The Vary header is one of the most critical HTTP response headers when working with CDNs or proxy caches. A misconfigured Vary header can severely impact caching efficiency and, in extreme cases, even overload the origin server, leading to service disruptions.

What is the Vary Header?

The Vary header instructs CDNs and caching proxies to store different versions of a cached response based on specific request headers.

For example, consider a request for:
🔗 https://www.iamacmp.com/logo.jpg

If the origin server responds with:

Vary: User-Agent

This tells the CDN to cache different versions of the image for different user agents (browsers). If 10 different browsers request the same image, the CDN will store 10 separate cache entries—one for each browser.

While this makes sense for delivering browser-specific optimizations, it still reduces cache efficiency. However, CDNs can still optimize by collapsing millions of requests into a manageable number of origin fetches, significantly reducing load.

When Vary Can Be Dangerous

Now, imagine if the origin server mistakenly included User-ID or User-IP in the Vary header:

Vary: User-ID, User-IP

In this scenario, the CDN would cache a unique version for every individual user. If you have millions of users, the cache becomes practically useless, and the CDN will flood the origin with requests instead of offloading traffic.

This can drastically reduce cache efficiency, overload the origin server, and even bring the entire service down.

The Bottom Line

• ✅ Use Vary headers carefully to balance caching efficiency and content customization.
• ❌ Avoid Vary headers on highly dynamic values like User-ID or User-IP, which can destroy CDN caching.
• ⚡ Always test and analyze cache hit ratios when using Vary to ensure optimal performance.

A well-optimized CDN can improve performance and scalability, but a poorly configured Vary header can turn it into a liability. Handle it with care!

CDNs like Akamai has a builtin capability to remove vary headers.