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I am running PHP on Apache, and am confused about how to implement server-side caching, in order to make the site load faster.

What is the difference between the Expires, Last-Modified and ETag headers, and which one should be used in what situation?

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Expires and Cache-Control are "strong caching headers"

Last-Modified and ETag are "weak caching headers"

First the browser checks Expires/Cache-Control to determine whether or not to make a request to the servers.

If it has to make a request, it will send Last-Modified/ETag in the HTTP request. If the Etag value of the document matches that, the server will send a 304 code instead of 200, and no content. The browser will load the contents from its cache.

I recommend using one of the strong caching headers, along with one of the weak caching headers.

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  • do you find any document support the "strong and weak" caching behavior? I couldn't find one, and my client browser now prioritize last-modified over expires actually, which I don't understand why.
    – Sam YC
    Commented Aug 20, 2015 at 2:15
  • Weak and strong validators are described in RFC 2616 section 13.3.3
    – user2418306
    Commented May 3, 2016 at 18:28
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    RFC 2616 says "Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak."
    – cquezel
    Commented Feb 9, 2017 at 18:06
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    I believe that the strong vs weak distinction of RFC2616 / ETags is a different one. I believe the poster of this answer means that Expires and Cache-Control are "strong" in the sense that they cause the browser to make no request at all, while Last-Modified and ETag are weaker in the sense that the browser still needs to make a request (but the server might not need to retransmit the entire resource).
    – Matthijs Kooijman
    Commented Jun 5, 2020 at 19:30
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You can use the Expires header in conjunction but regardless of the other two. It's universally supported by proxies and browser caches.

The difference between ETag and Last-Modified stamps is more semantic. ETags are opaque to clients. It's usually a checksum. Whereas a Last-Modified header can be interpreted by clients. It's understood that the last modified timestamp works linearly.

If a browser requests a resource with If-Unmodified-Since, then a wide range of timestamps in the past can match such a condition. If your pages change frequently then a Last-Modified timestamp might be advantageous.

The ETag approach, on the other hand, leads to clients that save one last fingerprint per resource. (I'm not sure if browser caches remember multiple ETags). On requests, only one or a few possible If-None-Match tokens are listed. This might mean more misses. Also, you have to compare multiple checksums, whereas with a Last-Modified timestamp you could have an arithmetic comparison.

The real advantage of ETags is that you can reliably compare fingerprints. The Last-Modified timestamps are a bit more vague, as they don't verify if the actual page content changed.

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  • so basically we should use mod_expire over eTags?
    – Avinash
    Commented Mar 16, 2011 at 7:23
  • then when we should use last modified header?
    – Avinash
    Commented Mar 16, 2011 at 8:02
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    I don't see why timestamps are advantageous to ETags if the page changes frequently. For both technologies, the server makes a simple comparison and sends the same response.
    – Torsten Bronger
    Commented Feb 10, 2015 at 15:39
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    @TorstenBronger - I think using a timestamp is faster on the server side. The If-None-Match request needs to read the entire file, digest it, then make a comparison. the If-Modified-Since request needs to stat the file, then compare to the mtime. Stat-ing a file takes far less time than reading and digesting, especially for large files.
    – Mjonir74
    Commented Sep 8, 2015 at 23:33
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    RFC 2616 states: "Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak."" and "The ETag ...This might allow more reliable validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates." and "the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value."
    – cquezel
    Commented Feb 9, 2017 at 18:14

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