AWS Cloudfront 0 Minimum TTL meaning & cost

Question

I read that Cloudfront now supports setting 0 as minimum TTL, I am wondering how it works though and if it affects the cost of Cloudfront/ AWS S3?

How does Cloudfront know when a js or css file has changed? We upload new file during deployments to S3. I have no idea if the modification time is correct or not but I think it is.

Main question is if it's cheaper to set it to 0 or keep 5 minutes as we have now and do an invalidation of js/css in Cloudfront after deployments. Sometimes when we golive with only backend changes we won't need to invalidate. Most of the time we have css/js changes also though.

Answer 1

Minimum TTL rarely needs to be customized.

Minimum TTL does not define the minimum amount of time that CloudFront will cache an object. That's what it sounds like, but that's a common misconception.

To help clarify that point, let me first clarify TTL.

The TTL (time-to-live) is a value CloudFront calculates, internally, for each individual object, for the purpose of determining whether an object should be cached at all, and then whether an object found in the cache is still considered fresh. If an object has been in the cache for less time than its TTL, it is said to be fresh, otherwise it is said to be stale.

A fresh object can be served to a viewer without checking with the origin.

A stale object should not be served to a viewer without verifying with the origin, and should eventually be purged.

CloudFront may retain a stale object in its cache for some period of time. (It can verify with the origin whether the stale object is still good, with a conditional request. It can also continue to use the stale object in limited conditions, including an outage on your origin).

CloudFront may also purge a fresh object from its cache at any time. Why would it do that? Cache space is free, so it wouldn't make sense for everything that CloudFront caches to continue to be stored in the cache if it isn't getting requested. CloudFront can purge an "unpopular" object from cache at any time.

From this, it is hopefully clear that TTL is not "how long will CloudFront cache the object?" but rather "how long will CloudFront consider a cached object fresh?"

If an object has a calculated TTL of 0, CloudFront won't cache the object.

So... how would an object get a calculated 0 TTL? At least one of these headers in the response from the origin:

Cache-Control: private
Cache-Control: no-cache
Cache-Control: no-store
Cache-Control: s-maxage=0
Cache-Control: max-age=0   // ignored when s-maxage is present

These are the only cases where Minimum TTL typically has a role to play. (Also, the Expires HTTP header can trigger it, but don't use Expires -- use Cache-Control).

Cache-Control -- as returned by the origin server -- is primarily intended for the browser, but CloudFront observes it as well, when trying to calculate an appropriate TTL for each object.

Minimum TTL establishes a lower boundary on the value CloudFront will extrapolate from the origin's Cache-Control header, from the conditions above, and smaller values are rounded up. If your origin returns (e.g.) Cache-Control: private, no-cache, no-store, that's a TTL that should be 0... but CloudFront sets its internal TTL for that object to the greater of 0 or Minimum TTL.

If the response had (e.g.) Cache-Control: max-age=15 then both browers and CloudFront would calculate a TTL for that object of 15 seconds. If (e.g.) Minimum TTL is set to 300 then CloudFront ignores the 15 seconds specified by the origin and sets its internal TTL for the object to 300 seconds. The browser will still use 15 seconds, because CloudFront does not modify the Cache-Control response header.

tl;dr: Minimum TTL is the minimum internal TTL value that CloudFront will assign to an object, regardless of values in the origin response that should cause it to calculate a lower value. Changing this setting from the default value 0 is an advanced option. It does not set a minimum amount of time that CloudFront is required to cache objects.

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