# Backend - Paper Dynasty Game Engine

## Overview

FastAPI-based real-time game backend handling WebSocket communication, game state management, and database persistence for both SBA and PD leagues.

## Technology Stack

- **Framework**: FastAPI (Python 3.11+)
- **WebSocket**: Socket.io (python-socketio)
- **Database**: PostgreSQL 14+ with SQLAlchemy 2.0 (async)
- **ORM**: SQLAlchemy with asyncpg driver
- **Validation**: Pydantic v2
- **Testing**: pytest with pytest-asyncio
- **Code Quality**: black, flake8, mypy

## Project Structure

```
backend/
├── app/
│   ├── main.py                 # FastAPI app + Socket.io initialization
│   ├── config.py               # Settings with pydantic-settings
│   │
│   ├── core/                   # Game logic (Phase 2+)
│   │   ├── game_engine.py      # Main game simulation
│   │   ├── state_manager.py    # In-memory state
│   │   ├── play_resolver.py    # Play outcome resolution
│   │   ├── dice.py             # Secure random rolls
│   │   └── validators.py       # Rule validation
│   │
│   ├── config/                 # League configurations (Phase 2+)
│   │   ├── base_config.py      # Shared configuration
│   │   ├── league_configs.py   # SBA/PD specific
│   │   └── result_charts.py    # d20 outcome tables
│   │
│   ├── models/                 # Data models
│   │   ├── db_models.py        # SQLAlchemy ORM models
│   │   ├── game_models.py      # Pydantic game state models (Phase 2+)
│   │   └── player_models.py    # Polymorphic player models (Phase 2+)
│   │
│   ├── websocket/              # WebSocket handling
│   │   ├── connection_manager.py  # Connection lifecycle
│   │   ├── handlers.py            # Event handlers
│   │   └── events.py              # Event definitions (Phase 2+)
│   │
│   ├── api/                    # REST API
│   │   ├── routes/
│   │   │   ├── health.py       # Health check endpoints
│   │   │   ├── auth.py         # Discord OAuth (Phase 1)
│   │   │   └── games.py        # Game CRUD (Phase 2+)
│   │   └── dependencies.py     # FastAPI dependencies
│   │
│   ├── database/               # Database layer
│   │   ├── session.py          # Async session management
│   │   └── operations.py       # DB operations (Phase 2+)
│   │
│   ├── data/                   # External data (Phase 2+)
│   │   ├── api_client.py       # League REST API client
│   │   └── cache.py            # Caching layer
│   │
│   └── utils/                  # Utilities
│       ├── logging.py          # Logging setup
│       └── auth.py             # JWT utilities (Phase 1)
│
├── tests/
│   ├── unit/                   # Unit tests
│   ├── integration/            # Integration tests
│   └── e2e/                    # End-to-end tests
│
├── logs/                       # Application logs (gitignored)
├── venv/                       # Virtual environment (gitignored)
├── .env                        # Environment variables (gitignored)
├── .env.example                # Environment template
├── requirements.txt            # Production dependencies
├── requirements-dev.txt        # Dev dependencies
├── Dockerfile                  # Container definition
├── docker-compose.yml          # Redis for local dev
└── pytest.ini                  # Pytest configuration
```

## Key Architectural Patterns

### 1. Hybrid State Management
- **In-Memory**: Active game states for fast access (<500ms response)
- **PostgreSQL**: Persistent storage for recovery and history
- **Pattern**: Write-through cache (update memory + async DB write)

### 2. Polymorphic Player Models
```python
# Base class with abstract methods
class BasePlayer(BaseModel, ABC):
    @abstractmethod
    def get_image_url(self) -> str: ...

# League-specific implementations
class SbaPlayer(BasePlayer): ...
class PdPlayer(BasePlayer): ...

# Factory pattern for instantiation
Lineup.from_api_data(config, data)
```

### 3. League-Agnostic Core
- Game engine works for any league
- League-specific logic in config classes
- Result charts loaded per league

### 4. Async-First
- All database operations use `async/await`
- Database writes don't block game logic
- Connection pooling for efficiency

## Development Workflow

### Daily Development
```bash
# Activate virtual environment
source venv/bin/activate

# Start Redis (in separate terminal)
docker-compose up

# Run backend with hot-reload
python -m app.main

# Backend available at http://localhost:8000
# API docs at http://localhost:8000/docs
```

### Testing
```bash
# Run all tests
pytest tests/ -v

# Run with coverage
pytest tests/ --cov=app --cov-report=html

# Run specific test file
pytest tests/unit/test_game_engine.py -v

# Type checking
mypy app/

# Code formatting
black app/ tests/

# Linting
flake8 app/ tests/
```

## Coding Standards

### Python Style
- **Formatting**: Black with default settings
- **Line Length**: 88 characters (black default)
- **Imports**: Group stdlib, third-party, local (isort compatible)
- **Type Hints**: Required for all public functions
- **Docstrings**: Google style for classes and public methods

### Logging Pattern
```python
import logging

logger = logging.getLogger(f'{__name__}.ClassName')

# Usage
logger.info(f"User {user_id} connected")
logger.error(f"Failed to process action: {error}", exc_info=True)
```

### Error Handling
- **Raise or Return**: Never return `Optional` unless specifically required
- **Custom Exceptions**: Use for domain-specific errors
- **Logging**: Always log exceptions with context

### Dataclasses
```python
from dataclasses import dataclass

@dataclass
class GameState:
    game_id: str
    inning: int
    outs: int
    # ... fields
```

## Database Patterns

### Async Session Usage
```python
from app.database.session import get_session

async def some_function():
    async with get_session() as session:
        result = await session.execute(query)
        # session.commit() happens automatically
```

### Model Definitions
```python
from app.database.session import Base
from sqlalchemy import Column, String, Integer

class Game(Base):
    __tablename__ = "games"

    id = Column(UUID(as_uuid=True), primary_key=True)
    # ... columns
```

## WebSocket Patterns

### Event Handler Registration
```python
@sio.event
async def some_event(sid, data):
    """Handle some_event from client"""
    try:
        # Validate data
        # Process action
        # Emit response
        await sio.emit('response_event', result, room=sid)
    except Exception as e:
        logger.error(f"Error handling event: {e}")
        await sio.emit('error', {'message': str(e)}, room=sid)
```

### Broadcasting
```python
# To specific game room
await connection_manager.broadcast_to_game(
    game_id,
    'game_state_update',
    state_data
)

# To specific user
await connection_manager.emit_to_user(
    sid,
    'decision_required',
    decision_data
)
```

## Environment Variables

Required in `.env`:
```bash
# Database
DATABASE_URL=postgresql+asyncpg://user:pass@host:5432/dbname

# Application
SECRET_KEY=your-secret-key-at-least-32-chars

# Discord OAuth
DISCORD_CLIENT_ID=your-client-id
DISCORD_CLIENT_SECRET=your-client-secret

# League APIs
SBA_API_URL=https://sba-api.example.com
SBA_API_KEY=your-api-key
PD_API_URL=https://pd-api.example.com
PD_API_KEY=your-api-key
```

## Performance Targets

- **Action Response**: < 500ms from user action to state update
- **WebSocket Delivery**: < 200ms
- **Database Write**: < 100ms (async, non-blocking)
- **State Recovery**: < 2 seconds
- **Concurrent Games**: Support 10+ simultaneous games
- **Memory**: < 1GB with 10 active games

## Security Considerations

- **Authentication**: All WebSocket connections require valid JWT
- **Authorization**: Verify team ownership before allowing actions
- **Input Validation**: Pydantic models validate all inputs
- **SQL Injection**: Prevented by SQLAlchemy ORM
- **Dice Rolls**: Cryptographically secure random generation
- **Server-Side Logic**: All game rules enforced server-side

## Common Tasks

### Adding a New API Endpoint
1. Create route in `app/api/routes/`
2. Define Pydantic request/response models
3. Add dependency injection if needed
4. Register router in `app/main.py`

### Adding a New WebSocket Event
1. Define event handler in `app/websocket/handlers.py`
2. Register with `@sio.event` decorator
3. Validate data with Pydantic
4. Add corresponding client handling in frontend

### Adding a New Database Model
1. Define SQLAlchemy model in `app/models/db_models.py`
2. Create Alembic migration: `alembic revision --autogenerate -m "description"`
3. Apply migration: `alembic upgrade head`

## Troubleshooting

### Import Errors
- Ensure virtual environment is activated
- Check `PYTHONPATH` if using custom structure
- Verify all `__init__.py` files exist

### Database Connection Issues
- Verify `DATABASE_URL` in `.env` is correct
- Test connection: `psql $DATABASE_URL`
- Check firewall/network access
- Verify database exists

### WebSocket Not Connecting
- Check CORS settings in `config.py`
- Verify token is being sent from client
- Check logs for connection errors
- Ensure Socket.io versions match (client/server)

## References

- **Implementation Guide**: `../.claude/implementation/01-infrastructure.md`
- **Backend Architecture**: `../.claude/implementation/backend-architecture.md`
- **WebSocket Protocol**: `../.claude/implementation/websocket-protocol.md`
- **Database Design**: `../.claude/implementation/database-design.md`
- **Full PRD**: `../prd-web-scorecard-1.1.md`

---

**Current Phase**: Phase 1 - Core Infrastructure
**Next Phase**: Phase 2 - Game Engine Core