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.

Traffic segmentation and forecasting

Forecasts should reflect seasonality, events, growth, and mix shifts such as more video or more API traffic. Segments include region, ASN clusters if relevant, protocol, and object class such as HTML, media, and large binaries. Ranges are more useful than single values. A base case, a low case, and a high case create guard rails for commit planning. Forecast error must be visible so that contract changes are requested before penalties accrue.

Pricing models and components

Providers publish rate cards or negotiated schedules that include per GB transfer rates, per request fees, tiered or staircase pricing, and special rates for features such as image transformation or edge functions. Some include minimum monthly charges or per property fees. Regional bands often differ by continent or by country group. Discounts can apply at the global level or per region. Understanding whether tiers are computed per region or globally is essential for an accurate model. Cloud egress, private interconnect, and marketplace fees belong in the same model to expose total delivered cost.

Commit strategy across providers

Commits reduce unit prices but increase take or pay risk. In multi-CDN, commits must be split so that each provider can be met under normal routing. A common plan sets a conservative floor at each provider that the deployment can meet even when traffic shifts for performance or incident response. Growth headroom lands as overage at discounted rates or as a soft commit with true-up. Co-termination across providers simplifies renewal and preserves optionality. Step down rights or ramp schedules reduce risk when mix changes.

Cost-aware routing

Routing on cost alone harms user outcomes. A safe approach evaluates constraints in order. Jurisdiction and allowlist come first, then health, then performance against service objectives, and only then cost within guard rails. The cost signal uses the same regional segmentation as the contract and caps exposure so that savings do not drop cache hit rate or overload origins. The control plane records when a cheaper route was chosen and what performance deltas were observed so that finance and engineering share a single record of decisions.

flowchart TD Start[Candidate routes] --> Policy[Apply constraints] Policy --> Health{Healthy} Health -- no --> Alt[Remove unhealthy routes] Health -- yes --> Perf{Meets SLO} Perf -- no --> Keep[Select best performer] Perf -- yes --> Cost{Within cost guard rails} Cost -- yes --> Cheap[Select lower cost route] Cost -- no --> Keep Cheap --> Record[Record decision with deltas] Keep --> Record

Regional and product differences

Prices and behavior differ by region and by product. HTTP/3 can change throughput and therefore bandwidth utilization. Image and video transforms incur compute and sometimes storage. Log delivery can be metered by volume or by request count. Negative caching defaults change upstream rates. A provider matrix that lists price points, default behaviors, and billing units prevents surprises when routing changes shift mix.

Egress and origin considerations

Total delivered cost includes origin egress from cloud or data centers. Private interconnect or colo near shields can reduce egress but adds fixed cost. Range requests for large files change request counts and cache behavior and should be modeled explicitly. For media, license and key servers are not cacheable and consume requests but little data. Moving sessions between providers can increase miss traffic temporarily and should be included in sensitivity analysis.

SLAs, service credits, and measurement

Availability and performance commitments matter only with clear definitions. The agreement should define what counts as an outage, how scope is measured, which time bases apply, and how credits are computed. Credits should apply to the affected region and product, not only to global totals, and should be claimable with logged evidence. Carve-outs and maintenance windows must be reviewed for practicality. Credits are not a substitute for performance; routing retains the right to remove a provider from candidacy when objectives are not met.

Contract structure and protections

Multi-CDN contracts benefit from co-termination dates, change of control clauses, assignment rights, and flexibility to add hostnames and properties without penalty. Most favored nation language, price review triggers, and inflation caps reduce long term drift. True-up options at year end reduce waste when growth outpaces plan. Exit rights for repeated objective failures reduce lock-in. Data handling and security obligations should align with origin and telemetry practices. Regional data residency promises must match routing capabilities.

Risk management and hedging

Risk comes from forecast error, sudden traffic shifts, vendor incidents, and regulatory change. Hedging uses conservative commits, cross-provider flexibility, and clauses that allow rebalancing within a defined window. A quarterly review compares actuals to plan by region and by provider and adjusts exposure. Incident history and error budgets inform whether to favor more capacity at a higher unit price or more aggressive commits with tighter guard rails.

Tooling and reporting

Useful tools include a bill of materials for traffic and requests by segment, a contract encoder that computes expected spend from volumes, and a reconciliation process that compares invoices to expected values. Anomalies such as sudden request inflation or unexpected regional rates should raise alerts to both engineering and finance. Reports show unit prices, effective prices after credits, and variance from plan. Records of routing decisions that considered cost allow post hoc validation that savings did not harm service.

Invoice reconciliation

Reconciliation aligns measured traffic with billed traffic. Differences arise from header normalization, protocol accounting, and sampling of telemetry. The process should map each invoice line item to a metric and a region. Any derived charges such as log delivery, purge API usage, or premium support should be tagged clearly. Disputes require per region evidence with timestamps and request ids. Closing the month includes an accrual for usage not yet invoiced, based on the same model used for commit planning.

Procurement process

RFPs for additional providers or renewals should state measurable requirements, test plans, and expected traffic mix. Evaluation should include performance variance, operational fit, and parity with existing controls as well as price. Pilot periods with defined success criteria reduce regret. Legal and security review proceeds in parallel so that the commercial window does not close before due diligence completes.

flowchart LR Forecast[Traffic forecast by region and product] --> Model[Cost model] Contracts[Rate cards and terms] --> Model Model --> Plan[Commit and overage plan] Plan --> Route[Routing guard rails] Actuals[Measured usage] --> Reco[Invoice reconciliation] Invoices --> Reco Reco --> Review[Quarterly review and adjustments] Review --> Plan

Operations

Day to day operations maintain parity and avoid surprises. Configuration changes that affect cache identity and route must include a cost impact note. Vendor changes to API limits, purge behavior, or log export pricing must be tracked and reflected in the model. During incidents that pin traffic to a single provider, a running estimate of incremental cost avoids unexpected spend. After restoration, exposure returns to planned levels and any temporary clauses or overrides are retired.

Routing behavior that applies the cost signal appears in /multicdn/traffic-steering/. Cache identity and purge behavior that influence origin load appear in /multicdn/cache-consistency/. Monitoring and reconciliation metrics appear in /multicdn/monitoring-slos/.

Further reading

Contract drafting references for service level agreements and true-up mechanisms provide structure for credits and risk sharing. Cloud egress and interconnect pricing documentation should be reviewed alongside CDN rate cards to confirm total delivered cost.