Why Does CDN Cache Fail? A Practical Troubleshooting Guide from Cache-Control to CDN Policies

Introduction

When using a CDN, one of the most frustrating situations is discovering that content is not being cached as expected. You deploy a CDN hoping to reduce server load and speed up access, yet every request still hits the origin server. Cache hit rates stay low, bandwidth costs remain high, and performance barely improves.

In practice, CDN cache failure is rarely caused by a single factor. It is often the result of multiple issues working together—improper HTTP headers, unreasonable CDN cache rules, dynamic content interference, or even subtle misconfigurations at the origin. This article walks through real-world troubleshooting steps, starting from Cache-Control headers and moving all the way to CDN-side strategies, helping you systematically identify why your CDN cache is not working.

1. Cache-Control Headers: The Most Common Root Cause

In most cases, CDN cache issues start at the origin server. The CDN does not “decide” caching behavior on its own—it primarily follows HTTP response headers sent by the origin. Among these, Cache-Control plays the most critical role.

Common problematic configurations include no-cache, no-store, or extremely short max-age values. Many frameworks and CMS platforms default to conservative cache headers to avoid serving stale content, especially for dynamic pages.

For example, if your server returns the following header:

Cache-Control: no-cache, no-store, must-revalidate

the CDN will treat the content as non-cacheable, regardless of any cache rules you configure on the CDN side. Similarly, a header like:

Cache-Control: max-age=0

effectively disables caching.

A practical checklist when inspecting headers:

• Ensure static resources (images, JS, CSS, videos) use a long max-age

• Avoid no-store unless absolutely necessary

• Separate cache policies for dynamic and static content

A commonly recommended configuration for static assets looks like this:

Cache-Control: public, max-age=31536000, immutable

This allows the CDN to cache content aggressively while relying on versioning (hash-based filenames) to handle updates safely.

2. CDN Cache Rules: When Headers Are Correct but Cache Still Misses

Even with proper headers, CDN-side rules can override expected behavior. Many cache failures happen because CDN policies are either too strict or incorrectly scoped.

Typical misconfigurations include:

• Cache rules applied only to specific paths, leaving other resources uncached

• Rules that ignore query strings when they should not—or cache by query when they shouldn’t

• Default behavior set to “respect origin headers” when the origin headers are not optimized

A useful way to analyze this is by mapping request types to cache strategies:

If you are using a self-built CDN such as 99CDN, you gain more granular control over cache rules, including path-based strategies, file extension matching, and flexible TTL overrides. This makes it easier to correct cache issues without touching application code. For teams that need fine-grained tuning, a self-operated CDN architecture can significantly reduce cache troubleshooting complexity. You can learn more about this approach at https://www.99cdn.com/.

3. Hidden Cache Killers: Cookies, Authorization, and Dynamic Responses

Another frequently overlooked reason for cache failure is request headers, not response headers. CDNs typically avoid caching responses that include sensitive or user-specific data.

Common examples include:

• Requests containing Cookie headers

• Responses requiring Authorization

• Pages generated differently for each user session

For instance, even a static page may fail to cache if your application framework automatically injects cookies into every response. From the CDN’s perspective, this content is potentially personalized and therefore unsafe to cache.

A practical solution is to:

• Strip unnecessary cookies at the CDN edge for static paths

• Separate static asset domains (e.g., static.example.com)

• Ensure APIs and dynamic endpoints are clearly isolated from cacheable resources

This separation alone can dramatically increase cache hit rates in real-world deployments.

4. Cache Expiration vs. Cache Invalidation: A Strategic Mistake

Many teams mistakenly rely on frequent cache purges instead of proper expiration strategies. Excessive cache invalidation defeats the purpose of using a CDN, as it forces repeated origin fetches.

A better approach is to:

• Use long TTLs for static content

• Apply versioned URLs for updates

• Reserve manual purge for emergencies only

For example, instead of purging /app.js after every update, deploy /app.2025.01.js and let the old version expire naturally. This approach reduces origin load and improves cache stability.

Modern CDN platforms, including self-hosted solutions like 99CDN, support batch purging and tag-based invalidation, allowing precise control without flushing the entire cache.

5. How to Verify Whether CDN Cache Is Actually Working

Blind configuration changes are risky. Always verify cache behavior using real requests. The most reliable method is to inspect response headers returned by the CDN.

Typical indicators include:

• X-Cache: HIT or MISS

• Age header increasing over time

• CDN-specific cache status headers

A simple test using curl:

curl -I https://your-domain.com/static/image.jpg

Run the command multiple times from different locations. If caching works correctly, you should see a transition from MISS to HIT, and the Age value should increase.

If every request remains a MISS, the problem is almost certainly one of the areas discussed above.

Conclusion

CDN cache failure is rarely caused by the CDN itself. In most real-world scenarios, it results from a combination of origin headers, cache rules, request characteristics, and content design. By systematically checking Cache-Control headers, reviewing CDN cache policies, eliminating hidden cache killers like cookies, and adopting proper expiration strategies, most cache issues can be resolved without drastic changes.

If your business relies heavily on performance, stability, and predictable cache behavior, a self-built CDN solution such as 99CDN offers greater transparency and control compared to black-box CDN services. With fine-grained cache rules and flexible strategies, cache problems become easier to diagnose and fix rather than a recurring mystery.

Ultimately, a CDN only works as well as the strategy behind it. Treat caching as a system, not a switch, and your CDN will finally deliver the performance gains it promises.