Synopsis
This chapter describes the main architecture patterns for multi-CDN and how to choose between them. It explains DNS-based steering, layer 7 proxy or aggregator designs, client-side selection, and hybrid approaches. It focuses on behavior, failure modes, latency impact, and operational complexity.
DNS-based steering
Authoritative DNS answers with records that point to different CDNs based on geography, network, or other criteria. This pattern has low per-request overhead because the routing decision happens before HTTP, but it reacts on the scale of DNS caching. Time to live and resolver behavior affect how quickly changes take effect. Geo and ASN databases must be maintained. Health-driven routing requires either short TTLs or resolver-aware mechanisms that may not be consistent across networks. DNS steering is simple to deploy and scales well, but it is limited in how fast it can respond to sudden failures and it has coarse visibility into per-request signals.
Layer 7 proxy or aggregator
A proxy that terminates TLS and forwards to a selected CDN enables real time decisions using application layer signals. It can consider HTTP headers, cookies, and recent measurements to route each request. This provides fine control and fast failover, but it adds an extra hop and becomes part of the critical path. The proxy must be anycasted or regionally deployed for low latency. It must be highly available and horizontally scalable. Caching behavior must be designed carefully to avoid double caching or reduced hit ratios. Security posture moves to the proxy and must match the CDNs. This pattern is flexible and reactive, but it increases operational responsibility.
Client-side selection
Client code or an SDK can select a CDN endpoint based on locally measured performance or signals provided by a control plane. This allows decisions that reflect the actual user network path. It can adapt quickly on mobile and in last mile networks where provider performance varies. The approach requires careful handling of privacy, telemetry sampling, versioning, and offline behavior. It can complicate cache keys and URL management if different endpoints are exposed to clients. Client logic should be simple, predictable, and resilient to noisy measurements.
Hybrid designs
Many deployments combine DNS-based steering for coarse placement with either a proxy or client logic for real time adjustments. For example, DNS can prefer a primary CDN per region, while a proxy fails over on health or cost signals. Hybrids can reduce operational risk by limiting where complex logic runs, but they require clear precedence rules so that layers do not conflict. Observability must span all layers.
Choosing a pattern
DNS is appropriate when simplicity and low overhead are priorities and slower reaction times are acceptable. A proxy suits deployments that require per-request control, fast failover, or complex policy evaluation across multiple signals. Client-side selection suits environments with dominant last mile variation and a safe path for shipping and maintaining client code. Hybrid designs fit cases that need combined strengths and the capacity to operate them together.
Failure modes and mitigation
DNS steering fails slowly when TTLs are long or resolvers cache aggressively. Health-integrated routing should use guard rails and tested resolver behavior. Proxies fail if capacity planning is poor or if they become a single point of failure. Anycast or regional sharding and surge planning reduce risk. Client logic fails when measurements are biased or when versions diverge. Algorithms should remain simple, with published defaults, dampening, and minimum dwell times.
Migration paths
A practical path is to begin with DNS steering to introduce a second CDN, then add a proxy or client logic in the regions that see the most variance. Traffic should migrate in controlled steps with recorded outcomes. Rollback remains straightforward by maintaining a working single-CDN path until confidence is high.
Telemetry integration
Steering quality depends on input signals. For DNS, independent synthetic probes from representative vantage points are preferred. For proxies, collect request outcomes and active health. For client-side, use sampled real user measurements and aggregate them in a privacy-safe way. Routing changes must improve user-centric metrics, not only synthetic scores.
Security and compliance
Decide where TLS terminates and how origin authentication is enforced. Proxies must protect keys and support certificate automation. Client-side endpoints must not leak internal hostnames. Where regional controls are required, routing and logging must comply with jurisdictional rules in every layer.
Operations and testing
Failure cases should be exercised regularly. For DNS, simulate a provider outage and observe propagation. For proxies, inject controlled failures and confirm reroute performance. For client-side, feature flags should enable pinning or splitting traffic safely for experiments. Procedures and diagrams should remain current so on-call engineers can trace routes quickly.
Related chapters
For policy design see /multicdn/traffic-steering/. For measurement see /multicdn/signals-telemetry/. For origin planning see /multicdn/origin-architecture/.
Further reading
RFC 1034 and RFC 1035 for DNS. RFC 9110 for HTTP semantics.