Caching Tutorial

for Web Authors and Webmasters

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.

1. What’s a Web Cache? Why do people use
   them?
2. Kinds of Web Caches
   1. Browser Caches
   2. Proxy Caches
3. Aren’t Web Caches bad for me? Why should I
   help them?
4. How Web Caches Work
5. How (and how not) to Control Caches
   1. HTML Meta Tags vs. HTTP
      Headers
   2. Pragma HTTP Headers (and why they
      don’t work)
   3. Controlling Freshness with the
      Expires HTTP Header
   4. Cache-Control HTTP
      Headers
   5. Validators and
      Validation
6. Tips for Building a Cache-Aware
   Site
7. Writing Cache-Aware Scripts
8. Frequently Asked Questions
9. Implementation Notes — Web
   Servers
10. Implementation Notes — Server-Side
    Scripting
11. References and Further Information
12. About This Document

What’s a Web Cache? Why do people
use them?

A Web cache sits between one or more Web servers (also known as
origin servers) and a client or many clients, and watches requests
come by, saving copies of the responses — like HTML pages, images and files
(collectively known as representations) — for itself. Then, if there
is another request for the same URL, it can use the response 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 it to get the representation and display it. This
  makes the Web seem more responsive.
• To reduce network traffic — Because representations are
  reused, it reduces the amount of bandwidth used by a client. This saves
  money if the client is paying for 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 Web browser (like
Internet Explorer, Safari or Mozilla), you’ll probably notice a “cache”
setting. This lets you set aside a section of your computer’s hard disk to
store representations that you’ve seen, just for you. The browser cache works
according to fairly simple rules. It will check to make sure that the
representations are fresh, usually once a session (that is, the once in the
current invocation of the browser).

This cache is especially useful when users hit the “back” button or click a
link to see a page they’ve just looked at. Also, if you use the same
navigation images throughout your site, they’ll be served from browsers’
caches 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 ISPs often set them up on their firewalls, or as standalone
devices (also known as intermediaries).

Because proxy caches aren’t part of the client or the origin server, but
instead are out on the network, requests have to be routed to them somehow.
One way to do this is to use your browser’s proxy setting to manually tell it
what proxy to use; another is using interception. Interception
proxies have Web requests redirected to them by the underlying
network itself, so that clients don’t need to be configured for them, or even
know about them.

Proxy caches are a type of shared cache; rather than just having
one person using them, they usually have a large number of users, and because
of this they are very good at reducing latency and network traffic. That’s
because popular representations are reused a number of times.

Gateway Caches

Also known as “reverse proxy caches” or “surrogate caches,” gateway caches
are also intermediaries, but instead of being deployed by network
administrators to save bandwidth, they’re typically deployed by Webmasters
themselves, to make their sites more scalable, reliable and better
performing.

Requests can be routed to gateway caches by a number of methods, but
typically some form of load balancer is used to make one or more of them look
like the origin server to clients.

Content delivery networks (CDNs) distribute gateway caches
throughout the Internet (or a part of it) and sell caching to interested Web
sites. Speedera and Akamai are examples of
CDNs.

This tutorial focuses mostly on browser and proxy caches, although some of
the information is suitable for those interested in gateway caches as
well.

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 proxy
cache can “hide” their users from them, making it difficult to see who’s using
the site.

Unfortunately for them, even if Web caches didn’t exist, 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
tutorial 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 tutorial can show you how to configure your
server to control how your content is cached.

CDNs
are an interesting development, because unlike many
proxy caches, their gateway caches are aligned with the interests of the
Web site being cached, so that these problems aren’t seen. However, even
when you use a CDN, you still have to consider that there will be proxy
and browser caches downstream.

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 proxy and browser 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 a
representation 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
(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 representation will be available
instantly from the cache, while a validated representation will avoid sending
the entire representation over again if it hasn’t changed.

How (and how not) to Control
Caches

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 your server’s 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 and HTTP Headers

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 only honored by a few browser caches, not proxy caches
(which almost never read the HTML in the document). While it may be tempting
to put a Pragma: no-cache meta tag into a Web page, it won’t necessarily
cause it to be kept fresh.

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.

On the other hand, true HTTP headers give you a lot of control
over how both browser caches and proxies handle your representations. 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 would follow these headers, separated by a blank
line. See the Implementation sections for information about how to set HTTP
headers.

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

Many people believe that assigning a Pragma: no-cache HTTP header to a
representation 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 a basic means of controlling caches; it tells
all caches how long the associated representation 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
cache.

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 retrieved the representation (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 representation 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
representation 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

It’s important to make sure that your Web
server’s clock is accurate if you use the Expires header.
One way to do this is using the Network Time
Protocol (NTP); talk to your local system administrator to find out
more.

Although the Expires header is useful, it has some limitations. First,
because there’s a date involved, the clocks on the Web server and the cache
must be synchronised; if they have a different idea of the time, the intended
results won’t be achieved, and caches might wrongly consider stale content as
fresh.

Another problem with Expires is that it’s easy to forget that you’ve set
some content to expire at a particular time. If you don’t update an Expires
time before it passes, each and every request will go back to your Web server,
increasing load and latency.

Cache-Control HTTP
Headers

HTTP 1.1 introduced a new class of headers, Cache-Control response
headers, to give Web publishers more control over their content, and
to address the limitations of Expires.

Useful Cache-Control response headers include:

• max-age=[seconds] — specifies the maximum amount of
  time that an representation will be considered fresh. Similar to Expires,
  this directive is relative to the time of the request, rather than absolute.
  [seconds] is the number of seconds from the time of the request you wish the
  representation to be fresh for.
• s-maxage=[seconds] — similar to max-age, except that it
  only applies to shared (e.g., proxy) caches.
• public — marks authenticated responses as cacheable;
  normally, if HTTP authentication is required, responses are automatically private.
• private — allows caches that are specific to one user (e.g., in a
  browser) to store the response; shared caches (e.g., in a proxy) may not.
• no-cache — forces caches to submit the request to the
  origin server for validation before releasing a cached copy, every time.
  This is useful to assure that authentication is respected (in combination
  with public), or to maintain rigid freshness, without sacrificing all of the
  benefits of caching.
• no-store — instructs caches not to keep a copy of the
  representation under any conditions.
• must-revalidate — tells caches that they must obey any
  freshness information you give them about a representation. HTTP allows
  caches to serve stale representations under special conditions; 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

When both Cache-Control and Expires are present,
Cache-Control takes precedence. If you plan to use the
Cache-Control headers, you should have a look at the excellent
documentation in HTTP 1.1; 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 representation has changed. By
using it, caches avoid having to download the entire representation 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, caches will
not store a representation at all.

The most common validator is the time that the document last changed, as
communicated in Last-Modified header. When a cache has an
representation stored that includes a Last-Modified header, it can use it to
ask the server if the representation 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 representation 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 representation really is the same.

Almost all caches use Last-Modified times as validators; ETag validation is also becoming prevalent.

Most modern Web servers will generate both ETag and Last-Modified
headers to use as validators for static content (i.e., files) 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.

• Use URLs 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 way 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 using a Cache-Control: max-age header with a large
  value.
• Make caches recognise regularly updated pages by
  specifying an appropriate max-age or 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.
• Check your pages with REDbot
  — 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 (a Last-Modified
or ETag response 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 cacheble; 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 trival task.

Some other tips;

• Don’t use POST unless it’s appropriate. Responses to
  the POST method aren’t kept by most caches; if you send information in the
  path or query (via GET), caches can store that information for the
  future.
• 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 clients to request
  multiple representations 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 representations
(especially images) and work with them first.

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

The most cacheable representation is one with a long freshness time set.
Validation does help reduce the time that it takes to see a representation,
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 item 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.

How can I see a representation’s HTTP headers?

Many Web browsers let you see the Expires and Last-Modified headers are in
a “page info” or similar interface. If available, this will give you a menu of
the page and any representations (like images) associated with it, along with
their details.

To see the full headers of a representation, you can manually connect to
the Web server using a Telnet client.

To do so, you may need to type the port (be default, 80) into a separate
field, or you may need to connect to www.example.com:80 or www.example.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
representation. For instance, if you want to see the headers for
http://www.example.com/foo.html, connect to www.example.com, port 80, and
type:

GET /foo.html HTTP/1.1 [return]
Host: www.example.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
representation. 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 considered private;
they will not be kept by shared caches. However, you can make authenticated
pages public with a Cache-Control: public header; HTTP 1.1-compliant caches will then
allow them to be cached.

If you’d like such 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 representation from the cache. This would look like:

Cache-Control: public, no-cache

Whether or not this is done, it’s best to minimize use of authentication;
for example, 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 people access my site through a
cache?

SSL pages are not cached (or decrypted) 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 about unsecured sites; an
unscrupulous administrator could conceivably gather information about their
users, especially in the URL.

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 use 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 representations, the cached
copy will be used until then.

The most effective solution is to change any links to them; that way,
completely new representations will be loaded fresh from the origin server.
Remember that any page that refers to these representations will be cached as
well. Because of this, it’s best to make static images and similar
representations very cacheable, while keeping the HTML pages that refer to
them on a tight leash.

If you want to reload an representation from a specific cache, you can
either force a reload (in Firefox, 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 representation
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
providing 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 representations 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.

I’ve marked my pages as cacheable, but my browser keeps requesting them
on every request. How do I force the cache to keep representations of them?

Caches aren’t required to keep a representation and reuse it; they’re only
required to not keep or use them under some conditions. All
caches make decisions about which representations to keep based upon their
size, type (e.g., image vs. html), or by how much space they have left to keep
local copies. Yours may not be considered worth keeping around, compared to
more popular or larger representations.

Some caches do allow their administrators to prioritize what kinds of
representations are kept, and some allow representations to be “pinned” in
cache, so that they’re always available.

Implementation Notes — Web
Servers

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 Server

Apache 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 (note that this only
lists compiled-in modules; on later versions of Apache, use httpd -M
to include dynamically loaded modules as well). The modules we’re
looking for are mod_expires and mod_headers.

• If they aren’t available, and you have administrative access, you can
  recompile Apache to include them. This can be done either by uncommenting
  the appropriate lines in the Configuration file, or using the
  -enable-module=expires and -enable-module=headers
  arguments to configure (1.3 or greater). Consult the INSTALL file found
  with the Apache distribution.

Once you have an Apache with the appropriate modules, you can use
mod_expires to specify when representations 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
the module
documentation 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 the
mod_headers documentation.

Here’s an example .htaccess file that demonstrates the use of some
headers.

• .htaccess files allow web publishers to use commands normally only
  found in configuration files. They affect the content of the directory
  they’re in and their subdirectories. Talk to your server administrator to
  find out if they’re enabled.

### 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>

• Note that mod_expires automatically calculates and inserts a
  Cache-Control:max-age header as appropriate.

Apache 2’s configuration is very similar to that of 1.3; see the 2.2 mod_expires and
mod_headers
documentation for more information.

Microsoft IIS

Microsoft’s
Internet Information Server 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.

Netscape/iPlanet Enterprise Server

As of version 3.6, Enterprise Server 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 the NES manual.

Implementation Notes — Server-Side
Scripting

One thing to keep in mind is that it may be easier to set
HTTP headers with your Web server rather than in the scripting language. Try
both.

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 a
Cache-Control: max-age header; 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

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_’ prepended
to its name. So, if a client makes an If-Modified-Since request, it will show
up as HTTP_IF_MODIFIED_SINCE.

See also the cgi_buffer
library, which automatically handles ETag generation and validation,
Content-Length generation and gzip content-coding for Perl and Python CGI
scripts with a one-line include. The Python version can also be used to wrap
arbitrary CGI scripts with.

Server Side Includes

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 SSI 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 the mod_include
documentation.

PHP

PHP 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, representations 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;
$ExpStr = "Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . " GMT";
Header($ExpStr);
?>

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 (although recent
versions have made it easier; see the PHP's date documentation). 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 the manual entry for
header.

See also the cgi_buffer library, which
automatically handles ETag generation and validation, Content-Length
generation and gzip content-coding for PHP scripts with a one-line
include.

Cold Fusion

Cold Fusion, by Macromedia is a commercial server-side
scripting engine, with support for several Web servers on Windows, Linux and
several flavors of Unix.

Cold Fusion makes setting arbitrary HTTP headers relatively easy, with the
CFHEADER
tag. Unfortunately, their example for setting an Expires header, as below, is a bit misleading.

<CFHEADER NAME="Expires" VALUE="#Now()#">

It doesn’t work like you might think, 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.

However, Cold Fusion does provide a date formatting function that will do the job;
GetHttpTimeString. In combination with
DateAdd, it’s easy to set Expires dates;
here, we set a header to declare that representations of the page expire in one month;

<cfheader name="Expires"
value="#GetHttpTimeString(DateAdd('m', 1, Now()))#">

You can also use the CFHEADER tag to set Cache-Control: max-age and other headers.

Remember that Web server headers are passed through in some deployments of Cold Fusion
(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.

ASP and ASP.NET

When setting HTTP headers from ASPs, make sure you either
place the Response method calls before any HTML generation, or use
Response.Buffer to buffer the output. Also, note that some versions of IIS set
a Cache-Control: private header on ASPs by default, and must be declared public
to be cacheable by shared caches.

Active Server Pages, built into IIS and also available for other Web
servers, also allows you to set HTTP headers. For instance, to set an expiry
time, you can use the properties of the Response object;

<% Response.Expires=1440 %>

specifying the number of minutes from the request to expire the
representation. Cache-Control headers can be added like this:

<% Response.CacheControl="public" %>

In ASP.NET, Response.Expires is deprecated; the proper way to set cache-related
headers is with Response.Cache;

Response.Cache.SetExpires ( DateTime.Now.AddMinutes ( 60 ) ) ;
Response.Cache.SetCacheability ( HttpCacheability.Public ) ;

See the MSDN documentation for more
information.

References and Further Information

HTTP 1.1 Specification

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.

Web-Caching.com

An excellent introduction to caching concepts, with links to other online
resources.

On Interpreting
Access Statistics

Jeff Goldberg’s informative rant on why you shouldn’t rely on access
statistics and hit counters.

REDbot

Examines HTTP resources to determine how they will interact with Web caches, and generally how well they use the protocol.

cgi_buffer Library

One-line include in Perl CGI, Python CGI and PHP scripts automatically
handles ETag generation and validation, Content-Length generation and gzip
Content-Encoding — correctly. The Python version can also be used as a
wrapper around arbitrary CGI scripts.

About This Document

This document is Copyright © 1998-2012 Mark Nottingham <mnot@mnot.net>.

This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.

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