The CDN Edge — Debug Headers Can Be Your Best Friend

Eric Klein
Dec 19, 2018 · 5 min read

A Background on Debug Headers

In a previous post, I discussed how to read the basic HTTP headers that may return with your HTTP response and how these impact caching. These are just the first building blocks needed in order to better understand where and how your objects are being cached within the disparate world of Content Delivery Networks. The true key to understand what is happening within these CDNs to your content is with Debug Headers.

These handy tools are in place in some way, shape or form on each Content Delivery Network that you may leverage. While some of these debug headers are easily published on every request, certain CDNs will only append these debug headers with the presence of a specific request header. For even more security, CDNs sometimes even require additional request headers and custom keys to be passed with a request in order to receive the appropriate debug headers.

While ultimately receiving these debug headers are relatively straightforward, the actual interpretation of the information return in the response can be quite challenging. In some instances, you’ll need to refer to detailed CDN guides that break down what each and every header means. In others, single characters may provide a wealth of data that can only be interpreted with internet specifications.

In this post, I’m going to explore the general benefits of these debug headers, some of the information they provide and how to best interpret this data. While we’re going to focus on Apache Traffic Server’s Debug Headers in this post, we’ll also provide links to some CDNs discussing their own debug headers in order to provide a well rounded guide for all things CDN debug related.

Debug Header Samples

By presenting “X-Debug” in the request for an object, the Apache Traffic Server — when setup with the appropriate package — will respond with a whole host of debug based options. Traditionally these options include a few critical elements of information that aid in debugging requests. These variables include: Cache State & Cache Location, Transaction ID, Cache Key

Cache State & Location — The Cache State and Location is typically revealed by the response contained within the “X-Cache” header. Here, you’ll receive information pertaining to the cache status of an object, as well as where that object is (for multi-tiered content delivery networks). The value of the cache status often includes:

  • Miss — Object Not In Cache. This typically means the object was served by going all the way back to origin to retrieve the asset.
  • Hit-Stale — Object in Cache, but is in need of freshness check. This typically precedes a 304 request to verify object freshness.
  • Hit-Fresh — Object in Cache and good to serve from cache.
  • Skipped — Cache Lookup Was Skipped. Typically done if the configuration defines cache bypass.

Additional information present in the “X-Cache” response is the name of the server responding with the cache information above. By appending multiple responses within the same response header, CDNs are typically able to present the entire request flow for an object.

For example, the below response would indicate an object that is not in cache at the first location, is stale in cache at the second location and is fresh in cache at the third location.

X-Cache: Miss; Hit-Stale; Hit-Fresh

As a result of this debug header, you can see the flow for this object through the CDN, and understand how things are behaving for your individual request, eventually being handled from a parent tier cache in a Toronto(YYZ) point of presence.

Transaction ID — Designated by “X-Transaction-ID” this value typically provides a unique string that is useful for identifying a transaction within logs. This enables retroactive analysis of a request to chase down exactly what happened to a specific request for a single asset. This feature is incredibly useful when trying to figure out how a specific object behaves within the entire CDN workflow and to diagnose any problems that may arise for a configuration.

Cache Key — Cache Key is perhaps the most useful debug header that you can evaluate. By providing the exact way that the CDN is interpreting and caching your object, you’re able to see whether things are appropriately configured on a specific request.

For example, let’s say you are removing query strings from a cacheable object in order to improve cache efficiency. While the request for the object may be, stripping of query parameters from the cache key should result in a different response from the x-cache-key header. In this instance, the cache key should be:


X-Cache-Key may also be revelatory if you have a more complex setup. Say that you’re using a different string match to replace a path within an object. By reviewing the cache key, you can ensure that this path replacement is occurring as expected, ensuring a smooth request flow for any asset requested against this specific setup.

One example of this scenario is if you’re configuration calls for /page/ in the URI above to be stripped on requests back to origin and replaced by /home/. In this scenario, the X-Cache-Key response should look like:


Add in complexity around regular expression configurations and advanced options and the X-Cache-Key feature becomes a crucial factor in being able to ensures CDNs are handling your objects appropriately.

More on Debug Headers

The usefulness and variety of CDN debug headers are numerous. Each CDN’s unique take makes memorizing these options and interpretations difficult.

While there is work underway to better standardize these requests and options as part of CDNi work in the Internet Engineering Task Force, the maturity of these headers across multiple different platforms still leaves a disjointed landscape of options for any content delivery specialist to navigate.

In order to better navigate these response header values, below are some examples from many of the popular commercial Content Delivery Networks. It is also recommended to always receive an education from your CDN of choice on their Debug Headers and all the different permutations when on boarding with that particular provider.

More to come from the CDN Edge in the future.

Useful Links

Akamai Pragma Headers:

Fastly Debug Headers:

Limelight Debug Headers:

Additional Debug Header samples from a Catchpoint blog post:


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