Caching Guide

Introduction

As of Apache HTTP server version 2.2 mod_cache and mod_file_cache are no longer marked experimental and are considered suitable for production use. These caching architectures provide a powerful means to accelerate HTTP handling, both as an origin webserver and as a proxy.

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 cachability of content. It can handle both local and proxied content. 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 which change with time.

mod_file_cache on the other hand presents a more basic, but sometimes useful, form of caching. Rather than maintain the complexity of actively ensuring the cachability of URLs, mod_file_cache offers file-handle and memory-mapping tricks to keep a cache of files as they were when httpd was last started. As such, mod_file_cache is aimed at improving the access time to local static files which do not change very often.

As mod_file_cache presents a relatively simple caching implementation, apart from the specific sections on CacheFile and MMapFile, the explanations in this guide cover the mod_cache caching architecture.

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.

Caching Overview

There are two main stages in mod_cache that can occur in the lifetime of a request. First, mod_cache is a URL mapping module, which means that if a URL has been cached, and the cached version of that URL has not expired, the request will be served directly by mod_cache.

This means that any other stages that might ordinarily happen in the process of serving a request -- for example being handled by mod_proxy, or mod_rewrite -- won't happen. But then this is the point of caching content in the first place.

If the URL is not found within the cache, mod_cache will add a filter to the request handling. After httpd has located the content by the usual means, the filter will be run as the content is served. If the content is determined to be cacheable, the content will be saved to the cache for future serving.

If the URL is found within the cache, but also found to have expired, the filter is added anyway, but mod_cache will create a conditional request to the backend, to determine if the cached version is still current. If the cached version is still current, its meta-information will be updated and the request will be served from the cache. If the cached version is no longer current, the cached version will be deleted and the filter will save the updated content to the cache as it is served.

Improving Cache Hits

When caching locally generated content, 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 forms a part of 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.

Because caching is performed within the URL to filename translation phase, cached documents will only be served in response to URL requests. Ordinarily this is of little consequence, but there is one circumstance in which it matters: If you are using Server Side Includes;

<!-- The following include can be cached -->
<!--#include virtual="/footer.html" --> 

<!-- The following include can not be cached -->
<!--#include file="/path/to/footer.html" -->

If you are using Server Side Includes, and want the benefit of speedy serves from the cache, you should use virtual include types.

Expiry Periods

The default expiry period for cached entities is one hour, however this can be easily over-ridden by using the CacheDefaultExpire directive. This default is only used when the original source of the content does not specify an expire time or time of last modification.

If a response does not include an Expires header but does include a Last-Modified header, mod_cache can infer an expiry period based on the use of the CacheLastModifiedFactor directive.

For local content, mod_expires may be used to fine-tune the expiry period.

The maximum expiry period may also be controlled by using the CacheMaxExpire.

A Brief Guide to Conditional Requests

When content expires from the cache and is re-requested from the backend or content provider, rather than pass on the original request, httpd will use a conditional request instead.

HTTP offers a number of headers which allow a client, or cache to discern between different versions of the same content. For example if a resource was served with an "Etag:" header, it is possible to make a conditional request with an "If-None-Match:" header. If a resource was served with a "Last-Modified:" header it is possible to make a conditional request with an "If-Modified-Since:" header, and so on.

When such a conditional request is made, the response differs depending on whether the content matches the conditions. If a request is made with an "If-Modified-Since:" header, and the content has not been modified since the time indicated in the request then a terse "304 Not Modified" response is issued.

If the content has changed, then it is served as if the request were not conditional to begin with.

The benefits of conditional requests in relation to caching are twofold. Firstly, when making such a request to the backend, if the content from the backend matches the content in the store, this can be determined easily and without the overhead of transferring the entire resource.

Secondly, conditional requests are usually less strenuous on the backend. 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 if httpd is caching local content, even expired content may still be served faster from the cache if it has not changed. As long as reading from the cache store is faster than reading from the backend (e.g. mod_cache_disk with memory disk compared to reading from disk).

What Can be Cached?

As mentioned already, the two styles of caching in httpd work differently, mod_file_cache caching maintains file contents as they were when httpd was started. When a request is made for a file that is cached by this module, it is intercepted and the cached file is served.

mod_cache caching on the other hand is more complex. When serving a request, if it has not been cached previously, the caching module will determine if the content is cacheable. The conditions for determining cachability of a response are;

1. Caching must be enabled for this URL. See the CacheEnable and CacheDisable directives.
2. The response must have a HTTP status code of 200, 203, 300, 301 or 410.
3. The request must be a HTTP GET request.
4. If the request contains an "Authorization:" header, the response will not be cached.
5. If the response contains an "Authorization:" header, it must also contain an "s-maxage", "must-revalidate" or "public" option in the "Cache-Control:" header.
6. If the URL included a query string (e.g. from a HTML form GET method) it will not be cached unless the response specifies an explicit expiration by including an "Expires:" header or the max-age or s-maxage directive of the "Cache-Control:" header, as per RFC2616 sections 13.9 and 13.2.1.
7. If the response has a status of 200 (OK), the response must also include at least one of the "Etag", "Last-Modified" or the "Expires" headers, or the max-age or s-maxage directive of the "Cache-Control:" header, unless the CacheIgnoreNoLastMod directive has been used to require otherwise.
8. If the response includes the "private" option in a "Cache-Control:" header, it will not be stored unless the CacheStorePrivate has been used to require otherwise.
9. Likewise, if the response includes the "no-store" option in a "Cache-Control:" header, it will not be stored unless the CacheStoreNoStore has been used.
10. A response will not be stored if it includes a "Vary:" header containing the match-all "*".

What Should Not be Cached?

In short, any content which is highly time-sensitive, or which varies depending on the particulars of the request that are not covered by HTTP negotiation, should not be cached.

If you have dynamic content which changes depending on the IP address of the requester, or changes every 5 minutes, it should almost certainly not be cached.

If on the other hand, the content served differs depending on the values of various HTTP headers, it might be possible to cache it intelligently through the use of a "Vary" header.

Variable/Negotiated Content

If a response with a "Vary" header is received by mod_cache when requesting content by the backend it will attempt to handle it intelligently. If possible, mod_cache will detect the headers attributed in the "Vary" response in future requests and serve the correct cached response.

If for example, a response is received with a vary header such as;

mod_cache will only serve the cached content to requesters with accept-language and accept-charset headers matching those of the original request.

Security Considerations

Authorization and Access Control

Using mod_cache is very much like having a built in reverse-proxy. Requests will be served by the caching module unless it determines that the backend should be queried. When caching local resources, 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.

Local exploits

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.

Cache Poisoning

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 backend.

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.

File-Handle Caching

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.

CacheFile

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.

In-Memory Caching

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.

Operating System Caching

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.

MMapFile Caching

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.

Disk-based Caching

mod_cache_disk provides a disk-based caching mechanism for mod_cache. This cache is intelligent and content will be served from the cache only as long as it is considered valid.

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.

Understanding the Cache-Store

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, to ensure that multiple URLs do not collide.

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.

Maintaining the Disk Cache

Although mod_cache_disk will remove cached content as it is expired, it does not maintain any information on the total size of the cache or how little free space may be left.

Instead, provided with httpd is the htcacheclean tool which, as the name suggests, 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.

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.