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Say I am serving 3 million requests of static and dynamic images per month from an EC2 machine (origin), and each image is about 15 KB, and my guess is that the approximate amount stored in the CloudFront cache will be about 15 GB of images, since 50% of the image requests are frequently repeated.

Using the AWS calculator, it says my monthly CouldFront bill will be only $4 per month.

$4 per month seems much too cheap, and I think I am missing some important piece in the cost puzzle?

On the AWS CloudFront Pricing sheet, I can't see how any pricing for CloudFront cache storage?

Is it possible that it is basically free to store 15 GB on the CloudFront cache? Or perhaps my understanding of the system is far off the mark?

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CloudFront will only keep data in its cache if it's "paying its way" through transfers. There is no specific fee for using cache. If your traffic is as low as you expect though, it's unlikely that as much information as you expect will remain in cache. This basically means you won't see as little traffic to your own server as you expect, since it will need to repopulate the cache more often than you might think.

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I thought the default object expiration in the cache , which is 24 hours, can be overridden by setting it from the origin server. So wouldn't setting the default for all image objects to 1 or more month enable me to limit 50% of the traffic to my server using CloudFront? I am sure I am missing something and appreciate your clarifications. (btw the images served don't change over a 1 year period, so dont have to be updated) –  indiehacker May 30 '12 at 19:59
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Expiration is not the only reason an object can be removed from the cache. It can also be removed because of inactivity. –  David Schwartz May 30 '12 at 20:08
    
Got it ....thanks. –  indiehacker May 30 '12 at 20:10

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