How Does CDN Caching Really Work? A Practical Guide You Can Actually Understand

Introduction

Many people start using a CDN with a simple expectation: “Once I enable it, my website will become faster.” But after deploying a CDN, questions quickly arise. Why are some resources cached while others always hit the origin? Why does updating a file sometimes take effect immediately, but other times requires a manual purge?

The truth is that CDN caching is not magic. It is a rule-based system built on HTTP standards, origin server behavior, and CDN-side strategies. If you don’t understand how these parts work together, CDN caching can feel unpredictable and even frustrating. In this article, we’ll break down how CDN caching actually works in real-world scenarios, helping you understand what is cached, why it’s cached, and when it won’t be cached at all.

1. From User Request to Cache Hit: What Really Happens

When a user requests a resource through a CDN, the request does not immediately reach your origin server. Instead, it first arrives at the nearest CDN edge node. The CDN then checks whether this resource already exists in its cache and whether the cached version is still valid.

If the cache is valid, the CDN responds directly to the user. This is a cache hit, which means faster response times and zero load on your origin server. If the cache does not exist or has expired, the CDN forwards the request to the origin server, retrieves the content, stores it according to caching rules, and then returns it to the user. This is a cache miss.

This simple process explains why CDNs can dramatically improve performance: repeated requests are served from the edge, not the origin. However, whether caching happens at all depends entirely on the rules defined by HTTP headers and CDN configuration.

2. Cache-Control and Expires: The Core of CDN Caching Logic

At the heart of CDN caching are HTTP response headers, especially Cache-Control and Expires. These headers tell the CDN how long a resource can be cached and under what conditions.

For example:

Cache-Control: public, max-age=3600

This tells the CDN that the resource can be cached publicly for one hour. During this time, the CDN can serve the cached content without contacting the origin server again.

On the other hand, headers like:

Cache-Control: no-cache

do not mean “do not cache at all,” but rather “revalidate before serving.” In practice, many CDNs treat this conservatively and will re-check with the origin on every request, which significantly reduces cache efficiency.

If the header is:

Cache-Control: no-store

the CDN will not cache the content under any circumstances. This is commonly seen on login pages, payment pages, or user-specific responses.

Understanding and intentionally setting these headers is essential. Many cache issues are not caused by the CDN, but by default settings in frameworks, reverse proxies, or application servers.

3. Static vs Dynamic Content: Why Not Everything Should Be Cached

A common misconception is that “more caching is always better.” In reality, CDN caching works best when content is clearly separated by type.

Static content such as images, JavaScript, CSS files, and video segments is ideal for CDN caching. These files change infrequently and are requested repeatedly by many users. Long cache lifetimes and aggressive caching strategies are recommended here.

Dynamic content, such as personalized pages, dashboards, or API responses, often depends on user identity, cookies, or authorization headers. CDNs usually avoid caching this type of content to prevent data leaks or incorrect responses.

A practical content separation strategy looks like this:

This separation alone can significantly improve cache hit rates and overall performance.

4. CDN Cache Is Not Permanent: Expiration and Revalidation

Caching does not mean content stays forever. Every cached resource has an expiration mechanism. Once the TTL (Time To Live) expires, the CDN must decide whether to re-fetch the content or revalidate it with the origin.

Some CDNs use conditional requests with headers like If-Modified-Since or ETag. If the origin responds with 304 Not Modified, the CDN continues serving the cached version without downloading the full content again. This reduces bandwidth usage while ensuring freshness.

This is why proper cache headers not only improve speed but also reduce origin traffic. A well-designed caching strategy balances freshness and performance instead of forcing frequent purges.

5. CDN Caching Is a Strategy, Not a Switch

Many users treat CDN caching as an on/off feature. In reality, it is an ongoing strategy that evolves with your website or application. Changes in content type, traffic patterns, or business logic all require cache policy adjustments.

Self-managed CDN solutions, such as 99CDN, are especially useful in this context because they allow fine-grained control over cache rules, TTLs, and purge behavior. Instead of relying on opaque defaults, you can design caching logic that matches your actual workload. This level of control is critical for high-traffic sites, download platforms, and video services. More details can be found at https://www.99cdn.com/.

Conclusion

CDN caching is fundamentally a collaboration between your origin server and the CDN edge. The CDN does not guess—it follows rules. If those rules are unclear, inconsistent, or overly conservative, caching will fail to deliver real benefits.

By understanding request flow, mastering Cache-Control behavior, separating static and dynamic content, and using expiration instead of constant purging, you can turn CDN caching from a black box into a predictable, powerful performance tool.

Once you stop asking “Why isn’t my CDN working?” and start asking “What rules am I giving it?”, CDN caching finally becomes something you can control, optimize, and rely on.