Website image caching with Apache

问题:

How can I get static content on Apache to be {cached by browser} and not {checked for freshness {with every request}}?

I'm working on a website hosted on Apache webserver. Recently, I was testing something with headers (Content-Type for different types of content) and saw a lot of conditional requests for images. Example:

200 /index.php?page=1234&action=list
304 /favicon.ico
304 /img/logo.png
304 /img/arrow.png
(etc.)

Although the image files are static content and are cached by the browser, every time an user opens a page that links to them, they are conditionally requested, to which they send "304 Not Modified". That's good (less data transferred), but it means 20+ more requests with every page load (longer page load due to all those round-trips, even with Keep-Alive and pipelining enabled).

How do I tell the browser to keep the existing file and not check for newer version?

EDIT: the mod_expires method works, even with the favicon.

回答1:

Expires module in Apache solves this - it needs to be loaded in server config, and set up in .htaccess (or in server config).

With an Expires header, the resource is only requested the first time. Before the expiration date, subsequent requests are fulfilled from browser cache. After the specified time expires and the resource is needed, only then it is requested again (conditionally - a 304 will be returned for an unchanged resource). The only reliable way to clear it from the cache before it expires is manually, or by forcing a refresh (usually Ctrl-F5). (This could be an issue if the resource changes in the meantime, but statical images don't change very often.)

# enable the directives - assuming they're not enabled globally
ExpiresActive on

# send an Expires: header for each of these mimetypes (as defined by server)
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"

# css may change a bit sometimes, so define shorter expiration
ExpiresByType text/css "access plus 1 days"

For favicon.ico, a bit more work is needed (Apache normally does not recognize Windows icon files, and sends this as the default text/plain).

# special MIME type for icons - see http://www.iana.org/assignments/media-types/image/vnd.microsoft.icon
AddType image/vnd.microsoft.icon .ico
# now we have icon MIME type, we can use it
# my favicon doesn't change much
ExpiresByType image/vnd.microsoft.icon "access plus 3 months"

And voila, It Works™!

回答2:

With the filesMatch directive, instead of ExpiresByType, you can group Content-Type by matching subtype (e.g. image/*), instead of listing each type/subtype pair, not subtype (e.g image/jpeg, image/png).

#Set caching on image files for 11 months
<filesMatch "\.(ico|gif|jpg|png)$">
  ExpiresActive On
  ExpiresDefault "access plus 11 month"
  Header append Cache-Control "public"
</filesMatch>

Acoording to this Google article I made expiration not longer than 1 year (access plus 11 month) and added Cache-Control "public" to enable HTTPS caching for Firefox.

For CSS and JS, Google recommends an expiry period of 1 week.

<filesMatch "\.(css|js)$">
  ExpiresActive On
  ExpiresDefault "access plus 1 week"
  Header append Cache-Control "public"
</filesMatch>

回答3:

If you set the Expires header on your http response for your static images, your server won't be checked again for that image after first download until the time specified has passed, e.g. if I download a file from your server now that gives it's Expires header as

Expires: Fri, 1 Jan 2010 00:00:01 GMT 

then my browser won't look for it from your server again until 2010, unless I clear my cache/do a force refresh (Ctrl+F5 on windows).

There's a simple introduction to setting this up here, and a list of other possibly helpful responses over at wikipedia

回答4:

Regarding favicon.ico, put it in your server document root say /var/www/html and add this to /etc/httpd/conf/httpd.conf in the Aliases section:-

Alias /favicon.ico "/var/www/html/favicon.ico"
<Directory "/var/www/html">
    <Files favicon.ico>
       ExpiresActive On
       ExpiresDefault "access plus 1 month"
    </Files>
</Directory>

Then a single favicon.ico will work for all the virtual hosted sites since you are aliasing it. After a user visits your site, any further visits will draw on the browser cache copy for one month, and not from the web.

I Could not get

ExpiresByType image/ico "access plus 1 month"

to work at all. Maybe it needs to be type text/plain as suggested above. In any case ExpiresDefault works OK.