# Backend — Social Proximity

## Technology Stack

| Component | Choice | Reason |
|---|---|---|
| Framework | FastAPI (Python) | Async, typed, WebSocket support, fast to build |
| Session store | Redis | Ephemeral data, TTL-based expiry, low latency |
| Database | PostgreSQL | Aggregate stats only — structured, durable |
| Real-time | WebSockets (FastAPI native) | Push nudge to app without polling |

## Project Structure

```
backend/
├── app/
│   ├── main.py               ← FastAPI app init, router registration
│   ├── api/
│   │   ├── session.py        ← POST /session — register BLE token + interests
│   │   ├── match.py          ← POST /match — attempt proximity match
│   │   ├── ws.py             ← WebSocket /ws/{token} — real-time nudge channel
│   │   ├── stats.py          ← GET /stats — anonymised aggregate stats (admin only)
│   │   └── health.py         ← GET /health
│   ├── models/
│   │   ├── session.py        ← Pydantic: SessionCreate, SessionResponse
│   │   ├── match.py          ← Pydantic: MatchRequest, MatchResult
│   │   └── stats.py          ← Pydantic: StatsResponse
│   ├── services/
│   │   ├── session_service.py ← create/get/expire sessions in Redis
│   │   ├── match_service.py   ← interest overlap logic + nudge dispatch
│   │   └── stats_service.py   ← aggregate stats queries (PostgreSQL)
│   └── core/
│       ├── config.py          ← Settings (env vars via pydantic-settings)
│       ├── redis.py           ← Redis connection pool
│       └── database.py        ← SQLAlchemy async engine + session
├── tests/
│   ├── test_session.py
│   ├── test_match.py
│   └── test_stats.py
├── requirements.txt
├── requirements-dev.txt
├── Dockerfile
└── .env.example
```

## API Endpoints

### `POST /session`
Register a new ephemeral session when user enables "open to talk".

**Request:**
```json
{
  "ble_token": "abc123",
  "interests": ["devops", "hardstyle", "philosophy"]
}
```

**Response:**
```json
{
  "session_id": "uuid",
  "expires_at": "2024-01-01T12:00:00Z"
}
```

- BLE token stored in Redis with 2-hour TTL
- Interest list hashed before storage — raw interests never persisted to DB

---

### `POST /match`
Called when a user's app detects a nearby BLE token.

**Request:**
```json
{
  "own_token": "abc123",
  "detected_token": "xyz789"
}
```

**Response:**
```json
{
  "match": true,
  "shared_interests": ["devops", "hardstyle"],
  "nudge_sent": true
}
```

- If match found: WebSocket nudge pushed to both parties
- Shared interests returned as category labels only — no raw profile data

---

### `WebSocket /ws/{ble_token}`
Persistent connection from the app. Used to receive real-time nudges.

**Nudge message (server → client):**
```json
{
  "type": "nudge",
  "shared_interests": ["devops", "hardstyle"],
  "message": "Someone nearby shares your interest in devops and hardstyle."
}
```

---

### `GET /health`
Standard health check. Returns `200 OK` with service status.

---

### `GET /stats`
Anonymised aggregate stats for admin dashboard. Internal access only.

**Response:**
```json
{
  "total_sessions_today": 142,
  "total_matches_today": 38,
  "top_interest_categories": ["tech", "music", "philosophy"],
  "active_sessions_now": 12
}
```

## Data Model (Redis)

Sessions are stored as Redis hashes:

```
KEY:   session:{ble_token}
VALUE: {
  "session_id": "uuid",
  "interests_hash": "sha256(...)",
  "interest_categories": ["tech", "music"],
  "created_at": "iso8601"
}
TTL:   7200 seconds (2 hours)
```

## Data Model (PostgreSQL)

Only aggregate counters — no user-level rows.

```sql
CREATE TABLE daily_stats (
    date          DATE PRIMARY KEY,
    total_sessions INTEGER DEFAULT 0,
    total_matches  INTEGER DEFAULT 0
);

CREATE TABLE interest_category_counts (
    date          DATE,
    category      TEXT,
    count         INTEGER DEFAULT 0,
    PRIMARY KEY (date, category)
);
```

## Environment Variables

```env
REDIS_URL=redis://localhost:6379
DATABASE_URL=postgresql+asyncpg://user:pass@localhost/sighej
SESSION_TTL_SECONDS=7200
ALLOWED_ORIGINS=http://localhost:3000
```

## Running Locally

```bash
cd backend
python -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt
uvicorn app.main:app --reload --port 8000
```

Or via Docker Compose from root:

```bash
docker-compose up
```