Caching in MediaWiki

1. Caching in MediaWiki

Caching is a fundamental concept in optimizing the performance of any web application, and MediaWiki is no exception. For wiki installations, especially those with significant traffic or complex templates, understanding and effectively configuring caching can dramatically improve response times, reduce server load, and enhance the overall user experience. This article provides a comprehensive overview of caching in MediaWiki, geared towards beginners, covering the various levels of caching available, how they work, and how to configure them.

1. 1. What is Caching?

At its core, caching is the process of storing copies of data in a faster, more accessible location. Instead of repeatedly fetching data from the original source (like a database or performing complex calculations), the system first checks if a cached copy exists. If it does, the cached copy is served, saving time and resources. Think of it like keeping frequently used ingredients within easy reach in the kitchen instead of going to the grocery store every time you need them.

In the context of MediaWiki, data that can be cached includes:

• **Parsed Pages:** The fully rendered HTML of a page after all templates and variables have been processed.
• **Database Queries:** Results of frequently executed database queries.
• **Object Cache:** PHP objects and data structures used by MediaWiki's code.
• **Output:** The complete HTML output generated by MediaWiki.
• **Images & Static Assets:** Images, CSS files, and JavaScript files. (This is often handled by the web server, but can be influenced by MediaWiki's configuration.)

1. 1. Levels of Caching in MediaWiki

MediaWiki employs several layers of caching, each addressing different aspects of performance:

1. 1. 1. 1. Browser Caching

This is the first line of defense. Web browsers store copies of static assets (images, CSS, JavaScript) locally on the user's computer. When the user revisits a page, the browser can retrieve these assets from its cache instead of downloading them again. This is controlled by HTTP headers sent by the web server. MediaWiki, through its `$wgUsePathInfo` and `$wgCachePages` settings combined with web server configuration, influences these headers. Properly configured browser caching significantly reduces bandwidth usage and page load times for returning visitors. Consider setting long expiration times for static assets that rarely change. Techniques like [cache busting](https://developers.google.com/speed/docs/best-practices/caching#cache-busting) (adding version numbers to filenames) are useful when you need to force browsers to download updated assets.

1. 1. 1. 2. Output Caching (Page Caching)

This is arguably the most impactful caching mechanism within MediaWiki itself. When enabled (`$wgCachePages = true;`), MediaWiki stores the *entire* rendered HTML output of a page in the cache. Subsequent requests for the same page are served directly from the cache, bypassing the database and template parsing entirely. This drastically reduces server load and improves response times, especially for popular pages.

• **Cache Key:** The cache key is based on the page title, the user's skin, and other relevant factors. Changes to any of these factors will result in a new cache entry being created.
• **Cache Invalidation:** The cache is automatically invalidated when a page is edited, or when changes are made to templates or variables that affect the page's output.
• **Configuration:** The `$wgCachePages` setting enables or disables output caching. The `$wgMainCacheType` setting determines the type of cache storage used (see section 3 below).
• **Considerations:** Output caching is not suitable for pages that are highly personalized or frequently updated, as it may serve stale content. Pages with dynamic content (e.g., user-specific watchlists) should generally *not* be cached. Special:Cache allows admins to manually clear the cache.

1. 1. 1. 3. Object Caching

Object caching focuses on storing the results of expensive operations within PHP. This includes database query results, parsed template fragments, and other data structures. MediaWiki uses a caching backend (like Memcached or Redis) to store these objects. When a function or method needs to retrieve data, it first checks the object cache. If the data is present, it's retrieved from the cache, avoiding the overhead of recalculating or re-querying.

• **Caching Backends:** Common object caching backends include:

   *   **Memcached:**  A distributed memory object caching system.  Highly performant and widely used. [Memcached Documentation](https://memcached.org/documentation)
   *   **Redis:**  An in-memory data structure store, often used as a cache.  Offers more features than Memcached, including persistence. [Redis Documentation](https://redis.io/documentation)
   *   **APC:** (Alternative PHP Cache) A PHP extension for caching bytecode and data.  Less flexible than Memcached or Redis, but can be useful for smaller installations.

• **Configuration:** The `$wgMainCacheType` setting determines the object cache backend. You'll also need to install and configure the appropriate PHP extension (e.g., `memcached`, `redis`) and configure MediaWiki's `$wgMemCachedServers` or `$wgRedisServers` settings to point to your cache servers.
• **Benefits:** Object caching significantly reduces database load and improves the performance of complex operations. It's particularly effective for caching frequently accessed data that doesn't change often.

1. 1. 1. 4. Database Query Caching

MediaWiki has a built-in database query caching mechanism, but it's generally less effective than object caching. It stores the results of simple database queries that are executed frequently. However, it's often bypassed by more complex queries or when the database schema changes. The `$wgQueryCacheType` setting controls this caching. While it can provide some performance improvements, focusing on object caching is typically more beneficial.

1. 1. 1. 5. Parser Cache

MediaWiki's parser is responsible for converting wikitext into HTML. Parsing can be a resource-intensive process, especially for pages with complex templates. The parser cache stores the results of parsing wikitext, so that subsequent requests for the same wikitext can be served directly from the cache. The `$wgParserCacheType` setting controls this caching. Like database query caching, it's less impactful than object caching but can still contribute to performance improvements.

1. 1. Configuring Caching in MediaWiki

Configuring caching involves modifying the `LocalSettings.php` file. Here's a breakdown of the key settings:

• `$wgCachePages = true;`: Enables output caching.
• `$wgMainCacheType = 'redis';`: Sets the object cache backend to Redis (or 'memcached', 'apc', etc.).
• `$wgMemCachedServers = array( '127.0.0.1:11211' );`: Specifies the Memcached server(s). Replace with your server's address and port.
• `$wgRedisServers = array( array( 'host' => '127.0.0.1', 'port' => 6379 ) );`: Specifies the Redis server(s). Replace with your server’s address and port.
• `$wgQueryCacheType = 'redis';`: Sets the database query cache backend.
• `$wgParserCacheType = 'redis';`: Sets the parser cache backend.
• `$wgCacheDirectory = '/path/to/cache/directory';`: Specifies the directory for storing cache files (if using file-based caching).

• • Important Considerations:**

• **Cache Size:** Ensure that your caching backend (Memcached, Redis) has sufficient memory allocated to store the cache. Monitor cache hit rates to determine if you need to increase the cache size. [Cache Hit Ratio Explained](https://www.datadoghq.com/blog/cache-hit-ratio/)
• **Cache Invalidation:** Understand how changes to templates, pages, and configuration files invalidate the cache. Use `Special:Cache` to manually clear the cache when necessary.
• **Distributed Caching:** For high-traffic wikis, consider using a distributed caching system (like Memcached or Redis) with multiple servers to increase capacity and redundancy. [Distributed Caching Architecture](https://www.cloudflare.com/learning/serverless/what-is-distributed-caching/)
• **Monitoring:** Monitor cache performance using tools provided by your caching backend (e.g., Memcached statistics, Redis INFO command). Pay attention to cache hit rates, eviction rates, and memory usage. [Redis Monitoring Tools](https://redis.io/docs/monitoring-and-metrics/)
• **Testing:** After making any changes to caching configuration, thoroughly test your wiki to ensure that caching is working as expected and that no unexpected issues arise.

1. 1. Advanced Caching Techniques

• **Varnish Cache:** A reverse proxy cache that sits in front of your web server. Varnish can cache entire web pages, reducing load on the MediaWiki server. [Varnish Cache Documentation](https://varnish-cache.org/docs/)
• **Content Delivery Network (CDN):** Distributes static assets (images, CSS, JavaScript) to servers around the world, reducing latency for users in different geographic locations. [CDN Explained](https://www.cloudflare.com/learning/cdn/)
• **Page Pre-caching:** Automatically cache popular pages in advance, ensuring that they are immediately available when requested. This can be implemented using cron jobs or other scheduling mechanisms.
• **Fragment Caching:** Cache specific portions of a page (e.g., a sidebar or a navigation menu) independently. This allows you to cache dynamic content without caching the entire page.

1. 1. Troubleshooting Caching Issues

• **Cache Not Updating:** If you're making changes to pages or templates but the changes aren't reflected on the live wiki, it's likely that the cache is not being invalidated correctly. Check your caching configuration and manually clear the cache using `Special:Cache`.
• **Slow Page Load Times:** If page load times are still slow despite enabling caching, it's possible that the cache is not being hit frequently enough. Check your cache hit rates and consider increasing the cache size or optimizing your caching configuration.
• **Error Messages:** Pay attention to any error messages related to caching in your MediaWiki error logs. These messages can provide clues about the cause of the problem.

1. 1. The Importance of Caching Strategies

Choosing the right caching strategy depends on your wiki’s specific needs and traffic patterns. A [layered caching approach](https://www.akamai.com/blog/security/layered-caching-architecture) – combining browser caching, output caching, and object caching – generally delivers the best results. Regularly analyze your wiki’s performance metrics, such as [page load times](https://web.dev/performance/metrics/), [time to first byte (TTFB)](https://www.keycdn.com/blog/time-to-first-byte/), and [cache hit ratio](https://www.datadoghq.com/blog/cache-hit-ratio/) to identify areas for improvement and refine your caching strategy. Understanding [web performance best practices](https://developers.google.com/speed/docs/best-practices) is crucial for maximizing the benefits of caching. Furthermore, employing [A/B testing](https://www.optimizely.com/optimization-glossary/a-b-testing/) can help you determine the most effective caching configurations for your specific user base. Analyzing [user behavior patterns](https://www.hotjar.com/blog/user-behavior-analytics/) can reveal which pages are most frequently accessed, allowing you to prioritize caching efforts accordingly. Consider using [performance monitoring tools](https://newrelic.com/) to gain deeper insights into your wiki’s performance and identify potential bottlenecks. Employing [predictive caching](https://aws.amazon.com/blogs/database/predictive-caching-with-amazon-memorydb-for-redis/) can further optimize performance by anticipating future data requests. Finally, staying abreast of [emerging caching technologies](https://www.infoworld.com/article/3686154/future-of-caching-what-s-next-for-performance-optimization.html) will ensure that your wiki remains efficient and responsive as your traffic grows. Understanding [HTTP caching headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control) is vital for fine-tuning browser caching behavior. Implementing a robust [cache invalidation strategy](https://www.stackpath.com/blog/cache-invalidation-strategies/) is crucial for ensuring that users always see the latest content. Leveraging [edge caching](https://www.akamai.com/blog/security/what-is-edge-caching) can significantly reduce latency for geographically dispersed users. Lastly, adopting a [cache-aware application design](https://www.infoq.com/articles/cache-aware-application-design/) can optimize your wiki’s code for efficient caching.

Help:Performance Manual:Configuration Manual:Configuring caching Special:Cache Extension:CacheLock Help:Templates Help:Variables Help:Database Help:Web server Help:PHP

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