This is an informational document. Although technical in nature, it attempts to make the concepts involved understandable and applicable in real-world situations. Because of this, some aspects of the material are simplified or omitted, for the sake of clarity. If you are interested in the minutia of the subject, please explore the References and Further Information at the end.
A Web cache sits between Web servers (or origin servers) and a client or many clients, and watches requests for HTML pages, images and files (collectively known as objects) come by, saving a copy for itself. Then, if there is another request for the same object, it will use the copy that it has, instead of asking the origin server for it again.
There are two main reasons that Web caches are used:
If you examine the preferences dialog of any modern browser (like Internet Explorer or Netscape), you'll probably notice a 'cache' setting. This lets you set aside a section of your computer's hard disk to store objects that you've seen, just for you. The browser cache works according to fairly simple rules. It will check to make sure that the objects are fresh, usually once a session (that is, the once in the current invocation of the browser).
This cache is useful when a client hits the 'back' button to go to a page they've already seen. Also, if you use the same navigation images throughout your site, they'll be served from the browser cache almost instantaneously.
Web proxy caches work on the same principle, but a much larger scale. Proxies serve hundreds or thousands of users in the same way; large corporations and ISP's often set them up on their firewalls.
Because proxy caches usually have a large number of users behind them, they are very good at reducing latency and traffic. That's because popular objects are requested only once, and served to a large number of clients.
Most proxy caches are deployed by large companies or ISPs that want to reduce the amount of Internet bandwidth that they use. Because the cache is shared by a large number of users, there are a large number of shared hits (objects that are requested by a number of clients). Hit rates of 50% efficiency or greater are not uncommon. Proxy caches are a type of shared cache.
Web caching is one of the most misunderstood technologies on the Internet. Webmasters in particular fear losing control of their site, because a cache can 'hide' their users from them, making it difficult to see who's using the site.
Unfortunately for them, even if no Web caches were used, there are too many variables on the Internet to assure that they'll be able to get an accurate picture of how users see their site. If this is a big concern for you, this document will teach you how to get the statistics you need without making your site cache-unfriendly.
Another concern is that caches can serve content that is out of date, or stale. However, this document can show you how to configure your server to control this, while making it more cacheable.
On the other hand, if you plan your site well, caches can help your Web site load faster, and save load on your server and Internet link. The difference can be dramatic; a site that is difficult to cache may take several seconds to load, while one that takes advantage of caching can seem instantaneous in comparison. Users will appreciate a fast-loading site, and will visit more often.
Think of it this way; many large Internet companies are spending millions of dollars setting up farms of servers around the world to replicate their content, in order to make it as fast to access as possible for their users. Caches do the same for you, and they're even closer to the end user. Best of all, you don't have to pay for them.
The fact is that caches will be used whether you like it or not. If you don't configure your site to be cached correctly, it will be cached using whatever defaults the cache's administrator decides upon.
All caches have a set of rules that they use to determine when to serve an object from the cache, if it's available. Some of these rules are set in the protocols (HTTP 1.0 and 1.1), and some are set by the administrator of the cache (either the user of the browser cache, or the proxy administrator).
Generally speaking, these are the most common rules that are followed for a particular request (don't worry if you don't understand the details, it will be explained below):
Together, freshness and validation are the most important ways that a cache works with content. A fresh object will be available instantly from the cache, while a validated object will avoid sending the entire object over again if it hasn't changed.
There are several tools that Web designers and Webmasters can use to fine-tune how caches will treat their sites. It may require getting your hands a little dirty with the server configuration, but the results are worth it. For details on how to use these tools with your server, see the Implementation sections below.
HTML authors can put tags in a document's <HEAD> section that describe its attributes. These Meta tags are often used in the belief that they can mark a document as uncacheable, or expire it at a certain time.
Meta tags are easy to use, but aren't very effective. That's because they're usually only honored by browser caches (which actually read the HTML), not proxy caches (which almost never read the HTML in the document). While it may be tempting to slap a Pragma: no-cache meta tag on a home page, it won't necessarily cause it to be kept fresh, if it goes through a shared cache.
On the other hand, true HTTP headers give you a lot of control over how both browser caches and proxies handle your objects. They can't be seen in the HTML, and are usually automatically generated by the Web server. However, you can control them to some degree, depending on the server you use. In the following sections, you'll see what HTTP headers are interesting, and how to apply them to your site.
HTTP headers are sent by the server before the HTML, and only seen by the browser and any intermediate caches. Typical HTTP 1.1 response headers might look like this:
HTTP/1.1 200 OK Date: Fri, 30 Oct 1998 13:19:41 GMT Server: Apache/1.3.3 (Unix) Cache-Control: max-age=3600, must-revalidate Expires: Fri, 30 Oct 1998 14:19:41 GMT Last-Modified: Mon, 29 Jun 1998 02:28:12 GMT ETag: "3e86-410-3596fbbc" Content-Length: 1040 Content-Type: text/html
The HTML document would follow these headers, separated by a blank line.
Many people believe that assigning a Pragma: no-cache HTTP header to an object will make it uncacheable. This is not necessarily true; the HTTP specification does not set any guidelines for Pragma response headers; instead, Pragma request headers (the headers that a browser sends to a server) are discussed. Although a few caches may honor this header, the majority won't, and it won't have any effect. Use the headers below instead.
The Expires HTTP header is the basic means of controlling caches; it tells all caches how long the object is fresh for; after that time, caches will always check back with the origin server to see if a document is changed. Expires headers are supported by practically every client.
Most Web servers allow you to set Expires response headers in a number of ways. Commonly, they will allow setting an absolute time to expire, a time based on the last time that the client saw the object (last access time), or a time based on the last time the document changed on your server (last modification time).
Expires headers are especially good for making static images (like navigation bars and buttons) cacheable. Because they don't change much, you can set extremely long expiry time on them, making your site appear much more responsive to your users. They're also useful for controlling caching of a page that is regularly changed. For instance, if you update a news page once a day at 6am, you can set the object to expire at that time, so caches will know when to get a fresh copy, without users having to hit 'reload'.
The only value valid in an Expires header is a HTTP date; anything else will most likely be interpreted as 'in the past', so that the object is uncacheable. Also, remember that the time in a HTTP date is Greenwich Mean Time (GMT), not local time.
Expires: Fri, 30 Oct 1998 14:19:41 GMT
Although the Expires header is useful, it is still somewhat limited; there are many situations where content is cacheable, but the HTTP 1.0 protocol lacks methods of telling caches what it is, or how to work with it.
HTTP 1.1 introduces a new class of headers, the Cache-Control response headers, which allow Web publishers to define how pages should be handled by caches. They include directives to declare what should be cacheable, what may be stored by caches, modifications of the expiration mechanism, and revalidation and reload controls.
Interesting Cache-Control response headers include:
Cache-Control: max-age=3600, must-revalidate
If you plan to use the Cache-Control headers, you should have a look at the excellent documentation in the HTTP 1.1 draft; see References and Further Information.
In How Web Caches Work, we said that validation is used by servers and caches to communicate when an object has changed. By using it, caches avoid having to download the entire object when they already have a copy locally, but they're not sure if it's still fresh.
Validators are very important; if one isn't present, and there isn't any freshness information (Expires or Cache-Control) available, most caches will not store an object at all.
The most common validator is the time that the document last changed, the Last-Modified time. When a cache has an object stored that includes a Last-Modified header, it can use it to ask the server if the object has changed since the last time it was seen, with an If-Modified-Since request.
HTTP 1.1 introduced a new kind of validator called the ETag. ETags are unique identifiers that are generated by the server and changed every time the object does. Because the server controls how the ETag is generated, caches can be surer that if the ETag matches when they make a If-None-Match request, the object really is the same.
Almost all caches use Last-Modified times in determining if an object is fresh; as more HTTP/1.1 caches come online, Etag headers will also be used.
Most modern Web servers will generate both ETag and Last-Modified validators for static content automatically; you won't have to do anything. However, they don't know enough about dynamic content (like CGI, ASP or database sites) to generate them; see Writing Cache-Aware Scripts.
Besides using freshness information and validation, there are a number of other things you can do to make your site more cache-friendly.
By default, most scripts won't return a validator (e.g., a Last-Modified or ETag HTTP header) or freshness information (Expires or Cache-Control). While some scripts really are dynamic (meaning that they return a different response for every request), many (like search engines and database-driven sites) can benefit from being cache-friendly.
Generally speaking, if a script produces output that is reproducable with the same request at a later time (whether it be minutes or days later), it should be cacheable. If the content of the script changes only depending on what's in the URL, it is cacheble; if the output depends on a cookie, authentication information or other external criteria, it probably isn't.
Some other tips;
See the Implementation Notes for more specific information.
A good strategy is to identify the most popular, largest objects (especially images) and work with them first.
The most cacheable object is one with a long freshness time set. Validation does help reduce the time that it takes to see an object, but the cache still has to contact the origin server to see if it's fresh. If the cache already knows it's fresh, it will be served directly.
If you must know every time a page is accessed, select ONE small object on a page (or the page itself), and make it uncacheable, by giving it a suitable headers. For example, you could refer to a 1x1 transparent uncacheable image from each page. The Referer header will contain information about what page called it.
Be aware that even this will not give truly accurate statistics about your users, and is unfriendly to the Internet and your users; it generates unnecessary traffic, and forces people to wait for that uncached item to be downloaded. For more information about this, see On Interpreting Access Statistics in the references.
The Expires header is the best way to do this. By setting the server to expire the document based on its modification time, you can automatically have caches mark it as stale a set amount of time after it is changed.
For example, if your site's home page changes every day at 8am, set the Expires header for 23 hours after the last modification time. This way, your users will always get a fresh copy of the page.
See also the Cache-Control: max-age header.
To see what the Expires and Last-Modified headers are, open the page with Netscape and select 'page info' from the View menu. This will give you a menu of the page and any objects (like images) associated with it, along with their details.
To see the full headers of an object, you'll need to manually connect to the Web server using a Telnet client. Depending on what program you use, you may need to type the port into a separate field, or you may need to connect to www.myhost.com:80 or www.myhost.com 80 (note the space). Consult your Telnet client's documentation.
Once you've opened a connection to the site, type a request for the object. For instance, if you want to see the headers for http://www.myhost.com/foo.html, connect to www.myhost.com, port 80, and type:
GET /foo.html HTTP/1.1 [return] Host: www.myhost.com [return][return]
Press the Return key every time you see [return]; make sure to press it twice at the end. This will print the headers, and then the full object. To see the headers only, substitute HEAD for GET.
By default, pages protected with HTTP authentication are marked private; they will not be cached by shared caches. However, you can mark authenticated pages public with a Cache-Control header; HTTP 1.1-compliant caches will then allow them to be cached.
If you'd like the pages to be cacheable, but still authenticated for every user, combine the Cache-Control: public and no-cache headers. This tells the cache that it must submit the new client's authentication information to the origin server before releasing the object from the cache.
Whether or not this is done, it's best to minimize use of authentication; for instance, if your images are not sensitive, put them in a separate directory and configure your server not to force authentication for it. That way, those images will be naturally cacheable.
SSL pages are not cached (or unencrypted) by proxy caches, so you don't have to worry about that. However, because caches store non-SSL requests and URLs fetched through them, you should be conscious of security on unsecured sites; an unscrupulous administrator could conceivably gather information about their users.
In fact, any administrator on the network between your server and your clients could gather this type of information. One particular problem is when CGI scripts put usernames and passwords in the URL itself; this makes it trivial for others to find and user their login.
If you're aware of the issues surrounding Web security in general, you shouldn't have any surprises from proxy caches.
It varies. Generally speaking, the more complex a solution is, the more difficult it is to cache. The worst are ones which dynamically generate all content and don't provide validators; they may not be cacheable at all. Speak with your vendor's technical staff for more information, and see the Implementation notes below.
The Expires header can't be circumvented; unless the cache (either browser or proxy) runs out of room and has to delete the objects, the cached copy will be used until then.
The most effective solution is to rename the files; that way, they will be completely new objects, and loaded fresh from the origin server. Remember that the page that refers to an object will be cached as well. Because of this, it's best to make static images and similar objects very cacheable, while keeping the HTML pages that refer to them on a tight leash.
If you want to reload an object from a specific cache, you can either force a reload (in Netscape, holding down shift while pressing 'reload' will do this by issuing a Pragma: no-cache request header) while using the cache. Or, you can have the cache administrator delete the object through their interface.
If you're using Apache, consider allowing them to use .htaccess files, and provide appropriate documentation.
Otherwise, you can establish predetermined areas for various caching attributes in each virtual server. For instance, you could specify a directory /cache-1m that will be cached for one month after access, and a /no-cache area that will be served with headers instructing caches not to store objects from it.
Whatever you are able to do, it is best to work with your largest customers first on caching. Most of the savings (in bandwidth and in load on your servers) will be realized from high-volume sites.
HTTP 1.1 compliance is mentioned several times in this document. As of the time it was written, the protocol is a work in progress. Because of this, it is virtually impossible for an application (whether a server, proxy or client) to be truly compliant. However, the protocol has been openly discussed for some time, and feature-frozen for enough time to allow developers to use the ideas contained in it, like Cache-Control and ETags. When HTTP 1.1 is final, expect more vendors to openly state that their applications are compliant.
Generally speaking, it's best to use the latest version of whatever Web server you've chosen to deploy. Not only will they likely contain more cache-friendly features, new versions also usually have important security and performance improvements.
Apache (http://www.apache.org/) uses optional modules to include headers, including both Expires and Cache-Control. Both modules are available in the 1.2 or greater distribution.
The modules need to be built into Apache; although they are included in the distribution, they are not turned on by default. To find out if the modules are enabled in your server, find the httpd binary and run httpd -l; this should print a list of the available modules. The modules we're looking for are mod_expires and mod_headers.
Once you have an Apache with the appropriate modules, you can use mod_expires to specify when objects should expire, either in .htaccess files or in the server's access.conf file. You can specify expiry from either access or modification time, and apply it to a file type or as a default. See http://www.apache.org/docs/mod/mod_expires.html for more information, and speak with your local Apache guru if you have trouble.
To apply Cache-Control headers, you'll need to use the mod_headers module, which allows you to specify arbitrary HTTP headers for a resource. See http://www.apache.org/docs/mod/mod_headers.html
Here's an example .htaccess file that demonstrates the use of some headers.
### activate mod_expires ExpiresActive On ### Expire .gif's 1 month from when they're accessed ExpiresByType image/gif A2592000 ### Expire everything else 1 day from when it's last modified ### (this uses the Alternative syntax) ExpiresDefault "modification plus 1 day" ### Apply a Cache-Control header to index.html <Files index.html> Header append Cache-Control "public, must-revalidate" </Files>
Netscape Enterprise Server (http://www.netscape.com/) does not provide any obvious way to set Expires headers. However, it has supported HTTP 1.1 features since version 3.0. This means that HTTP 1.1 caches (proxy and browser) will be able to take advantage of Cache-Control settings you make.
To use Cache-Control headers, choose Content Management | Cache Control Directives in the administration server. Then, using the Resource Picker, choose the directory where you want to set the headers. After setting the headers, click 'OK'. For more information, see http://developer.netscape.com/docs/manuals/enterprise/admnunix/content.htm#1006282
Microsoft's Internet Information Server (http://www.microsoft.com/) makes it very easy to set headers in a somewhat flexible way. Note that this is only possible in version 4 of the server, which will run only on NT Server.
To specify headers for an area of a site, select it in the Administration Tools interface, and bring up its properties. After selecting the HTTP Headers tab, you should see two interesting areas; Enable Content Expiration and Custom HTTP headers. The first should be self-explanatory, and the second can be used to apply Cache-Control headers.
See the ASP section below for information about setting headers in Active Server Pages. It is also possible to set headers from ISAPI modules; refer to MSDN for details.
Lotus' (http://www.lotus.com/) servers are notoriously difficult to cache; they don't provide any validators, so both browser and proxy caches can only use default mechanisms (i.e., once per session, and a few minutes of 'fresh' time, usually) to cache any content from them.
Even if this limitation is overcome, Notes' habit of referring to the same object by different URLs (depending on a variety of factors) bars any measurable gains. There is also no documented way to set an Expires, Cache-Control or other arbitrary HTTP header.
Because the emphasis in server-side scripting is on dynamic content, it doesn't make for very cacheable pages, even when the content could be cached. If your content changes often, but not on every page hit, consider setting an Expires header, even if just for a few hours. Most users access pages again in a relatively short period of time. For instance, when users hit the 'back' button, if there isn't any validator or freshness information available, they'll have to wait until the page is re-downloaded from the server to see it.
CGI scripts are one of the most popular ways to generate content. You can easily append HTTP response headers by adding them before you send the body; Most CGI implementations already require you to do this for the Content-Type header. For instance, in Perl;
#!/usr/bin/perl print "Content-type: text/html\n"; print "Expires: Thu, 29 Oct 1998 17:04:19 GMT\n"; print "\n"; ### the content body follows...
Since it's all text, you can easily generate Expires and other date-related headers with in-built functions. It's even easier if you use Cache-Control: max-age;
print "Cache-Control: max-age=600\n";
This will make the script cacheable for 10 minutes after the request, so that if the user hits the 'back' button, they won't be resubmitting the request.
The CGI specification also makes request headers that the client sends available in the environment of the script; each header has 'HTTP_' appended to its name. So, if a client makes an If-Modified-Since request, it may show up like this:
HTTP_IF_MODIFIED_SINCE = Fri, 30 Oct 1998 14:19:41 GMT
SSI (often used with the extension .shtml) is one of the first ways that Web publishers were able to get dynamic content into pages. By using special tags in the pages, a limited form of in-HTML scripting was available.
Most implementations of SSL do not set validators, and as such are not cacheable. However, Apache's implementation does allow users to specify which SSI files can be cached, by setting the group execute permissions on the appropriate files, combined with the XbitHack full directive. For more information, see http://www.apache.org/docs/mod/mod_include.html
PHP (http://www.php.net/) is a server-side scripting language that, when built into the server, can be used to embed scripts inside a page's HTML, much like SSI, but with a far larger number of options. PHP can be used as a CGI script on any Web server (Unix or Windows), or as an Apache module.
By default, objects processed by PHP are not assigned validators, and are therefore uncacheable. However, developers can set HTTP headers by using the Header() function.
For example, this will create a Cache-Control header, as well as an Expires header three days in the future:
<?php Header("Cache-Control: must-revalidate"); $offset = 60 * 60 * 24 * 3; $ExpireString = "Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . " GMT"; Header($ExpireString); ?>
Remember that the Header() function MUST come before any other output.
As you can see, you'll have to create the HTTP date for an Expires header by hand; PHP doesn't provide a function to do it for you. Of course, it's easy to set a Cache-Control: max-age header, which is just as good for most situations.
For more information, see http://www.php.net/manual/function.header.php3
Cold Fusion, by Allaire (http://www.allaire.com/) is a commercial server-side scripting engine, with support for several Web servers on Windows and Solaris.
Cold Fusion makes setting arbitrary HTTP headers relatively easy, with the CFHEADER tag. Unfortunately, setting date-related functions in Cold Fusion isn't easy as Allaire's documentation leads you to believe; their example for setting an Expires header, as below, won't work.
<CFHEADER NAME="Expires" VALUE="#Now()#">
It doesn't work because the time (in this case, when the request is made) doesn't get converted to a HTTP-valid date; instead, it just gets printed as a representation of Cold Fusion's Date/Time object. Most clients will either ignore such a value, or convert it to a default, like January 1, 1970.
Cold Fusion's date formatting functions make it difficult generate a date that is HTTP-valid; you'll need to either use a combination of DateFormat, Hour, Minute and Second, or roll your own. Of course, you can still use the CFHEADER tag to set Cache-Control: max-age and other headers.
Also, Remember that Web server headers are passed through with some implementations (such as CGI); check yours to determine whether you can use this to your advantage, by setting headers on the server instead of in Cold Fusion.
Active Server Pages, build into IIS and now becoming available in other implementations, also allow you to set HTTP headers. For instance, to set an expiry time, use the properties of the Response object in your page, like this:
<% Response.Expires=1440 %>
specifying the number of minutes from the request to expire the object. Likewise, absolute expiry time can be set like this (make sure you format HTTP date correctly):
<% Response.ExpiresAbsolute=#May 31,1996 13:30:15 GMT# %>
Cache-Control headers can be added like this:
<% Response.CacheControl="public" %>
The HTTP 1.1 spec has many extensions for making pages cacheable, and is the authoritative guide to implementing the protocol. See sections 13, 14.9, 14.21, and 14.25.
An excellent introduction to caching concepts, with links to other online resources.
Cache Now! is a campaign to raise awareness of caching, from all perspectives.
Jeff Goldberg's informative paper on why you shouldn't rely on access statistics and hit counters.
Examines Web pages to determine how they will interact with Web caches, the Engine is a good debugging tool, and a companion to this tutorial.
This document is Copyright © 1998, 1999 Mark Nottingham <email@example.com>. It may be freely distributed in any medium as long as the text (including this notice) is kept intact and the content is not modified, edited, added to or otherwise changed. Formatting and presentation may be modified. Small excerpts may be made as long as the full document is properly and conspicuously referenced.
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Although the author believes the contents to be accurate at the time of publication, no liability is assumed for them, their application or any consequences thereof. If any misrepresentations, errors or other need for clarification is found, please contact the author immediately.
The latest copy of this document can always be obtained from http://www.mnot.net/cache_docs/
Version 1.32 - June 19, 2000