Simon Willison's Weblog: etags

Add ETag header for static responses

2024-03-17T19:25:34+00:00

Add ETag header for static responses

I’ve been procrastinating on adding better caching headers for static assets (JavaScript and CSS) served by Datasette for several years, because I’ve been wanting to implement the perfect solution that sets far-future cache headers on every asset and ensures the URLs change when they are updated.

Agustin Bacigalup just submitted the best kind of pull request: he observed that adding ETag support for static assets would side-step the complexity while adding much of the benefit, and implemented it along with tests.

It’s a substantial performance improvement for any Datasette instance with a number of JavaScript plugins... like the ones we are building on Datasette Cloud. I’m just annoyed we didn’t ship something like this sooner!

Tags: etags, web-performance, datasette, datasette-cloud

ETags And Modification Times In Django

2008-12-13T09:49:42+00:00

ETags And Modification Times In Django

Part of Malcolm’s series of tutorials on implementing advanced HTTP concepts in Django.

Tags: caching, django, etags, http, malcolm-tredinnick

ETags, ETags, ETags

2007-08-07T14:51:31+00:00

ETags, ETags, ETags

They’re no magic bullet.

Tags: etags, http, mark-nottingham

Clever Caching

2007-07-05T00:56:33+00:00

Clever Caching

Instead of invalidating your cache directly, bump a version number on your model (blog entry or whatever) and use that as part of the cache key. This also gives you dynamic etags for free.

Tags: caching, etags, memcache, michael-koziarski

Supporting Conditional GET in PHP

2003-04-23T17:22:50+00:00

This site's RSS feeds now support Conditional GET. Since the feeds are dynamically generated on every request, adding support took a bit of hacking around with PHP. Here's the function I came up with (based on the excellent description provided by Charles Miller in the article linked above):

function doConditionalGet($timestamp) {
    // A PHP implementation of conditional get, see 
    //   https://fishbowl.pastiche.org/2002/10/21/http_conditional_get_for_rss_hackers
    $last_modified = substr(date('r', $timestamp), 0, -5).'GMT';
    $etag = '"'.md5($last_modified).'"';
    // Send the headers
    header("Last-Modified: $last_modified");
    header("ETag: $etag");
    // See if the client has provided the required headers
    $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ?
        stripslashes($_SERVER['HTTP_IF_MODIFIED_SINCE']) :
        false;
    $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ?
        stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : 
        false;
    if (!$if_modified_since && !$if_none_match) {
        return;
    }
    // At least one of the headers is there - check them
    if ($if_none_match && $if_none_match != $etag) {
        return; // etag is there but doesn't match
    }
    if ($if_modified_since && $if_modified_since != $last_modified) {
        return; // if-modified-since is there but doesn't match
    }
    // Nothing has changed since their last request - serve a 304 and exit
    header('HTTP/1.0 304 Not Modified');
    exit;
}

Usage is simple: Work out the timestamp that the page content was last modified and call doConditionalGet($timestamp);. It will send the 304 header for you and exit if the client claims to have seen the content already - otherwise control will return to your main script and you can serve content as normal. Slightly inelegant, but it does the job.

Unfortunately I don't have a Conditional-GET supporting RSS aggregator to hand so I have no idea if it works or not (so far I've only tested it by watching the headers sent with LiveHTTPHeaders). I'd be grateful if someone could confirm that this has had the desired effect.

Update: I've changed the above code sample (and my implementation) to send the ETag header as ETag rather than etag.

Tags: charles-miller, etags, php, rss