Synopsis#
This chapter explains the principles and practice of operating more than one content delivery network at the same time. Multi-CDN is used to improve availability, performance, and resilience. It also increases complexity and cost. This document describes when multi-CDN is appropriate, how to design it, and how to operate it reliably.
Fit analysis#
Multi-CDN is most useful when service level objectives require high availability across many regions, performance varies between providers in different areas, regulatory or commercial risk must be spread across vendors, or large traffic bursts cannot be absorbed by a single provider. It is less appropriate when traffic volume is low, the operational team is small, or application features depend on one vendor’s proprietary services.
Architecture patterns#
Common deployment patterns include DNS-based steering, layer 7 proxy or aggregator, client-side selection, and hybrid designs. DNS-based steering uses authoritative DNS to direct traffic to different CDNs based on geography, network, or other metrics. A layer 7 proxy or aggregator routes HTTP requests through a proxy that selects the target CDN in real time. Client-side selection uses code in the application or SDK to choose a CDN endpoint. Hybrid designs combine two or more of these methods.
flowchart LR
Client --|DNS lookup|--> Resolver
Resolver --> AuthoritativeDNS
AuthoritativeDNS --> CDN_A
Client --|HTTP(S)|--> L7_Proxy
L7_Proxy --> CDN_B
Client --|SDK decision|--> CDN_C
Traffic steering#
Traffic steering is the process that selects which CDN will serve a request. Policies can be based on geography or ASN, latency or throughput, real user measurement, synthetic measurement, or cost. These policies can be combined to balance performance, reliability, and spend, but they need clear precedence rules and guard rails to avoid oscillation.
Signals and data#
Reliable steering depends on accurate and timely data. Useful sources include synthetic probes from multiple regions, real user measurements embedded in pages or apps, BGP and routing signals, CDN health and status endpoints, and analysis of logs and metrics. Data collection should be continuous, representative of real users, and resilient to outliers.
Origin strategy#
The origin must be reachable by all CDNs in the configuration. Designs include a single origin with shielding, multiple origins in different regions, or a mix of both. Authentication at the edge can use signed URLs or tokens. Cache keys should be consistent across CDNs so that object identity is stable regardless of the serving provider.
Security parity#
All CDNs in the set should enforce the same security controls. That includes TLS configuration, web application firewall rules, bot mitigation, origin authentication, and any IP allowlists or rate limits. Changes should be applied and verified across providers to avoid gaps.
Feature parity#
Features like image optimization, video packaging, and edge functions vary by provider. Prefer portable features and deterministic URLs for any transforms. Where differences cannot be avoided, document them and test behavior in each CDN before enabling a rollout.
Operations#
Operating multi-CDN requires staged rollouts with canary traffic, incident playbooks that cover detection, isolation, and reroute, monitoring and alerting aligned with service level objectives, and regular configuration reviews across providers. Post-incident reviews should include routing decisions and data quality checks.
Cost and contracts#
Expect different commit structures, potential overage charges, and egress cost differences by region. Build a cost model that includes realistic traffic mixes and growth. Cost-aware routing can direct traffic to the most economical provider while meeting performance requirements if it is constrained by guard rails.
Compliance and data residency#
Some deployments require routing within jurisdictional boundaries and careful handling of logs and telemetry. Confirm where user data is processed and stored, and maintain audit records for routing decisions and configuration changes.
Implementation notes#
Confirm that multi-CDN solves a real requirement. Choose a steering architecture and the signals that feed it. Prepare origins and authentication for multiple CDNs. Align security controls. Test failover and rollback. Deploy monitoring and alerting with clear thresholds. Train operators. Review costs. Document the configuration and keep it current.
See the detailed chapters on architecture patterns at /multicdn/architecture-patterns/, traffic steering at /multicdn/traffic-steering/, and signals and telemetry at /multicdn/signals-telemetry/.
References#
RFC 1034 and RFC 1035 for DNS. RFC 9110 for HTTP semantics.
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.
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Synopsis This chapter explains how to keep caches consistent when more than one CDN serves the same content. It covers cache key design, HTTP caching headers, immutable assets, purge methods, sequencing across providers, and the operational controls that keep correctness and latency predictable.
Purpose and scope Multi-CDN changes cache behavior because different networks fetch, store, and expire objects using different rules and clocks. The goal is that users receive the same bytes for the same URL regardless of the serving CDN and that changes reach users in a controlled and explainable way. The guidance applies to static assets and dynamic responses that allow caching. It also covers the surrounding systems that push and purge content.
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Synopsis This chapter explains how compliance and data residency requirements shape a multi-CDN deployment. It describes how to constrain routing by jurisdiction, how to handle telemetry and logs without exporting sensitive data, how to align encryption and key custody with regional obligations, and how to verify behavior in production. The objective is predictable data location and lawful processing without degrading reliability or performance.
Scope and regulatory context Compliance spans content delivery, control planes, logs, real user measurement, and provider support systems. Obligations arise from privacy laws, sector rules, contracts, and customer commitments. Multi-CDN introduces additional processors and network paths. Architecture and operations must express where data is processed, where it is stored, who accesses it, and how long it is retained. The model treats providers as processors under documented agreements and keeps data flows narrowly defined.
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Synopsis This chapter describes cost modeling and contract structure for multi-CDN. It explains how to forecast traffic, choose commit levels, control overage risk, and align commercial terms across providers. It also covers cost-aware routing, regional and product differences, invoice reconciliation, and clauses that protect the deployment when conditions change.
Cost model foundations A simple model separates data transfer out, request charges, advanced product usage, and fixed fees. Each component varies by region and product family. The model expresses spend as a function of volume, mix, and route. Granularity by region and content type is required because prices and cache behavior differ. The model must match how providers bill so that reconciliation is possible without manual guesswork.
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Synopsis This chapter describes how image and asset optimization works in a multi-CDN environment. It explains deterministic transform URLs, format negotiation with Accept and client hints, parity across vendor image engines, cache identity and validation, quality targets, rollout controls, and telemetry that proves user visible improvements without fragmenting caches or breaking links.
Scope and objectives Many CDNs provide on the fly transforms for images and lightweight assets. Typical operations include resize, crop, format conversion, quality selection, and color space handling. Multi-CDN adds a requirement that identical requests produce identical bytes or at least identical perceptual quality, and that cache identity does not drift between providers. The objective is predictable output, stable cache behavior, and measurable gains in transfer size and rendering time.
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Synopsis This chapter provides a practical checklist for multi-CDN programs. It maps objectives to design, security, origin, caching, telemetry, traffic steering, testing, rollout, and ongoing operations. Items are written to be auditable and printable. The aim is stable behavior, predictable costs, and clear ownership.
Objectives and scope Document service objectives, success measures, and constraints. Define availability and latency targets per region. Record compliance and residency rules. Note content types, expected volumes, and seasonal peaks. Establish a decision owner for routing and for incidents. Record change control and rollback expectations.
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Synopsis This chapter provides standard playbooks for incidents in multi-CDN environments. It covers detection, triage, scoping by geography and network, isolation and reroute choices, change control during active events, communication, restoration, and post-incident analysis. The objective is to protect users first, keep changes reversible, and leave an audit trail that improves future responses.
Principles Incident handling favors user outcomes over internal metrics. Actions modify the smallest scope that achieves protection. All changes must be reversible. Each action records who acted, what changed, and why. Telemetry drives decisions and distinguishes symptoms from causes. Providers are treated as interchangeable routes unless a risk register documents exceptions.
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Synopsis This chapter describes monitoring and service level objectives for multi-CDN operation. It defines user facing indicators, explains stable aggregation and alerting, and outlines dashboards that connect routing decisions to outcomes. The aim is to detect harm early, confirm improvements with evidence, and support clear decisions during change and incident response.
Objectives and scope Monitoring must show whether content is correct, whether latency and reliability meet commitments, and whether routing decisions help users. It must separate symptoms from causes, include regional and network context, and expose differences between providers. It should remain simple enough that on-call engineers can act without guesswork and detailed enough to support post-incident analysis.
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Synopsis This glossary defines terms used across the multi-CDN handbook. Entries focus on functional meaning in production use.
ABR (Adaptive Bitrate) Streaming technique that lets the player switch among renditions of different bitrates during playback based on current conditions.
ACME Automated Certificate Management Environment. Protocol used to issue and renew certificates programmatically.
Aggregator (L7) A layer 7 proxy that accepts client TLS and HTTP, applies policy, and forwards to a selected CDN or origin.
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Synopsis This chapter provides a vendor-neutral request for proposal template for adding or renewing CDN providers in a multi-CDN deployment. The template covers scope, objectives, technical and operational requirements, test plans, pilot expectations, and an evaluation rubric. Fields are expressed as placeholders and may be tailored to regional or product constraints.
Usage notes The template is designed to be copied into a standalone document. Placeholders use angle brackets. Optional sections can be removed if out of scope. Terminology follows definitions in related chapters so responses remain comparable across providers.
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Synopsis This chapter explains how to design and operate origin infrastructure that can serve more than one CDN at the same time. It covers topology choices, origin shielding, authentication, cache key consistency, deployment and consistency models, failover behavior, and operational practices. The goal is to keep content correctness and performance stable while multiple CDNs fetch from the same source.
Role of the origin in multi-CDN The origin is the source of truth for content and APIs. In a multi-CDN setup more than one provider will fetch from it. The design must handle higher fan in, different retry behaviors, and different cache semantics without breaking correctness. It should also keep the number of variables low so that problems are diagnosable during incidents.
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Synopsis This chapter describes how to keep security controls equivalent when more than one CDN serves the same properties. It covers ownership of policy, alignment of web application firewall rules, parity in bot defenses, consistent rate limiting, origin authentication, secrets handling, configuration drift control, verification, logging normalisation, and incident procedures. The aim is a uniform security posture that does not depend on which provider handled a request.
Scope and objectives Security parity means that requests receive the same protection and the same outcomes independent of provider. Rules must be functionally equivalent, telemetry must be comparable, and emergency controls must have the same effect at all edges. Differences in vendor features are handled by choosing portable constructs first and by documenting exceptions that cannot be avoided.
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