# Health Check System Implementation

## Overview

This module implements a comprehensive health check system for the P2P Foundation with the following features:

### ✅ Implemented Features

1. **HTTP Endpoints**
   - `/health` - Basic liveness check
   - `/ready` - Readiness check with component status
   - `/metrics` - Prometheus-compatible metrics export
   - `/debug/vars` - Debug information endpoint

2. **Component Health Checkers**
   - `NetworkHealthChecker` - Monitors peer connections
   - `DhtHealthChecker` - Monitors DHT routing table
   - `StorageHealthChecker` - Monitors storage availability
   - `ResourceHealthChecker` - Monitors CPU/memory usage
   - `TransportHealthChecker` - Monitors transport status
   - `PeerHealthChecker` - Monitors peer count thresholds
   - `CompositeHealthChecker` - Combines multiple checkers

3. **Health Response Format**
   ```rust
   pub struct HealthResponse {
       pub status: String,      // "healthy", "degraded", or "unhealthy"
       pub version: String,
       pub uptime: Duration,
       pub checks: HashMap<String, ComponentHealth>,
       pub timestamp: SystemTime,
   }
   ```

4. **Performance Features**
   - Response time < 100ms achieved through:
     - Parallel component checks
     - 100ms response caching
     - 50ms timeout per component
   - Graceful degradation support
   - Zero-panic implementation

5. **Prometheus Metrics**
   - Node information
   - Uptime counter
   - Component health gauges
   - System resource metrics
   - Custom component metrics

## Architecture

```
health/
├── mod.rs          # Core types and HealthManager
├── checks.rs       # Component health checkers
├── endpoints.rs    # HTTP endpoint handlers
└── metrics.rs      # Prometheus metrics export
```

## Usage Example

```rust
use saorsa_core::health::{HealthManager, HealthServer, NetworkHealthChecker};
use std::sync::Arc;

// Create and configure health manager
let health_manager = Arc::new(HealthManager::new("1.0.0".to_string()));

// Register component checkers
health_manager.register_checker(
    "network",
    Box::new(NetworkHealthChecker::new(|| async { Ok(peer_count) }))
).await;

// Start health server
let (server, shutdown_tx) = HealthServer::new(health_manager, addr);
tokio::spawn(async move {
    server.run().await
});
```

## Production Readiness

- ✅ Zero `unwrap()` or `expect()` in production code
- ✅ Comprehensive error handling with proper types
- ✅ Full async/await support
- ✅ Thread-safe implementation
- ✅ Resource cleanup on shutdown
- ✅ Extensive test coverage

## Integration Points

The health checkers are designed to integrate with the actual P2P components once they're available:

- Network module for peer counts
- DHT module for routing table size
- Transport module for connection status
- Storage module for disk operations
- Resource manager for system metrics

Currently, the checkers use function callbacks to allow flexible integration without hard dependencies.