CloudCostCompareUsing a CDN to cut data transfer costs
A content delivery network, or CDN, keeps copies of your content in many locations close to users. When a user asks for a file, the CDN answers from its copy and only goes back to your servers when it has no copy. This can cut both the load on your servers and the data they send out.
Where the saving comes from
Data leaving your cloud servers to the internet is billed as egress. See data transfer costs. With a CDN in front:
- Requests the CDN answers from its cache never reach your servers, so they cost no egress from your origin.
- The CDN charges its own price for the data it delivers, which is often lower than origin egress and has its own tiers.
- Traffic from your origin to the CDN may be free or cheaper when both belong to the same vendor, depending on the vendor’s rules.
So the saving is the difference between your origin egress and the CDN’s delivery price, on the share of traffic the CDN answers from cache.
The number that decides everything: cache hit ratio
If the CDN answers 95 percent of requests from cache, your origin sends 5 percent of the data. If it answers 20 percent, it barely helps. The ratio depends on:
- What you serve. Images, scripts, styles, downloads and video segments cache well. Personalized pages and API responses mostly do not.
- Cache headers. If your server sends headers that forbid caching, the CDN passes every request through. Set sensible
Cache-Controltimes on static files. - File naming. Versioned file names, such as ones with a content hash, can be cached for a very long time safely.
- Query strings and cookies, which can make each request look unique unless you tell the CDN to ignore them.
What a CDN charges
- Data delivered to users, per gigabyte, usually in regional tiers.
- Requests, per ten thousand or million.
- Extras such as custom certificates, edge functions, log delivery and web application firewall rules.
Prices, free allowances and the way origin traffic is billed vary by vendor, so read the current page. Some CDNs are provided by the same cloud, and some by independent companies.
When it is worth it
A CDN is worth adding when most of your outbound data is static or cacheable, your users are spread out, or your servers struggle with the load. It adds less when almost everything is personalized, or when traffic is small enough to sit inside the free allowance.
How to test it
- Find what share of your outbound bytes is static. Your load balancer or server logs show this.
- Estimate the cache hit ratio you could reach with proper headers.
- Compute the origin egress saved, then subtract the CDN’s own charges.
- Try it on one site section and compare the real bill after a month.
In our estimates
The web app estimate includes data out to users as plain egress from the cloud, with no CDN, so it is the upper figure for static-heavy sites. Change the data out input to see the effect of a lower egress volume. The overall cost picture for a web app is in web app costs on Azure, AWS, Google Cloud and Oracle.