Manual:Varnish caching

Why Varnish?
Why not? Varnish is a lightweight, efficient reverse proxy server which reduces the time taken to serve often-requested pages.

Like Squid, Varnish is an HTTP accelerator which stores copies of the pages served by the web server. The next time the same page is requested, Varnish will serve the copy instead of requesting the page from the Apache server. This caching process removes the need for MediaWiki to regenerate that same page again, resulting in a tremendous performance boost.

Varnish has the advantage of being designed specifically for use as an HTTP accelerator (reverse proxy). It stores much of its cached data in memory, creating fewer disk files and fewer accesses to the filesystem than the larger, more multi-purpose Squid package. Like Squid, it serves often-requested pages to anonymous-IP users from cache instead of requesting them from the origin web server. This reduces both CPU usage and database access by the base MediaWiki server.

Because of this performance gain, MediaWiki has been designed to integrate closely with a web cache and will notify Squid or Varnish when a page should be purged from the cache in order to be regenerated.

From MediaWiki's point of view, a correctly-configured Varnish installation is interchangeable with its Squid counterpart.

The architecture
An example setup of Varnish, Apache and MediaWiki on a single server is outlined below. A more complex caching strategy may use multiple web servers behind the same Varnish caches (all of which can be made to appear to be a single host) or use independent servers to deliver wiki or image content.

To the outside world, Varnish appears to act as the web server. In reality it passes on requests to the Apache web server, but only when necessary. An Apache running on the same server only listens to requests from localhost (127.0.0.1) while Varnish only listens to requests on the server's external IP address. Both services run on port 80 without conflict as each is bound to different IP addresses.

/etc/sysconfig/varnish
This is the first configuration file loaded by Varnish on startup. It specifies the amount of memory to be allocated to the Varnish cache, the location of the main (*.vcl) configurations and the specific IP addresses to which Varnish must respond.

The remainder of the configuration data, including the address of the backend server(s), is listed in the main *.vcl file - not here.

A sample mediawiki.vcl
The address(es) of the backend server(s) must be specified here. In a simple installation, one server (localhost) is sufficient. Larger sites may operate multiple wiki or image servers behind a single Varnish cache :

If more than one backend webserver is available, a list of servers to be used may be selected here on a per-domain basis. This could allow multiple, relatively powerful servers to be used to respond to wiki page text requests while requests for static images are handled on a local web server. A simple one-server installation would simply pass all unhandled requests to the default web server.

Any requests other than a simple 'get' will be passed directly through to the web server, along with all requests from logged-in users.

Most common browsers do support compression (gzip or zip) of returned pages. While Varnish itself performs no compression, it is configured here to store separate copies of a page depending on whether the user's browser supports compression. If a browser accepts both gzip and zip (deflate), the gzip version of the page is served as it is smaller and therefore slightly quicker to display. The browser's reported capabilities are checked here and the gzip'ped version of pages is served wherever possible.

Varnish must pass the user's IP address as part of the 'x-forwarded-for' header, so that MediaWiki may be configured to display the user's address in special:recentchanges instead of Varnish's local IP address.

Varnish must be configured to allow a PURGE request from MediaWiki, instructing the cache to discard stored copies of pages which have been modified by user edits. These requests normally originate only from wiki servers within the local site.

If the page is not in the cache, a 200 (success) code is still returned as the objective is to remove the outdated page from the cache.

The web server may set default expiry times for various objects. As MediaWiki will indicate (via a PURGE request) when a page has been edited and therefore needs to be discarded from cache, the Apache-reported defaults for expiry time are best ignored or replaced with a significantly-longer expiry time.

Pages served to logged-in users (identified by MediaWiki setting browser cookies) or which require passwords to access are never cached.

In this example, the 'no-cache' flag is being ignored on pages served to anonymous-IP users. Such measures normally are only needed if a wiki is making extensive use of extensions which add this flag indiscriminately (such as a wiki packed with random &lt;choose&gt;/&lt;option&gt; Algorithm tags on the main page and various often-used templates).

Configuring MediaWiki
Since Varnish is doing the requests from localhost, Apache will receive "127.0.0.1" as the direct remote address. However, as Varnish forwards requests to Apache, it is configured to add the "X-Forwarded-For" header so that the remote address from the outside world is preserved. MediaWiki must be configured to use the "X-Forwarded-For" header in order to correctly display user addresses in special:recentchanges.

The required configuration is the same for Squid as for Varnish. Make sure the LocalSettings.php file contains the following lines:

Be sure to replace 'example.org' with the IP address on which your Varnish cache is listening. These settings serve two purposes:
 * If a request is received from the Varnish cache server, the MediaWiki logs need to display the IP address of the user, not that of Varnish. A special:recentchanges in which every edit is reported as '127.0.0.1' is all but useless; listing that address as a Squid/Varnish server tells MediaWiki to ignore the IP address and instead look at the 'x-forwarded-for' header for the user's IP.
 * If a page or image is changed on the wiki, MediaWiki will send notification to every server listed in $wgSquidServers telling it to discard (purge) the outdated stored page.

See also Squid configuration settings for all settings related to Squid/Varnish caching.

Some notes
As most of the traffic is handled by the Varnish cache, a statistics package will not give meaningful data if configured to analyse Apache's access_log. There are packages available to log Varnish access data to a file for analysis if needed. Counters on individual wiki pages will also severely underestimate the number of views to each page (and to the site overall) if a web cache is deployed. Many large sites will turn off the counters with $wgDisableCounters.

The display of the user's IP address in the user interface must also be disabled by setting $wgShowIPinHeader = false;

Note that Varnish is an alternative to Squid, but does not replace other portions of a complete MediaWiki caching strategy such as:


 * Pre-compiled PHP code: The default behaviour of PHP under Apache is to load and interpret PHP web scripts each time they are accessed. Installation of a cache such as APC (yum install php-pecl-apc, then allocate memory by setting apc.shm_size=128M or better in /etc/php.d/apc.ini) can greatly reduce the amount of CPU time required by Apache to serve PHP content.
 * Localisation/Internationalisation: By default, MediaWiki (as of version 1.16+) will create a huge l10n_cache database table and access it constantly - possibly more than doubling the load on the database server after an "upgrade" to the latest MediaWiki version. Set $wgLocalisationCacheConf to force the localisation information to be stored to the file system to remedy this.
 * Variables and session data: Storing variable data such as the MediaWiki sidebar, the list of namespaces or the spam blacklist to a memory cache will substantially increase the speed of a MediaWiki installation. Forcing user login data to be stored in a common location is also essential to any installation in which multiple, interchangeable Apache servers are hidden behind the same Varnish caches to serve pages for the same wikis. Install the memcached package and set the following options in LocalSettings.php to force both user login information and cached variables to use memcache:
 * $wgMainCacheType = CACHE_MEMCACHED;
 * $wgMemCachedServers = array ( '127.0.0.1:11211' );
 * $wgSessionsInMemcached = true;
 * $wgUseMemCached = true;</tt>
 * Note that, if you have multiple servers, the localhost address needs to be replaced with that of the shared memcached server(s), which must be the same for all of the matching web servers at your site. This ensures that logging a user into one server in the cluster logs them into the wiki on all the interchangeable web servers.

In many cases, there are multiple alternative caching approaches which will produce the same result. See Manual:Cache.

Log file
The Apache web server log, by default, shows only the address of the Varnish cache server, in this example "127.0.0.1:80"

Apache may be configured to log the original user's address by capturing "x-forwarded-for" information under a custom log file format.

An example for Apache's httpd.conf to configure logging of x-forwarded-for is:

 LogFormat "%{X-Forwarded-for}i %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" cached </tt>

Image hotlinking
If a site uses Apache's mod_rewrite</tt> to block attempts by other websites to hotlink images, this configuration will need to be removed and equivalent configuration added to Varnish's configuration files. Where an image server is located behind Varnish, typically 90% or more of common image requests never reach Apache and therefore will not be blocked by a "http_referer" check in Apache's configurations.