# Backend Architecture

## Overview

The Headquarter backend is built with **FastAPI** and follows a layered architecture pattern.

## Architecture Diagram

```
┌─────────────────────────────────────────────────────────────┐
│                         FastAPI App                          │
├─────────────────────────────────────────────────────────────┤
│  Middleware: CORS → Request Logging → Exception Logging      │
├─────────────────────────────────────────────────────────────┤
│  API Layer (src/api/)                                        │
│  ┌─────────┐ ┌──────────┐ ┌────────┐ ┌──────────┐          │
│  │  Auth   │ │ Projects │ │ Users  │ │   Git    │          │
│  │ Routes  │ │ Routes   │ │ Routes │ │ Repos    │          │
│  └────┬────┘ └────┬─────┘ └───┬────┘ └────┬─────┘          │
│  ┌──────────┐ ┌──────────┐                                  │
│  │ ToolInst │ │  Events  │                                  │
│  │ Routes   │ │ Routes   │                                  │
│  └────┬─────┘ └────┬─────┘                                  │
├───────┼───────────┼───────────┼───────────┼─────────────────┤
│       │           │           │           │                  │
│  Auth │  Project  │   User    │   Git     │                  │
│ Layer │  Service  │  Service  │ Service   │                  │
│       │           │           │           │                  │
├───────┴───────────┴───────────┴───────────┴─────────────────┤
│  Data Layer                                                  │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐                   │
│  │  Models  │  │ Database │  │   Config │                   │
│  │(SQLAlch) │  │(AsyncPG) │  │(Pydantic)│                   │
│  └──────────┘  └──────────┘  └──────────┘                   │
└─────────────────────────────────────────────────────────────┘
```

## Directory Structure

```
src/
├── api/                    # API Routes
│   ├── auth.py            # Authentication endpoints
│   ├── projects.py        # Project endpoints
│   ├── git_repositories.py # Repository endpoints
│   ├── users.py           # User endpoints
│   ├── tool_types.py      # Tool type endpoints
│   ├── tool_instances.py  # Tool instance endpoints
│   ├── ssh_keys.py        # SSH key endpoints
│   ├── events.py          # SSE streaming endpoint
│   └── dashboard.py       # Dashboard endpoints
├── auth/                   # Authentication
│   ├── session.py         # Session management
│   ├── oidc.py            # OAuth2 client
│   ├── dependencies.py    # Auth dependencies
│   └── cookies.py         # Cookie utilities
├── models/                 # Database Models
│   ├── user.py            # User model
│   ├── project.py         # Project model
│   ├── git_repository.py  # Repository model
│   ├── tool_type.py       # Tool type model
│   ├── ssh_key.py         # SSH key model
│   ├── instance_event.py  # Instance event audit model
│   ├── health_check.py    # Health check snapshot model
│   └── user_config.py     # User config model
├── services/               # Services
│   ├── docker.py          # Docker operations
│   ├── terminal_manager.py # Terminal session manager
│   ├── event_bus.py       # Instance event bus (pub/sub)
│   ├── health_monitor.py  # Background health monitoring
│   └── lifecycle_hooks.py # Instance lifecycle events
├── utils/                  # Utilities
│   ├── git_url_parser.py  # URL parsing
│   ├── git_files.py       # Git file operations
│   └── git_history.py     # Git history operations
├── config.py              # Configuration
├── database.py            # Database setup
└── main.py                # Application entry point
```

## Layers

### 1. API Layer (`src/api/`)

**Responsibilities:**
- Define HTTP endpoints
- Parse request parameters
- Return HTTP responses
- Use dependencies for auth and DB

**Pattern:**
```python
@router.get("/projects")
async def list_projects(
    session: AsyncSession = Depends(get_db_session),
    user_id: str = Depends(get_current_user_id),
):
    # Call service layer
    projects = await project_service.list(session, user_id)
    return projects
```

### 2. Auth Layer (`src/auth/`)

**Responsibilities:**
- Session management (create, verify, expire)
- OAuth2 flow (login, callback)
- User authentication dependencies

**Key Components:**
- `session.py`: HMAC-SHA256 signed cookies
- `oidc.py`: OAuth2 token exchange
- `dependencies.py`: FastAPI dependencies for auth

### 3. Data Layer (`src/models/`, `src/database.py`)

**Responsibilities:**
- Database schema definition
- Async database sessions
- Connection management

**Technology:**
- SQLAlchemy 2.0 with async support
- asyncpg driver for PostgreSQL
- Alembic for migrations

### 4. Utility Layer (`src/utils/`)

**Responsibilities:**
- Git operations (file browsing, history)
- URL parsing
- Helper functions

## Data Flow

### Request Lifecycle

```
1. Request arrives at FastAPI
2. Middleware processes (CORS, logging)
3. Auth dependency verifies session
4. Route handler processes request
5. Database session executes queries
6. Response returned to client
```

### Authentication Flow

```
1. User clicks login
2. Backend redirects to Authentik
3. User authenticates
4. Authentik redirects with code
5. Backend exchanges code for token
6. Backend fetches user info
7. Backend creates session cookie
8. User is authenticated
```

## Dependencies

### Database Session

```python
async def get_db_session() -> AsyncGenerator[AsyncSession, None]:
    async with SessionLocal() as session:
        yield session
```

### Current User

```python
async def get_current_user_id(
    request: Request,
    settings: Settings = Depends(get_settings),
) -> str:
    # Verify session cookie
    # Return user_id
```

## Configuration

Configuration is managed via Pydantic Settings:

```python
class Settings(BaseSettings):
    app_env: str = "development"
    database_url: str = "..."
    authentik_domain: str = "..."
    # ...
```

Environment variables are automatically loaded from `.env` files.

## Error Handling

Errors are handled at multiple levels:

1. **Validation**: Pydantic validates request bodies
2. **HTTP Exceptions**: FastAPI HTTPException for client errors
3. **Middleware**: ExceptionLoggingMiddleware logs server errors
4. **Database**: SQLAlchemy errors converted to HTTP responses

## Testing

- **Unit tests**: SQLite in-memory database
- **Integration tests**: PostgreSQL with transaction rollback
- **Fixtures**: Shared in `conftest.py`

## Monitoring & Notifications

The backend includes a real-time monitoring system:

### Components

- **InstanceEventBus** (`services/event_bus.py`): Typed pub/sub singleton for instance lifecycle events
- **HealthMonitor** (`services/health_monitor.py`): Asyncio background task polling container health every 15s
- **SSE Endpoint** (`api/events.py`): Server-Sent Events streaming for real-time frontend updates
- **Lifecycle Hooks** (`services/lifecycle_hooks.py`): Publishes events on create/start/stop/restart/delete

### Event Flow

```
Container Action → Lifecycle Hook → EventBus → SSE Stream → Frontend Toast
```

### Event Types

| Event | When Fired |
|-------|-----------|
| `instance.created` | After DB insert |
| `instance.starting` | Before docker compose up |
| `instance.running` | After readiness probe succeeds |
| `instance.error` | Build fail, crash, or probe fail |
| `instance.stopped` | After docker compose stop |

## Technology Stack

| Component | Technology | Version |
|-----------|-----------|---------|
| Web Framework | FastAPI | ^0.104 |
| ORM | SQLAlchemy | ^2.0 |
| Database Driver | asyncpg | ^0.29 |
| Validation | Pydantic | ^2.0 |
| Migrations | Alembic | ^1.12 |
| HTTP Client | httpx | ^0.25 |
| Testing | pytest | ^7.4 |

## Related Documentation

- [Database Schema](database.md)
- [Frontend Architecture](../frontend.md)
- [API Documentation](../api/)