Building a Private CDN: Architecture and Implementation
When to Build vs. Buy
Building a private CDN makes sense when: you have 100+ Tbps of monthly traffic (cost savings at scale), you need absolute control over edge logic, regulatory requirements prevent using commercial CDNs, or your content delivery requirements are highly specialized. For foundational context, read CDN Monitoring and Observability: Metrics That Matter — FlyFish NWA.
Most organizations should use commercial CDNs. Building a CDN is a significant engineering investment. Companies that operate private CDNs include Netflix (Open Connect), Apple, Facebook, and Google.
Architecture Components
1. Edge Servers
Deploy servers in colocation facilities or ISP networks worldwide:
| Metric | Edge Processing | Origin Processing | Improvement |
|---|---|---|---|
| Latency (p50) | 8–15 ms | 120–300 ms | 15–20x faster |
| Cache Hit Ratio | 85–95% | N/A | Reduces origin load |
| TLS Handshake | 1–5 ms (nearby PoP) | 50–150 ms | 10–30x faster |
| Bandwidth Cost | $0.01–0.04/GB | $0.05–0.12/GB | 3–5x cheaper |
| Availability | 99.99% (multi-PoP) | 99.9% (single DC) | 10x fewer outages |
- Server hardware: high storage (NVMe SSDs), high network I/O (25-100 Gbps NICs), moderate CPU
- Operating system: Linux (typically custom or hardened)
- Cache software: Nginx, Varnish, Apache Traffic Server (ATS), or custom
- Minimum viable network: 10-20 PoPs in your users' primary regions
2. Anycast Routing
Advertise the same IP address from all PoPs via BGP Anycast. Users automatically route to the nearest PoP. Requires: your own AS number, IP space, and BGP peering agreements with ISPs.
3. Cache Hierarchy
User → Edge PoP (L1 cache)
→ Regional cache (L2 cache / origin shield)
→ Origin serverL1 caches serve hot content. L2 aggregates requests from multiple L1 caches, reducing origin load. This hierarchy is essential for long-tail content that doesn't fit in every edge cache.
4. Health Checking and Failover
Global health checks monitor every edge server. If a PoP goes down, BGP withdrawal removes it from Anycast within seconds, and traffic automatically routes to the next-nearest PoP.
Software Stack
| Component | Options |
|---|---|
| Reverse proxy/cache | Nginx, Varnish, Apache Traffic Server, HAProxy |
| TLS termination | Nginx, HAProxy, custom (BoringSSL) |
| DNS | PowerDNS, BIND, custom (Lua-based) |
| Monitoring | Prometheus + Grafana, custom dashboards |
| Config management | Ansible, Puppet, Salt, custom |
| Log collection | Kafka + ELK, custom pipeline |
Cost Estimation
For a 20-PoP private CDN serving 50 PB/month:
- Server hardware: $1-2M (amortized over 3-5 years)
- Colocation: $20-50K/month (20 locations)
- Bandwidth (transit): $0.005-0.02/GB (peering reduces this further)
- Engineering team: 5-10 engineers dedicated to CDN operations
- Total: $1-3M/year (vs. $5-15M/year for commercial CDN at this scale)
The crossover point where building becomes cheaper than buying is typically 5-50 PB/month, depending on content type and geographic distribution.
Netflix Open Connect
Netflix's Open Connect Appliances (OCAs) are the most successful private CDN: embedded directly in ISP networks worldwide, serving 30%+ of peak internet traffic. Netflix provides hardware to ISPs for free — ISPs benefit from reduced transit costs. This model works because Netflix has the scale and content demand to justify it.