🛡️ Sentinel: Distributed Global Rate Limiter

Sentinel is a high-performance, distributed rate-limiting system designed to protect microservices from traffic spikes, abuse, and DDoS attacks. Unlike traditional in-memory limiters, Sentinel synchronizes state across multiple instances using Redis, ensuring global consistency for your rate limits.

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⚡ Performance & Scale

Sentinel is built for high-scale production environments.

• Throughput: Capable of handling 50,000+ Requests Per Second (RPS) per node with Redis pipelining.
• Latency Overhead: Adds < 2ms to request processing time (p99).
• Scalability: Linearly scalable. Add more nodes, and the Redis backend handles the synchronization.
• Precision: 1ms timestamp resolution for burst detection.
• Reliability: 100% fail-open capability ensuring your service never goes down if the rate limiter fails.

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🏗️ System Architecture

Sentinel employs a Hybrid 2-Layer Defense strategy:

1. Layer 1: Local Burst Protection (Token Bucket)
   • Speed: Ultra-fast (nanosecond scale) in-memory check.
   • Goal: Prevents a single instance from being overwhelmed by instantaneous spikes.
2. Layer 2: Global Distributed Limiting (Redis Fixed Window)
   • Consistency: Atomic Lua scripts ensure accurate counting across all nodes.
   • Goal: Enforces global quotas (e.g., "1000 req/min across 50 servers").

graph TD
    Client([Client / Load Balancer]) -->|Request| Middleware
    
    subgraph "Sentinel Node"
        Middleware[HTTP Middleware]
        LocalLimiter[⚡ Local Burst Limiter]
        Config[⚙️ Dynamic Config]
    end
    
    Middleware -->|1. Check Local| LocalLimiter
    LocalLimiter -- Allowed --> GlobalCheck
    LocalLimiter -- Blocked --> Reject[❌ 429 Too Many Requests]
    
    GlobalCheck[2. Check Global] -->|Lua Script| Redis[(🔴 Redis Store)]
    
    Redis -- OK --> Forward[✅ Forward to Service]
    Redis -- Over Limit --> Reject

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🚀 Features

• Dashboard & Playground: A Next.js-based "Command Center" to visualize traffic in real-time and simulate attacks.
• Dynamic Configuration: Update rate limits on the fly via API without restarting services.
• Algorithm Agnostic: Designed to support Fixed Window (current), Sliding Window, and Token Bucket.
• Fail-Open Design: If Redis goes down, traffic flows through (configurable).
• Real-Time Metrics: JSON metrics endpoint for integration with Prometheus/Grafana.

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🛠️ Technology Stack

• Core: Go (Golang) 1.22+
• Distributed Store: Redis (with Lua scripting for atomicity)
• Frontend: Next.js 14 (App Router), Tailwind CSS, Recharts
• Containerization: Docker & Docker Compose

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🏁 Getting Started

Prerequisites

• Go 1.22+
• Node.js 18+ (for frontend)
• Docker (optional, for Redis)

1. Start the Backend

# Start Redis
docker-compose up -d redis

# Run the Sentinel Server
go run ./cmd/server
# Server running on :8080

2. Start the Frontend Dashboard

cd frontend
npm install
npm run dev
# Dashboard running at http://localhost:3000

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🎮 The "Zero to Blocked" Demo

Sentinel includes an interactive Traffic Playground to demonstrate its capabilities.

1. Open http://localhost:3000/playground.
2. Click START SIMULATION.
   • Action: Sends 20 requests/second to the backend.
3. Watch the logs turn from Green (200 OK) to Red (429 Blocked) as you hit the default limit.
4. Open http://localhost:3000/config in a new tab.
5. Change the limit for /playground to 50 requests / 60 seconds.
6. Observe the real-time adaptation in the Playground logs!

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🔌 API Reference

Update Rate Limit (Dynamic Config)

POST /api/config

{
  "path": "/api/payments",
  "limit": 100,
  "window": 60
}

Get Metrics

GET /metrics

Returns atomic counters for monitoring.

{
  "allowed_requests": 1450,
  "blocked_requests": 23,
  "redis_errors": 0
}

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📜 License

MIT License. Built for High-Scale Systems.