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".

Tip

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.

Going further: Gherkin scenarios (optional but excellent)

Not required by the VCAA curriculum — but a powerful tool for surfacing entities you'd otherwise miss. Worth knowing if you want top-band detail.

A Gherkin scenario expands a user story into concrete Given / When / Then steps. Every noun in those steps is a candidate entity.

LitheSpeed — What Is Gherkin? (~1 min). The bare-bones intro; no need to learn the BDD tooling around it.

Example. A user story like "As an account holder I want to withdraw cash so that I have money for the day" expands to:

Feature: Account Holder withdraws cash

Scenario: Account has sufficient funds
   Given the account balance is $100
     And the card is valid
     And the machine contains enough money
    When the Account Holder requests $20
    Then the ATM should dispense $20
     And the account balance should be $80
     And the card should be returned

Entities this scenario surfaces (each underlined noun is an external system the ATM software talks to):

Account Holder — the human user (you'd already have this).
Card / Authentication system — "the card is valid" implies an external check (bank network or token validator).
Bank account database — "balance is $100 → $80" means your software reads and writes to an external account store.
Cash dispenser hardware — "dispense $20" is a physical subsystem outside your software.
Card reader hardware — "card should be returned" is another physical subsystem.

A user story alone might surface only Account Holder. The Gherkin expansion forces you to confront every Given precondition and every Then effect — that's where the missing entities live.

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.