Manual:File cache
From MediaWiki.org
MediaWiki has an optional simplistic scheme for caching the rendered HTML of article pages.
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[edit] Operation and usage
The file cache is enabled by setting three variables in LocalSettings.php:
- $wgUseFileCache = true; /* default: false */
- $wgFileCacheDirectory = "$IP/cache";
- $wgShowIPinHeader = false;
This causes the rendered HTML webpage for each page of the wiki to be stored in an individual file on the hard disk. Any subsequent requests from anonymous users are met not by rendering the page again, but by simply sending the stored HTML version which is on the disk. This saves time.
The generated HTML web text is stored to disk in directories under the $wgFileCacheDirectory and looks somewhat like:
-rw-r--r-- 1 apache apache 57421 Jan 7 01:58 cache/0/04/P%C3%A1gina_principal.html -rw-r--r-- 1 apache apache 29608 Jan 4 16:37 cache/1/17/P%C3%A1gina_Riscada.html -rw-r--r-- 1 apache apache 21592 Jan 3 07:27 cache/1/1c/P%C3%A1gina_Duplicada.html -rw-r--r-- 1 apache apache 36088 Jan 7 02:03 cache/2/24/P%C3%A1gina_principal_alternativa.html -rw-r--r-- 1 apache apache 44205 Jan 7 06:10 cache/a/a4/P%C3%A1gina_linkada.html -rw-r--r-- 1 apache apache 24686 Jan 3 07:27 cache/d/db/P%C3%A1gina_Invertida.html -rw-r--r-- 1 apache apache 17222 Jan 3 06:28 cache/f/f0/P%C3%A1gina_n%C3%A3o_encontrada.html
In this example, the file cache directory is cache/ and the stored wiki pages are named Página ..., yielding one corresponding Página_....html file for each. These files will be created as users visit articles on the wiki; you can create them all in one go with the rebuildFileCache.php maintenance script.
The file cache tends to cache aggressively; there is no set expiry date for the cached pages and pages are cached unconditionally even if they contain variables, extensions and other changeable output. Some extensions disable file cache for pages with dynamic content.
For larger sites, the use of an external cache such as Squid or Varnish is preferable to enabling the file cache.
[edit] Domain and range
Caching is only done for users who:
- are not logged in.
- do not have their user_newtalk flag active.
Caching is only done for pages which:
- are not special pages.
- are not redirects.
- are being viewed in current version, plain view, no url parameters
This covers the majority of requests to the wiki, but it is still important to set up bytecode and application caching to handle the rest.
[edit] Validation
The modification time of the cached file is compared with the global $wgCacheEpoch timestamp set in LocalSettings.php on each view.
If the file is at least as new as both of these, it is considered valid and is sent directly to the client. If it is older or does not exist, parsing and rendering continues and the results are saved for future use.
[edit] Invalidation
The entire cache can be invalidated by setting $wgCacheEpoch to the current time, or of course one could delete all files in the cache.
Individual pages are invalidated by updating their page.page_touched fields, conveniently done by calling Title::invalidateCache(). This should be done on article creation, edit saves, renames, and creation and deletion of linked articles (in order to update edit links). Invalidation operations that occur when editing templates, for example, check pages that use template to compare the cache date against page_touched.
Some cases are not yet handled properly, which probably includes:
- browser 'reload' or 'refresh' (just reloads same cached page without updates)
- output variables from extensions such as Extension:DynamicPageList disable filecache for those pages to avoid stale data. Those that don't will return stale data even on browser refresh.
- certain long error messages may be cached indefinitely as if they were valid page content; only ?action=purge will remove these from the file cache once stored. A minimum output size threshold is used to avoid caching most errors, like fatal PHP errors.
[edit] Expiration
There should probably be some method of expiration of cache pages, particularly for pages containing variables (it is X date, we have X articles, etc).
If $parserOutput->getCacheTime() is -1, file caching will be disabled for the output of the page. There is no provision to set an expiry time, so all HTML for all pages is cached forever. An explicit ?action=purge command (or an edit to the page) will regenerate that one page, but neither the MediaWiki internal no-cache flags nor the browser refresh will remove outdated extension output once it has been stored as part of a file-cached page.
[edit] Refresh tab
It is possible to force one individual page to be invalidated and refreshed by using ?action=purge using the Purge page extension.
[edit] Compression
Optionally, the cache may be compressed to save space and bandwidth. (This requires that zlib be enabled in the PHP config.)
- $wgUseGzip = true;
If compression is enabled, the cache files are saved as .html.gz. Browsers that advertise support for gzip in their Accept-Encoding field will be given the gzipped version straight; for those browsers that don't, we unzip the data on the fly and send them the plaintext.
A "Vary: User-agent" header will be sent to tell proxy caches to be more careful about who it resends data to. ("Vary: Accept-encoding" would be more appropriate, but Internet Explorer refuses to cache pages so marked.)
Note: The default behaviour of $wgUseFileCache is that enabling file cache will turn off served page compression which would have otherwise taken place. This can substantially increase the cost of hosting a site with a provider which charges per-gigabyte or per-Mbps transferred.
[edit] Emergency fallback
If the wiki can't contact the database server, it will try to show the cached version of whatever page was requested, regardless of whether it may be current or not, with a "database is down" message tacked into it.
This has some limitations:
- special pages are not covered in any way, there's just a warning message
- redirect pages are not cached, so clicking a link to a redirect doesn't go through to the final destination
- attempts to use non-view actions result in a plain page view, which may be confusing
- there may be issues with the MySQL connection timeout which make it take prohibitively long before giving up, particularly if using persistent connections and the db dies later.
- pages with colons in the title will still fail due to MediaWiki checking the DB to see if the title has a valid interwiki prefix
- fallback may fail if $wgDisableCounters is not "true" due to attempting a DB query
