# Backend Architecture (Go)

The LLM Proxy backend is implemented in Go, focusing on high performance, clear concurrency patterns, and maintainability.

## Core Technologies

- **Runtime:** Go 1.22+
- **Web Framework:** [Gin Gonic](https://github.com/gin-gonic/gin) - Fast and lightweight HTTP routing.
- **Database:** [sqlx](https://github.com/jmoiron/sqlx) - Lightweight wrapper for standard `database/sql`.
- **SQLite Driver:** [modernc.org/sqlite](https://modernc.org/sqlite) - CGO-free SQLite implementation for ease of cross-compilation.
- **Config:** [Viper](https://github.com/spf13/viper) - Robust configuration management supporting environment variables and files.

## Project Structure

```text
├── cmd/
│   └── llm-proxy/          # Entry point (main.go)
├── internal/
│   ├── config/             # Configuration loading and validation
│   ├── db/                 # Database schema, migrations, and models
│   ├── middleware/         # Auth and logging middleware
│   ├── models/             # Unified request/response structs
│   ├── providers/          # LLM provider implementations (OpenAI, Gemini, etc.)
│   ├── server/             # HTTP server, dashboard handlers, and WebSocket hub
│   └── utils/              # Common utilities (multimodal, etc.)
└── static/                 # Frontend assets (served by the backend)
```

## Key Components

### 1. Provider Interface (`internal/providers/provider.go`)
Standardized interface for all LLM backends:
```go
type Provider interface {
	Name() string
	ChatCompletion(ctx context.Context, req *models.UnifiedRequest) (*models.ChatCompletionResponse, error)
	ChatCompletionStream(ctx context.Context, req *models.UnifiedRequest) (<-chan *models.ChatCompletionStreamResponse, error)
}
```

### 2. Asynchronous Logging (`internal/server/logging.go`)
Uses a buffered channel and background worker to log every request to SQLite without blocking the client response. It also broadcasts logs to the WebSocket hub for real-time dashboard updates.

### 3. Session Management (`internal/server/sessions.go`)
Implements HMAC-SHA256 signed tokens for dashboard authentication. Sessions are stored in-memory with configurable TTL.

### 4. WebSocket Hub (`internal/server/websocket.go`)
A centralized hub for managing WebSocket connections, allowing real-time broadcast of system events and request logs to the dashboard.

## Concurrency Model

Go's goroutines and channels are used extensively:
- **Streaming:** Each streaming request uses a goroutine to read and parse the provider's response, feeding chunks into a channel.
- **Logging:** A single background worker processes the `logChan` to perform database writes.
- **WebSocket:** The `Hub` runs in a dedicated goroutine, handling registration and broadcasting.

## Security

- **Encryption Key:** A mandatory 32-byte key is used for both session signing and encryption of sensitive data in the database.
- **Auth Middleware:** Verifies client API keys against the database before proxying requests to LLM providers.
- **Bcrypt:** Passwords for dashboard users are hashed using Bcrypt with a work factor of 12.