How to Build a Multi-CDN Over Your CDN Vendor and Your Private CDN?
You build a Multi-CDN on top of your CDN vendor and your private CDN by adding one thin “traffic steering” layer in front of both, then forcing both CDNs to behave the same way for caching, TLS, headers, tokens, and logging.
The steering layer decides, per request, whether the user hits your vendor edge or your private edge, based on health, performance, geography, and cost. When one path degrades, traffic automatically shifts to the other without you changing app code or asking users to refresh anything.
If you do this right, you get two big wins at once: reliability (instant failover) and control (you can gradually move traffic to your private footprint where it makes sense, without ripping out the vendor).
Build A Multi-CDN Solution Without Making Your Stack Fragile
The easiest way to think about a multi CDN solution is: one control plane, two delivery planes.
- Control plane: “Where should this request go right now?”
- Delivery planes: “If it goes there, does it cache correctly, authenticate correctly, and log correctly?”
Most teams get stuck because they obsess over steering first, and only later realize their two CDNs don’t act the same. Then “failover” becomes “sudden cache miss storm” and you learn about it the hard way.
I treat this as a compatibility project more than a routing project. When both CDNs look identical from the client’s point of view, steering becomes boring, which is exactly what you want.
Choose A Multi-CDN Strategy That Matches Your Traffic
You’re basically deciding what your private CDN is for:
- Failover-only: vendor is primary, private is safety net
- Cost and capacity relief: private serves predictable regions or long-tail assets
- Performance optimization: private serves regions where you have strong PoPs and peering
- Regulatory or data residency: private serves specific countries or networks
The best multi CDN strategy is usually hybrid: start with failover-only, then earn the right to shift real traffic once you’ve proven parity.
Here’s the key rule you want to internalize: routing is easy, parity is hard. So you design for parity first.
Pick Your Traffic Steering Layer For Multi-CDN Deployment
You have three practical steering options. All work. The “best” one depends on how fast you need to react and how much control you want at request time.
If you’re serving a normal website or API-heavy product, DNS steering is usually the first step. If you’re doing serious CDN media delivery (HLS/DASH, large objects, global audiences), you’ll often end up with a blend of DNS steering and client-side logic, because the player can react faster than DNS.
One detail people skip: whatever steering you choose, you need a way to force a route for debugging (cookie, header, query param, or dedicated hostname). Otherwise, every incident becomes guesswork.
Design Private CDN Architecture That Doesn’t Collapse Under Real Load
Your private CDN architecture does not need to beat a vendor everywhere. It needs to be predictable, observable, and safe to fail over to.
A practical private CDN edge design usually includes:
- Edge cache nodes close to users (PoPs or regional sites)
- Mid-tier cache or “shield” layer that protects origin from edge miss storms
- Origin(s) (object storage, media packager, API origin) with strict rate limits and autoscaling
- Routing: anycast BGP, regional load balancers, or a combination
- Operational basics: deployment automation, config management, and fast rollback
If you’re building this from scratch, the single most underrated feature is tiered caching (edge -> shield -> origin). Without it, a multi-CDN failover event becomes an origin meltdown event.
Make Your Vendor And Private CDN Behave The Same
This is where most “multi-CDN” projects secretly die. Your steering layer can be perfect, and you’ll still have problems if:
- cache keys differ
- headers differ
- compression differs
- TLS/SNI differs
- authentication differs
- range requests behave differently
- redirects differ
- error codes differ
So you normalize.
- Hostnames: Use the same customer-facing hostname everywhere, like cdn.yoursite.com.
- TLS: Same cert chain behavior, same TLS versions, same SNI expectations.
- Cache key policy: Decide what varies the cache (path, query params, headers) and make both CDNs match it.
- Compression and brotli/gzip: Align it or you’ll see weird cache fragmentation.
- HTTP caching headers: Decide your truth for Cache-Control, Surrogate-Control, Vary, ETag.
- Range requests: Especially for video and large files, this must match.
- Default TTLs and stale rules: If one CDN serves stale-while-revalidate and the other doesn’t, users will notice during incidents.
If you only do one thing, do this: make cache keys identical across both delivery paths. Otherwise you’re not failing over, but are forcing a cold-start.
Build Steering Policies
Your steering logic should be boring and explicit. A good default policy looks like this:
- Route by health first
- Then by performance
- Then by cost
- Then by experiments / canary rules
A clean starting point for web delivery:
- 95% vendor, 5% private (only for static assets)
- If private errors exceed threshold, go 100% vendor
- If vendor errors exceed threshold, go 100% private for specific paths you trust
Then expand.
You can implement this with a DNS traffic manager, a global load balancer, or an edge worker. The tooling matters less than the discipline.
Logging And Debugging Across Vendor And Private
When someone says “the CDN is slow,” you need to answer in minutes, not hours. Multi-CDN makes that harder unless you standardize identifiers.
Make sure both CDNs emit:
- a shared request ID header
- cache status headers (hit/miss/stale)
- upstream timing headers (edge time, origin time)
- a consistent log schema you can ship into one place
If you don’t unify logs, you’ll end up debugging vendor in one dashboard and private in another, and you’ll miss correlations.
Common Ways This Goes Sideways
A few sharp edges you can avoid up front:
- Cache key mismatch: you fail over into a cold cache and blame the steering layer
- TTL mismatch: one CDN serves stale, the other hammers origin
- Token mismatch: private CDN returns 403 and you think it’s a network issue
- DNS TTL too high: you can’t fail over quickly when it matters
- No shielding: origin collapses during failover
- No forced routing: you can’t reproduce issues because routing keeps changing
If you handle parity, shielding, and observability, the rest becomes a straightforward engineering project instead of a series of late-night incidents.
And once it’s running, multi CDN deployment stops being a one-time migration and becomes a knob you can turn: performance, resilience, cost, region by region, path by path, without drama.
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