问题
I already asked this question at <img> tag not respecting max-age, and unnecessarily reloading images and thought the comments provided a satisfactory answer, but turns out I was mistaken. In the six months since, I think I've thought up a solution, but want to check it out here before implementing it. Will it work? Is it worth implementing? Any simpler solution? etc.
To briefly re-describe the problem, I have a cgi program that generates gif images for html pages with tags like the following
<img src="http://example.com/cgi-bin/gifprogram.cgi?query_string">
The ?query_string specifies the image to be generated, and once the program generates it, the image is cached in a cachedirectory/filename.gif with filename chosen as the md5-hash of the ?query_string. Then subsequent tags calling for the same image just get that same cached file.
So browsers can use their own cache, but treat the tag as dynamic, and re-download the same, often-large, images again-and-again, every time they see the tag, regardless of max-age or any other http-header info (that I could think to try). Okay, I understand why "dynamic" is necessary for actually-dynamic cgi applications. But there ought to be a way to tell the browser "static". There doesn't seem to be any standard way, so here's my kludge...
On the server side, I'm going to write a short cgi program called checkcache.cgi?expression that checks the cache directory for a filename that's md5hash(expression), i.e., it will check whether or not a desired image is already cached. And its output to stdout will be the usual http-headers, ending with text/text, followed by either the full path/filename.ext to the cached file corresponding to expression, or "notfound" if the desired image doesn't yet exist.
On the client side, rather than directly writing <img> tags, I'll write a php script that takes the desired expression and does the following. First, it uses curl to curl_exec() checkcache.cgi. Then, if the cached file already exists, it echo's a static <img> tag, with src="http://etc/filename.gif", using the filename returned by checkcache. But if checkcache returns "not found", then it echo's the dynamic tag to run the gifprogram.cgi dynamically.
Since checkcache only emits a few dozen bytes (rather than some of these gifs which can sometimes run half a MB or more), it should go real quick. So, my question again -- the above strategy will work, right? Or what's wrong with it? Or can you suggest a simpler idea? Thanks.
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Edit Update: Yeah, that works. Finally. Just an awfully elaborate way of accomplishing what I'd think ought to be a simple task. And I'd still be interested to learn some simpler procedure.
来源：https://stackoverflow.com/questions/44719626/dynamic-img-tags-unnecessarily-reloading-static-images