What MultiCDN is
MultiCDN uses more than one CDN provider at the same time. The goal is better reach, higher availability, and lower tail latency. A steering layer decides which provider should serve each request based on performance, health, geography, or cost.
When to use it
Pick MultiCDN when a single provider leaves gaps in coverage, when outages are costly, or when large events create sharp traffic spikes. It adds control and resilience, but also complexity. If one CDN already meets your SLOs, start there and add more only for clear benefits.
The three roles in a MultiCDN stack
CDN providers
They cache and deliver your content. Each has its own PoP footprint, peering, features, and pricing. Expect small differences in cache behavior, TLS, WAF, and logging. Normalize where possible so failover does not change user experience.
Data providers
They supply measurements that inform decisions. Inputs include RUM (real user monitoring), synthetic probes, provider telemetry, status pages, and cost data. Fresh and regionally diverse data makes steering accurate. Stale or sparse data causes flapping.
Routing or steering providers
They turn data into per-request or per-group choices. Common mechanisms are DNS answers, HTTP redirects, client-side selection, or network controls. Good steering enforces policy, respects SLOs, and fails safe when inputs are missing.
How it works (high level)
A user requests content. The steering layer picks a provider using recent signals and policy. The chosen CDN serves from cache or fetches from origin. Feedback loops update the next decision.
Steering methods at a glance
| Method | Control granularity | Pros | Cons | Typical use |
|---|---|---|---|---|
| DNS steering | Many users per answer | Simple, global, low overhead | Resolver caching, TTL lag | Web, streaming |
| HTTP redirect | Per request | Fast pivots, finer control | Extra round trip | APIs, sites with high TTL DNS |
| Client-side | Per user | Last-mile truth, precise | Client complexity, privacy concerns | Web apps with JS control |
| Anycast/BGP | Per prefix/region | Network-level control | Operationally heavy | Custom or large-scale builds |
Key data signals
Focus on latency percentiles (p95, p99), error rates, availability, throughput, and regional coverage. Add cost and contract commits if spend matters. Prefer RUM for last-mile reality; use synthetic to fill coverage gaps and run canaries.
Cache and origin considerations
Traffic spread across providers can dilute cache. Use consistent cache keys, larger TTLs where safe, and an origin shield to protect the origin. Enable validation caching, stale-while-revalidate, and request coalescing consistently to avoid stampedes during traffic shifts.
Security across providers
Align TLS versions, ciphers, and certificates. Keep WAF/WAAP rules, bot controls, and DDoS postures in sync so failover does not change risk. If you use signed URLs or tokens, standardize clock skew and key rotation across vendors.
Observability
Unify logs and metrics. Standard fields help: request ID, user ID (hashed), PoP/colo, cache status, and origin timings. Correlate RUM, synthetic, and edge logs to confirm that steering improved p95/p99 and reduced errors.
Cost and contracts
Balance commits and burst pricing across providers. Include cost as a steering input so small latency wins do not trigger large egress bills. Watch hidden costs like log egress and TLS handshakes at scale.
Operations and runbooks
Document when to shift traffic off a provider, how to cap a region or ASN, and how to roll back. Define safe ramp rates for global moves. Rehearse failover and back-to-normal. Keep a simple manual override for incidents.
Rollout plan
Start small.
- Pick a pilot region or traffic slice.
- Measure before/after: cache hit ratio, TTFB, p95/p99 latency, error rate.
- Tune policies and normalizations.
- Expand coverage once gains are proven.
Common patterns
- Web and APIs: prioritize tail latency, TLS, and WAF consistency.
- Live events: pre-warm caches, strict ramps, real-time health gates.
- VOD and large files: long TTLs, shielded origins, coalescing to avoid origin thundering herds.
Risks and tradeoffs
More vendors mean more moving parts. Cache dilution, misaligned features, and unclear logs can erase gains. Keep policies simple, revisit them often, and remove unused paths to reduce toil.
Related pages
- Background and milestones: see the history page.
- Answers to common questions: read the FAQ.
- Considering alternatives: see the comparison in DSA vs MultiCDN.