MediaWiki Caching

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  1. MediaWiki Caching: A Beginner's Guide

This article provides a comprehensive introduction to caching in MediaWiki, specifically geared towards users new to wiki administration and performance optimization. Caching is a crucial technique for improving the speed and responsiveness of a MediaWiki installation, especially those with high traffic or complex templates. We will cover what caching is, why it's important, the various caching mechanisms available in MediaWiki, how to configure them, and troubleshooting common issues.

What is Caching?

At its core, caching is the process of storing copies of frequently accessed data in a faster storage location. Instead of re-calculating or re-fetching information every time it's needed, the system retrieves it from the cache. This drastically reduces server load and improves response times for users. Think of it like this: instead of constantly going to the library to find the same information in a book, you make a photocopy and keep it handy.

In the context of a web application like MediaWiki, caching can apply to various parts of the system, including:

  • **Page content:** The rendered HTML of a wiki page.
  • **Query results:** Results from database queries.
  • **Parsed templates:** The output of complex template structures.
  • **Images and other assets:** Logos, icons, and other static files.

Why is Caching Important for MediaWiki?

MediaWiki, especially with many extensions and a large amount of content, can be resource-intensive. Each page view can trigger multiple database queries, template parsing, and other operations. Without caching, every user request puts a significant strain on the server. Here's why caching is important:

  • **Improved Performance:** The most obvious benefit. Caching dramatically reduces page load times, leading to a better user experience. Faster loading pages encourage more user engagement.
  • **Reduced Server Load:** By serving content from the cache, the server handles fewer requests, reducing CPU usage, memory consumption, and database load. This is particularly important during peak traffic times.
  • **Increased Scalability:** Caching allows a MediaWiki installation to handle a larger number of concurrent users without performance degradation. This is crucial for growing wikis.
  • **Lower Hosting Costs:** Reduced server load can translate to lower hosting costs, as you may be able to use a smaller or less expensive server.
  • **Better SEO:** Search engines favor faster-loading websites. Caching can improve your wiki's search engine ranking. Consider using a Content Delivery Network (CDN) alongside caching for even better results.

MediaWiki's Caching Mechanisms

MediaWiki offers several different caching mechanisms, each with its own strengths and weaknesses. Understanding these differences is key to configuring caching effectively.

  • **Parser Cache:** This is perhaps the most important cache for MediaWiki. It stores the parsed output of wiki pages, including the results of template expansions and function calls. When a page is requested, MediaWiki first checks the parser cache. If a recent version exists, it's served directly, bypassing the parsing process. This is a significant performance boost. The `ParserCache` class handles this functionality.
  • **Output Cache:** This cache stores the fully rendered HTML output of pages. It's even more aggressive than the parser cache, as it skips the entire page rendering process. However, it's more sensitive to changes and requires careful configuration to avoid serving stale content. It's particularly effective for pages that don't change frequently.
  • **Query Cache:** This cache stores the results of database queries. When a query is executed, MediaWiki checks the query cache. If a matching query result exists, it's retrieved directly from the cache, avoiding a database hit. This is extremely beneficial for frequently executed queries.
  • **Object Cache (Memcached/Redis):** MediaWiki supports integration with external object caching systems like Memcached and Redis. These systems provide a shared cache that can be used by multiple applications, including MediaWiki. They're typically used to cache database query results, parser cache data, and other frequently accessed objects. Using an object cache is highly recommended for high-traffic wikis.
  • **Transformer Cache:** This cache stores the transformed output of pages by transformers, such as those used for mobile views.
  • **ResourceLoader Cache:** The ResourceLoader is responsible for loading JavaScript and CSS files. It has its own caching mechanism to reduce the number of HTTP requests.
  • **File Cache:** Caches images and other uploaded files. While largely handled by the web server, MediaWiki's configuration can influence its behavior.

Configuring Caching in MediaWiki

Caching configuration is done primarily through the `LocalSettings.php` file. Here's a breakdown of the key settings:

  • **$wgParserCacheType:** Determines the type of parser cache to use. Common options include:
   *   `'REDIS'`: Uses Redis for the parser cache (recommended for high-traffic wikis).
   *   `'MEMCACHED'`: Uses Memcached for the parser cache.
   *   `'database'`: Stores the parser cache in the database (least efficient, suitable for small wikis).
  • **$wgMainCacheType:** Determines the type of main cache to use (typically used for the output cache and query cache). Options are similar to `$wgParserCacheType`.
  • **$wgMemCachedServers:** An array of Memcached servers to use. Example: `$wgMemCachedServers = array( '127.0.0.1:11211' );`
  • **$wgRedisServers:** An array of Redis servers to use. Example: `$wgRedisServers = array( '127.0.0.1:6379' );`
  • **$wgCachePages:** Enables or disables the output cache. Set to `true` to enable.
  • **$wgCacheEpochs:** Controls how long cached pages are considered valid. Higher values mean longer cache lifetimes but also a higher risk of serving stale content.
  • **$wgQueryCacheTTL:** Controls the time-to-live (TTL) for query cache entries, in seconds. A longer TTL reduces database load but may result in stale data.
  • **$wgObjectCacheEnabled:** Enables or disables the object cache. Set to `true` to enable.
  • **$wgCacheDirectory:** Specifies the directory where MediaWiki stores its cache files (for database-based caches). Ensure this directory is writable by the web server.
    • Example Configuration (using Redis):**

```php $wgParserCacheType = 'REDIS'; $wgMainCacheType = 'REDIS'; $wgRedisServers = array( '127.0.0.1:6379' ); $wgCachePages = true; $wgCacheEpochs = 3600; // 1 hour $wgQueryCacheTTL = 600; // 10 minutes $wgObjectCacheEnabled = true; ```

    • Important Considerations:**
  • **Cache Invalidation:** When content is updated, the corresponding cache entries must be invalidated to ensure that users see the latest version. MediaWiki handles this automatically for most cases, but it's important to be aware of it. Tools like Purge can be used to manually invalidate the cache for specific pages.
  • **Cache Size:** Monitor the size of your cache to ensure that it doesn't exceed available memory. If the cache becomes too large, it can lead to performance degradation. Adjust the cache TTLs and consider using a larger cache server if necessary.
  • **Shared Caches:** When using shared caches like Memcached or Redis, be mindful of potential conflicts with other applications. Use namespaces or keys to isolate MediaWiki's cache data.
  • **Configuration Testing:** Always test your caching configuration thoroughly after making changes. Use browser developer tools to verify that pages are being served from the cache.

Troubleshooting Caching Issues

Here are some common caching issues and how to troubleshoot them:

  • **Stale Content:** If users are seeing outdated content, it's likely that the cache hasn't been invalidated properly. Try purging the page or increasing the cache TTL.
  • **Cache Misses:** If pages are consistently being served directly from the server, it indicates that the cache isn't being used effectively. Check your caching configuration and ensure that caching is enabled for the appropriate components. Also check the server logs for errors related to the cache.
  • **Cache Server Errors:** If you're using Memcached or Redis, check the cache server logs for errors. Ensure that the cache server is running and accessible from the MediaWiki server.
  • **Permissions Issues:** If MediaWiki is unable to write to the cache directory, it will be unable to store cached data. Verify that the web server has write permissions to the cache directory.
  • **Extension Conflicts:** Some extensions may interfere with caching. Try disabling extensions one by one to identify any conflicts.
  • **Slow Performance Despite Caching:** This may indicate that the bottleneck isn’t the caching itself, but rather the underlying database queries or template complexity. Analyze database query times using tools like MySQL Workbench or equivalent for your database system. Optimize complex templates using techniques like conditional statements and function calls to reduce parsing overhead.

Advanced Caching Techniques

  • **Varnish Cache:** Varnish is a powerful HTTP accelerator that can be used to cache entire websites, including MediaWiki installations. It sits in front of the web server and caches frequently accessed content, further reducing server load. [1](https://varnish-cache.org/)
  • **Content Delivery Network (CDN):** A CDN distributes your wiki's content across multiple servers around the world, reducing latency for users in different geographic locations. [2](https://www.cloudflare.com/)
  • **Page Pre-caching:** Some extensions allow you to pre-cache frequently accessed pages, ensuring that they're always available in the cache.
  • **Database Query Optimization:** Optimize your database queries to reduce the amount of time it takes to retrieve data. Use indexes, avoid unnecessary joins, and rewrite complex queries. [3](https://www.percona.com/)
  • **Template Optimization:** Simplify complex templates and avoid unnecessary function calls. Use parser functions sparingly and consider using pre-compiled templates. [4](https://www.mediawiki.org/wiki/Manual:Templates)
  • **Using HTTP Headers:** Configure your web server to send appropriate HTTP caching headers (e.g., `Cache-Control`, `Expires`) to instruct browsers to cache content. [5](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control)
  • **Analyzing Cache Hit Rates:** Monitor your cache hit rates to assess the effectiveness of your caching configuration. Low hit rates indicate that the cache isn't being used effectively. Tools like `redis-cli` (for Redis) and `memcached-tool` (for Memcached) can provide cache statistics.

Resources


MediaWiki Administration MediaWiki Configuration MediaWiki Performance Extensions ResourceLoader Memcached Redis Database Administration Web Server Configuration Purge

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