What is Cloudfront Minimum TTL for?

Question

I am trying to understand Minimum TTL, Maximum TTL and Default TTL with this document.

As my understanding, Maximum TTL is used when HTTP cache header appears in respons to limit maximum cache time and Default TTL is used when there is no HTTP cache header to use as default cache time.

However, for Maximum TTL, there is no specific mention.

In addition, It mentions the relation with forwarding head. Does it mean that if I set any HTTP header to forward to an origin and Minimum TTL is not 0, it doesn't cache anything?

Minimum TTL Specify the minimum amount of time, in seconds, that you want objects to stay in CloudFront caches before CloudFront forwards another request to your origin to determine whether the object has been updated. The default value for Minimum TTL is 0 seconds.

Important

. If you configure CloudFront to forward all headers to your origin for a cache behavior, CloudFront never caches the associated objects. Instead, CloudFront forwards all requests for those objects to the origin. In that configuration, the value of Minimum TTL must be 0.

Answer 1

When deciding whether and for how long to cache an object, CloudFront uses the following logic:

Check for any Cache-Control response header with these values:

• no-cache
• no-store
• private

If any of these is encountered, stop, and set the object's TTL¹ to the configured value of Minimum TTL. A non-zero value means CloudFront will cache objects that it would not otherwise cache.

Otherwise, find the origin's directive for how long the object may be cached. In order, find one of these response headers:

• Cache-Control: s-maxage=x
• Cache-Control: max-age=x
• Expires

Stop on the first value encountered using this ordering, then continue to the next step.

If no value was found, use Default TTL. Stop.

Otherwise, using the value discovered in the previous step:

• If smaller than Minimum TTL, then set the object's TTL to Minimum TTL; otherwise,
• If larger than Maximum TTL, then set the object's TTL to Maximum TTL; otherwise,
• Use the value found in the previous step as the object's TTL.

See https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/Expiration.html.

It's important to note that the TTL determines how long CloudFront is allowed to cache the response. It does not dictate how long CloudFront is required to cache the response. CloudFront can evict objects from cache before TTL expires, if the object is rarely accessed.

Whitelisting some (but not all) headers for forwarding to the origin does not change any of the above logic.

What it does change is how objects are evaluated to determine whether a cached response is available.

For example, if you forward the Origin header to the origin, then each unique value for an Origin header creates a different cache entry. Two requests that are identical, except for their Origin header, are then considered different objects... so a cached response for Origin: https://one.example.com would not be used if a later request for the same resource included Origin: https://two.example.com. Both would be sent to the origin, and both would be cached independently, for use in serving future requests with same the matching request header.

CloudFront does this because if you need to forward headers to the origin, then this implies that the origin will potentially react differently to different values for the whitelisted headers... so they are cached separately.

Forwarding headers unnecessarily will thus reduce your cache hit rate unnecessarily.

There is no documented limit to the number of different copies of the same resource that CloudFront can cache, based on varying headers.

But forwarding all headers to the origin reduces to almost zero the chance of any future request being truly identical. This would potentially consume a lot of cache storage, storing objects that would never again be reused, so CloudFront treats this as a special case, and does not allow any caching under this condition. As a result, you are required to set Minimum TTL to 0 for consistency.

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¹the object's TTL as used here refers to CloudFront's internal timer for each cached object that tracks how long it is allowed to continue to serve the cached object without checking back with the origin. The object's TTL inside CloudFront is known only to CloudFront, so this value does not impact browser caching.