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Expiration of cached adaptive images

I'm trying out an adaptive image solution, wherein image requests are routed to a php file by htaccess.  The php file serves the visitor with a downsized version of the image, if the native image's width exceeds the visitor's screen width.  The downsized image is also cached.  My question pertains to some code in the php file, which removes the cached image after a configurable length of time.  So, is it desirable to keep cached images for a longer time, since other visitors with small screens might request the same image?
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ddantes
Asked:
ddantes
2 Solutions
 
Gary DavisDir Internet SvcsCommented:
There is a trade-off of course. Caching longer than needed uses up resources and shorter than needed uses bandwidth.

A sliding expiration is desirable though I am not sure if PHP has it (DotNET does). This resets the cache expiration time every time the cached object is requested keeping oft-needed images in cache as long as they are needed and other fall out.

Another possibility is to use the "max-age" header to let the browser know the image should be cached there and not bother to check with the server to see if it has changed. This saves transaction hits to the server.

Gary Davis
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ddantesAuthor Commented:
Thank you for your comment.  Would I include the max-age header in the <HEAD> section of my pages?

If the PHP file has a cache expiration of "365", does that keep images there in perpetuity, or do they still expire after a year?
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David S.Commented:
I'd say it depends on how many images you're potentially dealing with and how much disk space is available on your server. Performance issues like this usually involve a trade-off.

I got the impression that this question is about server-side caching. Gary's point about "max-age" is a client-side caching feature, so it's not clear to me that it has much relevance to this question.

365 might well be in days, but I would not call that a safe assumption. When cached files actually would be removed after the specified time period depends on how it was programmed. The script might do it itself the next time it runs, or it might use a CRON-job that runs at a set interval to handle that.
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