Apache HTTP Server Version 2.4
Apache HTTP Server Version 2.4
This document supplements the mod_cache,
mod_cache_disk, mod_file_cache and htcacheclean reference documentation.
It describes how to use the Apache HTTP Server's caching features to accelerate web and
proxy serving, while avoiding common problems and misconfigurations.
The Apache HTTP server offers a range of caching features that are designed to improve the performance of the server in various ways.
mod_cache
and its provider modules
mod_cache_disk
provide intelligent, HTTP-aware caching. The content itself is stored
in the cache, and mod_cache aims to honor all of the various HTTP
headers and options that control the cacheability of content
as described in
Section
13 of RFC2616.
mod_cache
is aimed at both simple and complex caching configurations, where
you are dealing with proxied content, dynamic local content or
have a need to speed up access to local files on a potentially
slow disk.
mod_file_cache
offers the ability to pre-load
files into memory on server startup, and can improve access
times and save file handles on files that are accessed often,
as there is no need to go to disk on each request.
To get the most from this document, you should be familiar with the basics of HTTP, and have read the Users' Guides to Mapping URLs to the Filesystem and Content negotiation.
| Related Modules | Related Directives |
|---|---|
The HTTP protocol contains built in support for an in-line caching
mechanism
described by section 13 of RFC2616, and the
mod_cache module can be used to take advantage of
this.
Unlike a simple two state key/value cache where the content disappears completely when no longer fresh, an HTTP cache includes a mechanism to retain stale content, and to ask the origin server whether this stale content has changed and if not, make it fresh again.
An entry in an HTTP cache exists in one of three states:
If the content is too old (older than its freshness lifetime), it is considered stale. An HTTP cache should contact the origin server and check whether the content is still fresh before serving stale content to a client. The origin server will either respond with replacement content if not still valid, or ideally, the origin server will respond with a code to tell the cache the content is still fresh, without the need to generate or send the content again. The content becomes fresh again and the cycle continues.
The HTTP protocol does allow the cache to serve stale data
under certain circumstances, such as when an attempt to freshen
the data with an origin server has failed with a 5xx error, or
when another request is already in the process of freshening
the given entry. In these cases a Warning header
is added to the response.
Full details of how HTTP caching works can be found in Section 13 of RFC2616.
The mod_cache module hooks into the server in two
possible places depending on the value of the
CacheQuickHandler directive:
This phase happens very early on during the request processing, just after the request has been parsed. If the content is found within the cache, it is served immediately and almost all request processing is bypassed.
In this scenario, the cache behaves as if it has been "bolted on" to the front of the server.
This mode offers the best performance, as the majority of server processing is bypassed. This mode however also bypasses the authentication and authorization phases of server processing, so this mode should be chosen with care when this is important.
Requests with an "Authorization" header (for example, HTTP Basic
Authentication) are neither cacheable nor served from the cache
when mod_cache is running in this phase.
This phase happens late in the request processing, after all the request phases have completed.
In this scenario, the cache behaves as if it has been "bolted on" to the back of the server.
This mode offers the most flexibility, as the potential exists for caching to occur at a precisely controlled point in the filter chain, and cached content can be filtered or personalized before being sent to the client.
If the URL is not found within the cache, mod_cache
will add a filter to the filter stack in order
to record the response to the cache, and then stand down, allowing normal
request processing to continue. If the content is determined to be
cacheable, the content will be saved to the cache for future serving,
otherwise the content will be ignored.
If the content found within the cache is stale, the
mod_cache module converts the request into a
conditional request. If the origin server responds with
a normal response, the normal response is cached, replacing the content
already cached. If the origin server responds with a 304 Not Modified
response, the content is marked as fresh again, and the cached content
is served by the filter instead of saving it.
When a virtual host is known by one of many different server aliases,
ensuring that UseCanonicalName is
set to On can dramatically improve the ratio of cache hits.
This is because the hostname of the virtual-host serving the content is
used within the cache key. With the setting set to On
virtual-hosts with multiple server names or aliases will not produce
differently cached entities, and instead content will be cached as
per the canonical hostname.
Well formed content that is intended to be cached should declare an
explicit freshness lifetime with the Cache-Control
header's max-age or s-maxage fields, or
by including an Expires header.
At the same time, the origin server defined freshness lifetime can
be overridden by a client when the client presents their own
Cache-Control header within the request. In this case,
the lowest freshness lifetime between request and response wins.
When this freshness lifetime is missing from the request or the
response, a default freshness lifetime is applied. The default
freshness lifetime for cached entities is one hour, however
this can be easily over-ridden by using the CacheDefaultExpire directive.
If a response does not include an Expires header but does
include a Last-Modified header, mod_cache
can infer a freshness lifetime based on a heuristic, which can be
controlled through the use of the CacheLastModifiedFactor directive.
For local content, or for remote content that does not define its own
Expires header, mod_expires may be used to
fine-tune the freshness lifetime by adding max-age and
Expires.
The maximum freshness lifetime may also be controlled by using the
CacheMaxExpire.
When content expires from the cache and becomes stale, rather than pass on the original request, httpd will modify the request to make it conditional instead.
When an ETag header exists in the original cached
response, mod_cache will add an
If-None-Match header to the request to the origin server.
When a Last-Modified header exists in the original
cached response, mod_cache will add an
If-Modified-Since header to the request to the origin
server. Performing either of these actions makes the request
conditional.
When a conditional request is received by an origin server, the origin server should check whether the ETag or the Last-Modified parameter has changed, as appropriate for the request. If not, the origin should respond with a terse "304 Not Modified" response. This signals to the cache that the stale content is still fresh should be used for subsequent requests until the content's new freshness lifetime is reached again.
If the content has changed, then the content is served as if the request were not conditional to begin with.
Conditional requests offer two benefits. Firstly, when making such a request to the origin server, if the content from the origin matches the content in the cache, this can be determined easily and without the overhead of transferring the entire resource.
Secondly, a well designed origin server will be designed in such
a way that conditional requests will be significantly cheaper to
produce than a full response. For static files, typically all that is
involved is a call to stat() or similar system call, to
see if the file has changed in size or modification time. As such, even
local content may still be served faster from the cache if it has not
changed.
Origin servers should make every effort to support conditional requests as is practical, however if conditional requests are not supported, the origin will respond as if the request was not conditional, and the cache will respond as if the content had changed and save the new content to the cache. In this case, the cache will behave like a simple two state cache, where content is effectively either fresh or deleted.
The full definition of which responses can be cached by an HTTP cache is defined in RFC2616 Section 13.4 Response Cacheability, and can be summed up as follows:
CacheEnable and CacheDisable directives.CacheIgnoreNoLastMod
directive has been used to require otherwise.CacheStorePrivate has been
used to require otherwise.CacheStoreNoStore has been
used.It should be up to the client creating the request, or the origin
server constructing the response to decide whether or not the content
should be cacheable or not by correctly setting the
Cache-Control header, and mod_cache should
be left alone to honor the wishes of the client or server as appropriate.
Content that is time sensitive, or which varies depending on the
particulars of the request that are not covered by HTTP negotiation,
should not be cached. This content should declare itself uncacheable
using the Cache-Control header.
If content changes often, expressed by a freshness lifetime of minutes or seconds, the content can still be cached, however it is highly desirable that the origin server supports conditional requests correctly to ensure that full responses do not have to be generated on a regular basis.
Content that varies based on client provided request headers can be
cached through intelligent use of the Vary response
header.
When the origin server is designed to respond with different content based on the value of headers in the request, for example to serve multiple languages at the same URL, HTTP's caching mechanism makes it possible to cache multiple variants of the same page at the same URL.
This is done by the origin server adding a Vary header
to indicate which headers must be taken into account by a cache when
determining whether two variants are different from one another.
If for example, a response is received with a vary header such as;
Vary: negotiate,accept-language,accept-charset
mod_cache will only serve the cached content to
requesters with accept-language and accept-charset headers
matching those of the original request.
Multiple variants of the content can be cached side by side,
mod_cache uses the Vary header and the
corresponding values of the request headers listed by Vary
to decide on which of many variants to return to the client.
The mod_cache module relies on specific backend store
implementations in order to manage the cache, and for caching to disk
mod_cache_disk is provided to support this.
Typically the module will be configured as so;
CacheRoot "/var/cache/apache/" CacheEnable disk / CacheDirLevels 2 CacheDirLength 1
Importantly, as the cached files are locally stored, operating system in-memory caching will typically be applied to their access also. So although the files are stored on disk, if they are frequently accessed it is likely the operating system will ensure that they are actually served from memory.
To store items in the cache, mod_cache_disk creates
a 22 character hash of the URL being requested. This hash incorporates
the hostname, protocol, port, path and any CGI arguments to the URL,
as well as elements defined by the Vary header to ensure that multiple
URLs do not collide with one another.
Each character may be any one of 64-different characters, which mean
that overall there are 64^22 possible hashes. For example, a URL might
be hashed to xyTGxSMO2b68mBCykqkp1w. This hash is used
as a prefix for the naming of the files specific to that URL within
the cache, however first it is split up into directories as per
the CacheDirLevels and
CacheDirLength
directives.
CacheDirLevels
specifies how many levels of subdirectory there should be, and
CacheDirLength
specifies how many characters should be in each directory. With
the example settings given above, the hash would be turned into
a filename prefix as
/var/cache/apache/x/y/TGxSMO2b68mBCykqkp1w.
The overall aim of this technique is to reduce the number of
subdirectories or files that may be in a particular directory,
as most file-systems slow down as this number increases. With
setting of "1" for
CacheDirLength
there can at most be 64 subdirectories at any particular level.
With a setting of 2 there can be 64 * 64 subdirectories, and so on.
Unless you have a good reason not to, using a setting of "1"
for CacheDirLength
is recommended.
Setting
CacheDirLevels
depends on how many files you anticipate to store in the cache.
With the setting of "2" used in the above example, a grand
total of 4096 subdirectories can ultimately be created. With
1 million files cached, this works out at roughly 245 cached
URLs per directory.
Each URL uses at least two files in the cache-store. Typically there is a ".header" file, which includes meta-information about the URL, such as when it is due to expire and a ".data" file which is a verbatim copy of the content to be served.
In the case of a content negotiated via the "Vary" header, a ".vary" directory will be created for the URL in question. This directory will have multiple ".data" files corresponding to the differently negotiated content.
The mod_cache_disk module makes no attempt to
regulate the amount of disk space used by the cache, although it
will gracefully stand down on any disk error and behave as if the
cache was never present.
Instead, provided with httpd is the htcacheclean tool which allows you to clean the cache periodically. Determining how frequently to run htcacheclean and what target size to use for the cache is somewhat complex and trial and error may be needed to select optimal values.
htcacheclean has two modes of operation. It can be run as persistent daemon, or periodically from cron. htcacheclean can take up to an hour or more to process very large (tens of gigabytes) caches and if you are running it from cron it is recommended that you determine how long a typical run takes, to avoid running more than one instance at a time.
It is also recommended that an appropriate "nice" level is chosen for htcacheclean so that the tool does not cause excessive disk io while the server is running.
Figure 1: Typical
cache growth / clean sequence.
Because mod_cache_disk does not itself pay attention
to how much space is used you should ensure that
htcacheclean is configured to
leave enough "grow room" following a clean.
| Related Modules | Related Directives |
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The Apache HTTP server offers a low level shared object cache for caching information such as SSL sessions, or authentication credentials, within the socache interface.
Additional modules are provided for each implementation, offering the following backends:
mod_socache_dbmmod_socache_dcmod_socache_memcachemod_socache_shmcb| Related Modules | Related Directives |
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The mod_authn_socache module allows the result of
authentication to be cached, relieving load on authentication backends.
| Related Modules | Related Directives |
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The mod_ssl module uses the socache interface
to provide a session cache and a stapling cache.
| Related Modules | Related Directives |
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On platforms where a filesystem might be slow, or where file handles are expensive, the option exists to pre-load files into memory on startup.
On systems where opening files is slow, the option exists to open the file on startup and cache the file handle. These options can help on systems where access to static files is slow.
The act of opening a file can itself be a source of delay, particularly on network filesystems. By maintaining a cache of open file descriptors for commonly served files, httpd can avoid this delay. Currently httpd provides one implementation of File-Handle Caching.
The most basic form of caching present in httpd is the file-handle
caching provided by mod_file_cache. Rather than caching
file-contents, this cache maintains a table of open file descriptors. Files
to be cached in this manner are specified in the configuration file using
the CacheFile
directive.
The
CacheFile directive
instructs httpd to open the file when it is started and to re-use
this file-handle for all subsequent access to this file.
CacheFile /usr/local/apache2/htdocs/index.html
If you intend to cache a large number of files in this manner, you must ensure that your operating system's limit for the number of open files is set appropriately.
Although using CacheFile
does not cause the file-contents to be cached per-se, it does mean
that if the file changes while httpd is running these changes will
not be picked up. The file will be consistently served as it was
when httpd was started.
If the file is removed while httpd is running, it will continue to maintain an open file descriptor and serve the file as it was when httpd was started. This usually also means that although the file will have been deleted, and not show up on the filesystem, extra free space will not be recovered until httpd is stopped and the file descriptor closed.
Serving directly from system memory is universally the fastest method of serving content. Reading files from a disk controller or, even worse, from a remote network is orders of magnitude slower. Disk controllers usually involve physical processes, and network access is limited by your available bandwidth. Memory access on the other hand can take mere nano-seconds.
System memory isn't cheap though, byte for byte it's by far the most expensive type of storage and it's important to ensure that it is used efficiently. By caching files in memory you decrease the amount of memory available on the system. As we'll see, in the case of operating system caching, this is not so much of an issue, but when using httpd's own in-memory caching it is important to make sure that you do not allocate too much memory to a cache. Otherwise the system will be forced to swap out memory, which will likely degrade performance.
Almost all modern operating systems cache file-data in memory managed directly by the kernel. This is a powerful feature, and for the most part operating systems get it right. For example, on Linux, let's look at the difference in the time it takes to read a file for the first time and the second time;
colm@coroebus:~$ time cat testfile > /dev/null real 0m0.065s user 0m0.000s sys 0m0.001s colm@coroebus:~$ time cat testfile > /dev/null real 0m0.003s user 0m0.003s sys 0m0.000s
Even for this small file, there is a huge difference in the amount of time it takes to read the file. This is because the kernel has cached the file contents in memory.
By ensuring there is "spare" memory on your system, you can ensure that more and more file-contents will be stored in this cache. This can be a very efficient means of in-memory caching, and involves no extra configuration of httpd at all.
Additionally, because the operating system knows when files are deleted or modified, it can automatically remove file contents from the cache when necessary. This is a big advantage over httpd's in-memory caching which has no way of knowing when a file has changed.
Despite the performance and advantages of automatic operating system caching there are some circumstances in which in-memory caching may be better performed by httpd.
mod_file_cache provides the
MMapFile directive, which
allows you to have httpd map a static file's contents into memory at
start time (using the mmap system call). httpd will use the in-memory
contents for all subsequent accesses to this file.
MMapFile /usr/local/apache2/htdocs/index.html
As with the
CacheFile directive, any
changes in these files will not be picked up by httpd after it has
started.
The MMapFile
directive does not keep track of how much memory it allocates, so
you must ensure not to over-use the directive. Each httpd child
process will replicate this memory, so it is critically important
to ensure that the files mapped are not so large as to cause the
system to swap memory.
Using mod_cache in its default state where
CacheQuickHandler is set to
On is very much like having a caching reverse-proxy bolted
to the front of the server. Requests will be served by the caching module
unless it determines that the origin server should be queried just as an
external cache would, and this drastically changes the security model of
httpd.
As traversing a filesystem hierarchy to examine potential
.htaccess files would be a very expensive operation,
partially defeating the point of caching (to speed up requests),
mod_cache makes no decision about whether a cached
entity is authorised for serving. In other words; if
mod_cache has cached some content, it will be served
from the cache as long as that content has not expired.
If, for example, your configuration permits access to a resource by IP
address you should ensure that this content is not cached. You can do this
by using the CacheDisable
directive, or mod_expires. Left unchecked,
mod_cache - very much like a reverse proxy - would cache
the content when served and then serve it to any client, on any IP
address.
When the CacheQuickHandler
directive is set to Off, the full set of request processing
phases are executed and the security model remains unchanged.
As requests to end-users can be served from the cache, the cache itself can become a target for those wishing to deface or interfere with content. It is important to bear in mind that the cache must at all times be writable by the user which httpd is running as. This is in stark contrast to the usually recommended situation of maintaining all content unwritable by the Apache user.
If the Apache user is compromised, for example through a flaw in
a CGI process, it is possible that the cache may be targeted. When
using mod_cache_disk, it is relatively easy to
insert or modify a cached entity.
This presents a somewhat elevated risk in comparison to the other
types of attack it is possible to make as the Apache user. If you are
using mod_cache_disk you should bear this in mind -
ensure you upgrade httpd when security upgrades are announced and
run CGI processes as a non-Apache user using suEXEC if possible.
When running httpd as a caching proxy server, there is also the potential for so-called cache poisoning. Cache Poisoning is a broad term for attacks in which an attacker causes the proxy server to retrieve incorrect (and usually undesirable) content from the origin server.
For example if the DNS servers used by your system running httpd are vulnerable to DNS cache poisoning, an attacker may be able to control where httpd connects to when requesting content from the origin server. Another example is so-called HTTP request-smuggling attacks.
This document is not the correct place for an in-depth discussion of HTTP request smuggling (instead, try your favourite search engine) however it is important to be aware that it is possible to make a series of requests, and to exploit a vulnerability on an origin webserver such that the attacker can entirely control the content retrieved by the proxy.
The Vary mechanism allows multiple variants of the same URL to be
cached side by side. Depending on header values provided by the client,
the cache will select the correct variant to return to the client. This
mechanism can become a problem when an attempt is made to vary on a
header that is known to contain a wide range of possible values under
normal use, for example the User-Agent header. Depending
on the popularity of the particular web site thousands or millions of
duplicate cache entries could be created for the same URL, crowding
out other entries in the cache.
In other cases, there may be a need to change the URL of a particular
resource on every request, usually by adding a "cachebuster" string to
the URL. If this content is declared cacheable by a server for a
significant freshness lifetime, these entries can crowd out
legitimate entries in a cache. While mod_cache
provides a
CacheIgnoreURLSessionIdentifiers
directive, this directive should be used with care to ensure that
downstream proxy or browser caches aren't subjected to the same denial
of service issue.