Content Delivery Network (CDN)
A content delivery network, or CDN, is a distributed network of servers used to deliver website resources from infrastructure closer to the person requesting them.
CDNs can reduce network distance and offload work from the website's original server.
What It Means in Practice
Without a CDN, every visitor may need to request resources from the same origin server.
With a CDN, copies of cacheable resources can be distributed across multiple geographic locations.
A visitor can then receive those resources from a nearby CDN location rather than travelling across the full network path to the origin every time.
How It Works
The CDN sits between the visitor and the website's origin server.
When a resource is requested, the CDN may:
- Serve an existing cached copy
- Retrieve the resource from the origin
- Store it according to caching rules
- Deliver it to the visitor
Many CDNs also provide features such as image transformation, compression, traffic routing, and security controls.
Why It Matters
Physical distance and network latency affect how quickly requests can complete.
A CDN can improve delivery by shortening that path and reducing repeated origin requests.
This can be especially valuable for websites serving visitors across a large geographic area.
Common Problems
Common CDN problems include:
- Assuming a CDN automatically makes every site fast
- Incorrect caching rules
- Stale content
- Origin and CDN configuration conflicts
- Cache misses for resources that could be reused
- Large assets remaining inefficient even when delivered nearby
- Multiple proxy or CDN layers adding unnecessary complexity
A CDN can improve delivery, but it does not fix an unnecessarily heavy page.
How to Improve
Use a CDN as part of a broader performance strategy.
Cache appropriate resources, compress responses, optimize images, and keep the origin server healthy.
When debugging, test whether the requested resource was served from the CDN cache or fetched from the origin.