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Caching Explained
Quick Guide
 
What's a Web Cache? Why do people use them?

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:

  • To reduce latency - Because the request is satisfied from the cache (which is closer to the client) instead of the origin server, it takes less time for the client to get the object and display it. This makes Web sites seem more responsive.
  • To reduce traffic - Because each object is only gotten from the server once, it reduces the amount of bandwidth used by a client. This saves money if the client is paying by traffic, and keeps their bandwidth requirements lower and more manageable.
Kinds of Web Caches

Browser Caches

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.

Proxy Caches

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.

Aren't Web Caches bad for me? Why should I help them?

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.

How Web Caches Work

All caches have a set of rules that they use to determine when to serve an object from the cache, if its 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):

  1. If the object's headers tell the cache not to keep the object, it won't. Also, if no validator is present, most caches will mark the object as uncacheable.
  2. If the object is authenticated or secure, it won't be cached.
  3. A cached object is considered fresh (that is, able to be sent to a client without checking with the origin server) if:
    • It has an expiry time or other age-controlling directive set, and is still within the fresh period..
    • If a browser cache has already seen the object, and has been set to check once a session.
    • If a proxy cache has seen the object recently, and it was modified relatively long ago. Fresh documents are served directly from the cache, without checking with the origin server.
  4. If an object is stale, the origin server will be asked to validate the object, or tell the cache whether the copy that it has is still good.

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 Meta Tags vs. HTTP Headers

HTML authors can put tags in a document's 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.

  • If your site is hosted at an ISP or hosting farm and they don't give you the ability to set arbitrary HTTP headers (like Expires and Cache-Control), complain loudly; these are tools necessary for doing your job.

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.

Pragma HTTP Headers (and why they don't work)

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.

Controlling Freshness with the Expires HTTP Header

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.

For example:

Expires: Fri, 30 Oct 1998 14:19:41 GMT

Cache-Control HTTP Headers

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:

  • max-age=[seconds] - specifies the maximum amount of that an object will be considered fresh.Similar to Expires, this directive allows more flexibility. [seconds] is the number of seconds from the time of the request you wish the object to be fresh for.
  • s-maxage=[seconds] - similar to max-age, except that it only applies to proxy (shared) caches.
  • public - marks the response as cacheable, even if it would normally be uncacheable. For instance, if your pages are authenticated, the public directive makes them cacheable.
  • no-cache - forces caches (both proxy and browser) to submit the request to the origin server for validation before releasing a cached copy, every time. This is useful for to assure that authentication is respected (in combination with public), or to maintain rigid object freshness, without sacrificing all of the benefits of caching.
  • must-revalidate - tells caches that they must obey any freshness information you give them about an object. The HTTP allows caches to take liberties with the freshness of objects; by specifying this header, you're telling the cache that you want it to strictly follow your rules.
  • proxy-revalidate - similar to must-revalidate, except that it only applies to proxy caches.

For example:
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.

Validators and Validation

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.

Tips for Building a Cache-Aware Site

Besides using freshness information and validation, there are a number of other things you can do to make your site more cache-friendly.

  • Refer to objects consistently - this is the golden rule of caching. If you serve the same content on different pages, to different users, or from different sites, it should use the same URL. This is the easiest and most effective may to make your site cache-friendly. For example, if you use /index.html in your HTML as a reference once, always use it that way.
  • Use a common library of images and other elements and refer back to them from different places.
  • Make caches store images and pages that don't change often by specifying either a far-away Expires header.
  • Make caches recognize regularly updated pages by specifying an appropriate expiration time.
  • If a resource (especially a downloadable file) changes, change its name. That way, you can make it expire far in the future, and still guarantee that the correct version is served; the page that links to it is the only one that will need a short expiry time.
  • Don't change files unnecessarily. If you do, everything will have a falsely young Last-Modified date. For instance, when updating your site, don't copy over the entire site; just move the files that you've changed.
  • Use cookies only where necessary - cookies are difficult to cache, and aren't needed in most situations. If you must use a cookie, limit its use to dynamic pages.
  • Minimize use of SSL - because encrypted pages are not stored by shared caches, use them only when you have to, and use images on SSL pages sparingly.
  • Use the Cacheability Engine - it can help you apply many of the concepts in this tutorial.

Writing Cache-Aware Scripts

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 reproducible 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 cacheable; if the output depends on a cookie, authentication information or other external criteria, it probably isn't.

  • The best way to make a script cache-friendly (as well as perform better) is to dump its content to a plain file whenever it changes. The Web server can then treat it like any other Web page, generating and using validators, which makes your life easier. Remember to only write files that have changed, so the Last-Modified times are preserved.
  • Another way to make a script cacheable in a limited fashion is to set an age-related header for as far in the future as practical. Although this can be done with Expires, it's probably easiest to do so with Cache-Control: max-age, which will make the request fresh for an amount of time after the request.
  • If you can't do that, you'll need to make the script generate a validator, and then respond to If-Modified-Since and/or If-None-Match requests. This can be done by parsing the HTTP headers, and then responding with 304 Not Modified when appropriate. Unfortunately, this is not a trivial task.
Some other tips:
  • If you have to use scripting, don't POST unless it's appropriate. The POST method is (practically) impossible to cache; if you send information in the path or query (via GET), caches can store that information for the future. POST, on the other hand, is good for sending large amount of information to the server (which is why it won't be cached; it's very unlikely that the same exact POST will be made twice).
  • Don't embed user-specific information in the URL unless the content generated is completely unique to that user.
  • Don't count on all requests from a user coming from the same host, because caches often work together.
  • Generate Content-Length response headers. It's easy to do, and it will allow the response of your script to be used in a persistent connection. This allows a client (whether a proxy or a browser) to request multiple objects on one TCP/IP connection, instead of setting up a connection for every request. It makes your site seem much faster.
See the Implementation Notes for more specific information.

Frequently Asked Questions

What are the most important things to make cacheable?
A good strategy is to identify the most popular, largest objects (especially images) and work with them first.

How can I make my pages as fast as possible with caches?
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.

I understand that caching is good, but I need to keep statistics on how many people visit my page!
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.

I've got a page that is updated often. How do I keep caches from giving my users a stale copy?
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.

How can I see which HTTP headers are set for an object?
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 an 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.

My pages are password-protected; how do proxy caches deal with them?
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.

Should I worry about security if my users access my site through a cache?
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.

I'm looking for an integrated Web publishing solution. Which ones are cache-aware?
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.

My images expire a month from now, but I need to change them in the caches now!
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.

I run a Web Hosting service. How can I let my users publish cache-friendly pages?
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