# Domain Entities — Unit 4: Dashboard

## Entities

### AvailabilityStatus (enumeration)

A union type representing the three possible system states returned by the API.

| Value | Meaning |
|---|---|
| `Available` | System is fully operational |
| `Maintenance` | System is in planned maintenance mode |
| `Unavailable` | System is not accessible |

---

### AvailabilityResponse (API response shape)

Returned by `GET /api/v1/Availability/status`.

| Field | Type | Description |
|---|---|---|
| `status` | `AvailabilityStatus` | Current system status |
| `checkedAt` | `string` (ISO 8601) | Timestamp when status was last evaluated |
| `message` | `string` | Optional human-readable reason or note (may be empty string) |

---

## Entity Relationship Diagram

```mermaid
classDiagram
    class AvailabilityResponse {
        +status: AvailabilityStatus
        +checkedAt: string
        +message: string
    }
    class AvailabilityStatus {
        <<enumeration>>
        Available
        Maintenance
        Unavailable
    }
    AvailabilityResponse --> AvailabilityStatus : status

    style AvailabilityResponse fill:#4CAF50,stroke:#2E7D32,color:#000
    style AvailabilityStatus fill:#2196F3,stroke:#0D47A1,color:#000
```

AvailabilityResponse holds a status (one of three enumeration values) plus a timestamp and message.

---

## Type Placement

Both `AvailabilityStatus` and `AvailabilityResponse` are added to `frontend/src/api/types.ts` — the shared domain types file, consistent with `User`, `AuthResponse`, and `SetupStatus`.

---

## TanStack Query Dependency

`@tanstack/react-query` is added as a new production dependency. A shared `QueryClient` is configured in `main.tsx` and provided via `QueryClientProvider`. This enables `staleTime`, background refetching, and persistent cached data across error states.