Caching HTTP Headers, Cache-Control: max-age

Caching speeds up repeated page views and saves a lot of traffic by preventing downloading of unchanged content every page view.

We can use Cache-Control: max-age=… to inform browser that the component won’t be changed for defined period. This way we avoid unneeded further requests if browser already has the component in its cache and therefore primed-cache page views will be performed faster.

Modern browsers able to cache static files even without any cache control headers using some heuristic methods but they will do it more efficient if we define caching headers implicitly.

For Apache2 you can enable max-age using mod_expires:

ExpiresActive On
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType text/css "access plus 1 month"
ExpiresByType text/javascript "access plus 1 month"
ExpiresByType application/x-javascript "access plus 1 month"
ExpiresByType application/x-shockwave-flash "access plus 1 month"

For Lighttpd there is mod_expire module. Enable it in server.modules section:

server.modules = (
...
"mod_expire",
...
)
Then add following directives for directories with static files:
$HTTP["url"] =~ "^/images/" {
expire.url = ( "" => "access 30 days" )
}

Max-age for Nginx server can be enabled using ngx_http_headers_module:

expires max;

Now web server sends the caching header for static files:

Cache-Control: max-age=2592000

In case of design change we should prevent using outdated content that browsers have in their caches. This can be done by adding file versions to filenames:

script.js -> script1.js -> script2.js -> ... etc

Cache-control: max-age can be useful also when we output HTML. Imagine pages generated by PHP that changed not so often, once per day or even longer. But browsers still have to download HTML every page view.

We can improve it by sending max-age value in PHP.

header('Cache-Control: max-age=28800');

This way we set desirable cache lifetime to 8 hours. Now if someone is clicking a link for second time within 8 hours period he gets the page instantly.

Max-age also helps to make proxy servers more efficient. We can easily organize transparent server-side caching by adding proxy server to web frontend.

Note that there is not easy case if pages have content that changes often and that’s relevant.

For example, there can be difficulties in caching pages with login form that transforms into some box with «Hello username» after user login or if there are user comments, the user who posted commentary will not see it. Because we cannot ask browser to destroy cache entry, it will still get the old page from cache.

The solution can be using Javascript to generate login box (requires enabled Javascript). If we set a cookie after user logged in, we can check it on client-side and generate suitable content for the logged in user. This way the content will be the same from server side view and can be cached.

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