---
title: "CDN Cost Optimization: Egress, Cache Hits and Origin Shielding"
description: "Model CDN delivery, request, origin, shielding, and observability costs. Measure byte offload and cache fragmentation before changing policy."
canonical_url: https://optimi.com/en/guides/cdn-cost-optimization
md_url: https://optimi.com/en/guides/cdn-cost-optimization.md
last_updated: 2026-07-15
---

# CDN Cost Optimization: Egress, Cache Hits and Origin Shielding

A higher cache-hit ratio is not automatically a lower bill. Attribute delivery, origin, shield, compute, and log meters to the request paths that produce them.

CDN cost optimisation is a measurement problem, not a universal TTL recommendation. A cache hit may avoid an origin fetch, but its financial impact depends on object size, contracted rates, origin work, shielding topology, edge products, and observability retention.

## Outcome

Build a cost-to-serve model and safe experiment loop that can distinguish a real cost change from a local metric improvement or a shifted bill.

## Model the complete delivery path

**Representative fully loaded cost model**

```
total = delivery bytes + CDN requests + shield or inter-POP traffic
      + edge products + origin egress + origin requests and compute
      + logging, storage, queries + allocated fixed commitments
```

Use effective, region-specific contract rates rather than public-list-price examples. Separate avoided usage from financial savings: fewer origin requests do not reduce a fixed reserved-capacity bill until a capacity decision changes.

**Where a cache miss can cost**

![Left-to-right flow: Viewer edge to Shield tier to Origin to Telemetry, each a metered cost stage in the delivery path.](/diagrams/cdn-cost-optimization/cache-miss-cost.svg)

*Viewer delivery, edge policy, shield, origin transfer, application work, and telemetry can each be metered independently.*

## Measure bytes and origin work, not one cache ratio

Request cache-hit ratio gives a cached thumbnail and a cached video segment equal weight. Byte offload shows how much delivered body volume was served by cache. Origin-request avoidance is often closer to API and compute impact. Define the cacheable population and report `PASS`, private, write, error, and revalidation traffic separately.

| Metric | Calculation | Decision use |
| --- | --- | --- |
| Request hit ratio | hits / eligible requests | Diagnose cache behaviour |
| Byte offload | cache-served bytes / eligible delivered bytes | Estimate transfer avoided |
| Origin avoidance | 1 - origin fetches / eligible requests | Model API and compute pressure |
| Cost per unit | attributed cost / GB, request, stream, order, or tenant | Connect technical use to business value |

## Fix fragmentation before extending freshness

Query order, tracking parameters, cookies, headers, image transformations, locale, experiments, and signed URLs can divide a reusable representation into many keys. Remove or normalise a dimension only after proving it does not change the representation or weaken authorisation. Never raise a hit ratio by exposing private data.

## Treat shielding as a break-even experiment

Origin shielding can consolidate duplicate misses, especially for globally requested cacheable objects and constrained origins. It can also add inter-POP requests, bandwidth, and double edge execution. Measure pre/post origin cost, added shield cost, latency, errors, freshness, and fallback behaviour before retaining it.

## Cost optimisation guardrails

- Do not add local hit ratios across CDNs; use end-user and shared-origin denominators.
  - Do not extend a public cache policy to authenticated, personalised, or transaction state.
  - Do not retain raw logs indefinitely just to preserve a future optimisation option.
  - Do not declare a saving until billing and capacity evidence support it.

## Related guides

- [Multi-CDN Strategy](/en/guides/multi-cdn-strategy)
- [Fastly Origin Shielding](/en/guides/fastly-origin-shielding)
- [CDN Cache Misses](/en/guides/cdn-cache-misses)

## Authoritative references

- [FinOps Foundation: Unit Economics](https://www.finops.org/framework/capabilities/unit-economics/)
- [AWS: Improve CloudFront cache-hit ratio](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/cache-hit-ratio.html)
- [AWS: CloudFront Origin Shield](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/origin-shield.html)
- [Cloudflare: How charges accrue](https://developers.cloudflare.com/billing/understand/how-charges-accrue/)
- [Fastly: Shielding](https://www.fastly.com/documentation/guides/concepts/shielding/)

[Assess CDN cost drivers](/en/contact): Measure delivery economics before changing cache policy — Optimi can help join edge, origin, and usage evidence into a cost model that preserves correctness and reliability.