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I have:

ExpiresActive On
ExpiresByType image/jpg "access 2 years"

And let's say:

<filesMatch ".(jpg|jpeg|png|gif)$">
    Header set Cache-Control "max-age=31536000, public"
</filesMatch>

When will jpg files expire ? In 2 years or in one year (31536000 seconds) ?

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ExpiresByType image/jpg "access 2 years"

First... to clarify, this should probably be image/jpeg, not image/jpg. image/jpeg is the official mime-type for JPEG files. Check the Content-Type HTTP response header associated with this response. For instance, if your server is sending JPEG files with a image/jpeg mime-type (as it should be), then the above directive is not doing anything.

Note that image/jpg has nothing to do with the file extension (eg. .jpg) that might be on the underlying filename. It refers to the mime-type the server is sending the resource as.

So, for the remainder of my answer, I assume this is the correct mime-type that your server is sending.

When will jpg files expire ? In 2 years or in one year (31536000 seconds) ?

The answer to this does depend on the type of request/response and what you are doing server-side. ie. Does the request map directly to a physical file or not? Since you mention "jpg files" then #1 below probably applies here (the most common scenario) and the "jpg file" will expire in 1 year.

1. Request maps directly to a physical .jpg file

The request maps directly to a physical JPEG file on disk (either directly or via an internal rewrite*1). eg. /myimage.jpg - which would indeed be the most common use case (ie. you are linking directly to your static resources) then the JPEG file will expire in 1 year, by the Header directive. This is because:

  1. <FilesMatch> (and <Files>) containers only match physical files.
  2. The Header directive is processed later and so will always override the Cache-Control header that mod_expires might otherwise set.

*1 Note that you could link to (ie. request) /get-image.php, but providing the request is internally rewritten directly to an underlying .jpg file on disk, eg. /some-image.jpg then the same applies.

2. Request does NOT map directly to a physical file

HOWEVER, if the request does not map to a physical file. For example, if either of the following scenarios is true (both are really the same):

  • You are linking to a PHP script that serves the JPEG image (either by creating it, or reading it from a different location, etc.) and there is no internal rewrite to a physical file.

    href="send-image.php?name=myimage.jpg&size=medium"
    
  • You are linking to a .jpg URL, eg. href="/myotherimage-medium.jpg" and you are internally rewriting this to a script (like above) that generates this image and returns it to the client.

Then the JPEG image will expire in 2 years since the <FilesMatch> directive will not apply, because the request does not map directly to a physical file.

Note that in both cases (scenario #1 and #2 above), the ExpiresByType directive will apply, but the Header directive will always override this if it is applied.

Actually, there is a further complication here... mod_expires sets 2 response headers Cache-Control: max-age and Expires. Expires is only for old browsers, all modern browsers prioritise the Cache-Control: max-age header. So, in case #1 above, where the request maps directly to a physical file then you'll have a conflict of headers... Cache-Control: max-age will state that it expires in 1 year (overridden by the Header directive), whereas the Expires header will state that it expires in 2 years (set my mod_expires).

So, unless you have specific caching requirements*2 then use mod_expires only.

*2 mod_expires only sets the max-age directive on the Cache-Control header. If you need to set other directives, eg. no-store or must-revalidate etc. then you will need to use the Header directive instead (or as well as).

Reference:

0

Meanwhile I found a subject on stackoverflow: https://stackoverflow.com/questions/5799906/what-s-the-difference-between-expires-and-cache-control-headers

There, one user also gives a link to w3: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html which says:

If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be useful if certain HTTP/1.0 caches improperly calculate ages or expiration times, perhaps due to desynchronized clocks.

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