" — it is an entity. Add it to your diagram.
If it is inside your system (a function you are writing, a class in your code, a database table your code owns) — it is not an entity. It belongs on the DFD as a process or data store.
Commonly missed entities — by project type
These are the entities students most often miss. Check whether any apply to your project before submitting.
| Project type | Commonly missed entity | Why it is missed |
|---|---|---|
| Any project with login | Authentication provider (e.g. Google OAuth) | Students think login is "inside" their app |
| Any project that emails users | Email server / SMTP service | Students don't model outgoing system calls |
| School management tool | Admin user or IT department | Students only think about the primary user |
| App using external data | External database or API (e.g. weather API) | Students model the API as a feature, not an entity |
| App with notifications | Push notification service | Treated as internal, but it is external |
User stories → entities: the reliable method
Your data-collection-plan.md user stories are the best source of entities. For each story:
- "As a [role]" → that role is probably an entity (if they interact with your system directly).
- "I want [goal] so that [benefit]" → what external systems does achieving this goal require? Each one is a candidate entity.
Work through every user story before finalising your context diagram.
Why VCAA cares
The C2-2 rubric at 5–6 requires "illustrates the relationships between the existing system, entities and data flows". "Some errors or omissions exist" is still acceptable at 5–6. But at 9–10, "no errors, inconsistencies or omissions exist" — a missing entity is an omission, and omissions cost you the top band.
The most common C2-2 omission flagged in past years: the email server or notification service that every project needs but few students draw.
See also
- Context Diagram vs Data Flow Diagram
- C02 Resources: external reading and videos
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