🌍 CDN System Design

A Content Delivery Network (CDN) prototype designed to serve static assets (images, CSS/JS, video segments) with low latency, high availability, and secure delivery across multiple edge locations.

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🚀 Goals / Scope (v1)

• Serve static assets efficiently with caching and origin protection.
• Support HTTPS, HTTP/2, and QUIC-ready architecture.
• Global edge presence via Geo-DNS routing.
• Implement cache invalidation APIs, rate limiting, and DDoS protection.
• Provide metrics, logging, autoscaling, and cert management.

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🏗️ High-Level Architecture

             +-----------------+
             |   Control Plane  |  <-- (Management APIs, UI, Cert Manager)
             +-----------------+
                      |
      +---------------+---------------+
      |                               |
+-------------+               +----------------+
|  Origin(s)  |               | Origin Storage  |
|  (App/API)  |               |  (S3 / Blob)    |
+-------------+               +----------------+
      ^                               ^
      |                               |
  +---+-------------------------------+---+
  |     Global DNS / Anycast / Geo-DNS     |
  +----------------------------------------+
         |         |         |         |
         v         v         v         v
       Edge A    Edge B    Edge C    Edge D

Each Edge POP handles:

• TLS termination
• HTTP/2 & QUIC parsing
• Caching (RAM + SSD)
• Rate limiting, WAF hooks
• Metrics & logging

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⚙️ Components

🧠 Control Plane

• Manages configs, certificates, POP registry, and cache purge APIs.

• APIs:

  • POST /purge – Purge by URL or cache key
  • POST /invalidate – Pattern invalidation
  • GET /config – Fetch edge configuration
  • POST /origin/register – Register origin details

• Stores configs in Postgres or etcd.

• Includes ACME client for certificate management.

🌐 Edge Servers (POP)

• Handle connections, caching, and origin fetches.
• Implements stale-while-revalidate, conditional GET, and request coalescing.
• Collects metrics (Prometheus format) and logs (JSON structured).

🗂️ Cache System

• Hot Cache: In-memory LRU store for small objects.

• Persistent Cache: SSD-based for larger assets.

• Eviction Policy: LRU (upgradeable to LFU/ARC).

• Cache Key:

  method:host:normalized_path:sorted_query
  

🛡️ Security

• Edge-level rate limiting (per IP, per domain).
• WAF hooks for common exploit filtering.
• DDoS detection with basic anomaly triggers.
• TLS + HSTS + secure cipher suites.

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📊 Observability

• Metrics: Prometheus-compatible /metrics endpoint.
• Logs: Structured JSON → Elastic / Loki / S3.
• Dashboards: Grafana + Alertmanager.
• Tracing: Optional Jaeger integration.

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🌍 DNS / Routing

• v1: Geo-DNS + health-check failover.
• v2: Add Anycast (BGP) routing for faster convergence.

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🧩 APIs

Purge API Example

POST /v1/purge
Content-Type: application/json

{
  "token": "your_api_key",
  "urls": ["https://cdn.example.com/a.jpg"],
  "hard": true
}

Response: 202 Accepted with job ID.

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🧪 Testing & Load Tools

• Unit tests: Cache correctness, eviction.
• Integration tests: Origin outage simulation, stale revalidation.
• Load tests: wrk2, vegeta, hey.
• Chaos tests: POP saturation, origin unavailability.

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📦 Deployment & Infrastructure

• Containerized (Docker / Kubernetes).
• Each POP runs a fleet behind local load balancer (Nginx or built-in).
• CI/CD: Git → Build → Deploy → Rolling updates.
• Supports Blue/Green deployments.

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📈 Capacity Planning (Example)

• Peak egress: 10 Gbps
• Edge node: 1 Gbps
• Redundancy (x3): 30 edge instances globally
• RAM cache: 16 GB
• SSD cache: 1 TB
• Max object size: 100 MB

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🧭 Roadmap (v2+)

• Add Anycast routing with BGP.
• Implement hierarchical caching (regional parent POPs).
• Support HTTP/3 (QUIC) fully.
• Add edge compute (Lambda@Edge-style functions).
• Introduce smart prefetching and ML-based cache warming.

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🔒 Security & Privacy

• Support signed URLs / token-based access for private content.
• Rotate TLS keys and protect private key storage.
• Sanitize logs (strip cookies/auth headers).

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🧰 Modules

Module	Description
edge-server	Core HTTP listener, cache, metrics
control-plane	Configuration, purge API, certificate manager
deploy	Deployment scripts for POP provisioning
observability	Prometheus exporters, Grafana dashboards
tests	Unit, integration, and load testing tools

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📄 License

MIT License