Caching strategies

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  1. Caching Strategies for MediaWiki

Caching is a critical component of optimizing the performance of any MediaWiki installation, especially those experiencing moderate to high traffic. It significantly reduces server load, improves page load times, and enhances the overall user experience. This article provides a comprehensive overview of caching strategies available in MediaWiki 1.40 and beyond, geared towards beginners. We will cover different layers of caching, configuration options, and best practices.

    1. What is Caching?

At its core, caching is the process of storing copies of frequently accessed data in a faster storage location. When a user requests a page, MediaWiki first checks if a cached version exists. If it does, the cached version is served, bypassing the need to rebuild the page from the database and PHP code. This dramatically reduces the time it takes to deliver content to the user. Without caching, every page request would require substantial server resources, leading to slow response times and potential server crashes under heavy load. Understanding Scalability is crucial when considering caching.

    1. Layers of Caching in MediaWiki

MediaWiki employs several layers of caching, each targeting a different aspect of the content delivery process.

      1. 1. Browser Caching

This is the first line of defense. Browsers store static assets like images, CSS, and JavaScript files locally. When a user revisits a page, the browser can retrieve these assets from its cache instead of downloading them again. Configuring appropriate HTTP headers (e.g., `Cache-Control`, `Expires`) in your web server (Apache, Nginx) controls how long browsers cache these assets. Longer cache durations are suitable for infrequently updated files, while shorter durations are better for frequently changing content. Consider using a Content Delivery Network (CDN) to further optimize browser caching by distributing assets geographically closer to users. Content delivery network can greatly enhance performance.

      1. 2. Opcode Caching

PHP, the language MediaWiki is written in, needs to be compiled into executable code (opcode) each time a script is run. Opcode caching stores the compiled code in memory, eliminating the need for recompilation on subsequent requests. This is a *significant* performance booster. Popular PHP opcode caches include:

  • **OPcache:** The default opcode cache in PHP 5.5 and later. It’s built-in and generally performs very well. Configuration is done via `php.ini`.
  • **APC (Alternative PHP Cache):** An older opcode cache, still usable but generally superseded by OPcache.

Configuring OPcache involves adjusting settings in `php.ini` such as `opcache.memory_consumption` (amount of memory allocated to the cache) and `opcache.validate_timestamps` (whether to check for file modifications). Proper configuration is essential; incorrect settings can negate the benefits. [PHP Documentation on OPcache](https://www.php.net/manual/en/opcache.configuration.php) provides detailed information.

      1. 3. DataBase Query Caching

MediaWiki frequently queries the database to retrieve content. Database query caching stores the results of these queries in memory. When the same query is executed again, the cached result is returned, avoiding a database lookup. This is especially effective for queries that return relatively static data.

  • **MySQL Query Cache:** While available in older MySQL versions, the query cache has been deprecated and removed in MySQL 8.0. It often introduced more overhead than benefit, especially under high concurrency.
  • **Redis/Memcached:** These are in-memory data stores commonly used for caching database query results. MediaWiki supports using them as a backend for its caching system. They offer better scalability and performance than the MySQL query cache. Configuration is done through the `$wgMemCachedServers` variable in `LocalSettings.php`. [Redis Documentation](https://redis.io/docs/) and [Memcached Documentation](https://memcached.org/documentation) are valuable resources.
      1. 4. Object Caching

Object caching stores pre-processed objects, such as parsed templates or frequently accessed data structures, in memory. This avoids the need to re-parse or re-calculate these objects on every request. Redis and Memcached are also commonly used for object caching in MediaWiki.

      1. 5. Page Caching

This is the most visible form of caching. Page caching stores the complete rendered HTML output of a page in a cache. When a user requests the page, the cached HTML is served directly, bypassing the entire PHP processing pipeline. This provides the most significant performance gains but requires careful consideration of cache invalidation.

  • **Squid:** A popular HTTP proxy cache that can cache entire web pages. It sits between your web server and users, intercepting requests and serving cached content when available. [Squid Documentation](https://www.squid-cache.org/Documentation/)
  • **Varnish:** Another powerful HTTP accelerator designed for caching HTTP content. It’s known for its flexibility and performance. [Varnish Documentation](https://varnish-cache.org/docs/)
  • **MediaWiki’s Built-in Cache:** MediaWiki has its own internal page caching mechanism, configurable through the `$wgCachePages` variable in `LocalSettings.php`. This caches pages based on their content and expiration settings.
    1. Configuring Caching in MediaWiki (LocalSettings.php)

The primary configuration file for caching in MediaWiki is `LocalSettings.php`. Here are some key variables:

  • `$wgCacheDirectory = "/path/to/cache";`: Specifies the directory where MediaWiki stores its internal cache files. Ensure this directory has appropriate permissions.
  • `$wgCachePages = true;`: Enables MediaWiki’s built-in page caching.
  • `$wgMainCacheType = 'redis';`: Sets the default caching backend. Options include 'redis', 'memcached', 'memcached-persistent', 'apc', and 'hash'. 'redis' and 'memcached' are generally recommended for production environments.
  • `$wgMemCachedServers = array( '127.0.0.1:11211' );`: Specifies the address(es) of your Memcached or Redis servers.
  • `$wgObjectCacheEnabled = true;`: Enables object caching.
  • `$wgUseTrackbacks = false;`: Disabling Trackbacks reduces database load.
  • `$wgParserCacheType = 'redis';`: Sets the cache type for the parser output.
  • `$wgEnableParserCache = true;`: Enables parser caching.
    • Important:** After modifying `LocalSettings.php`, you need to re-run the `update.php` script to apply the changes. This script rebuilds the configuration and clears existing caches. Maintenance tasks are essential for a healthy wiki.
    1. Cache Invalidation Strategies

Caching is beneficial only if the cached content is up-to-date. Cache invalidation is the process of removing or updating cached content when the underlying data changes. Common strategies include:

  • **Time-Based Expiration:** Setting a time-to-live (TTL) for cached content. After the TTL expires, the content is considered stale and is refreshed. This is simple to implement but may serve stale content for a period.
  • **Event-Based Invalidation:** Invalidating the cache when specific events occur, such as a page edit, a template change, or a category update. This ensures that the cache always contains the most current data. MediaWiki's job queue can be used to handle event-based invalidation efficiently. Jobs and queues are crucial for asynchronous tasks.
  • **Tag-Based Invalidation:** Associating tags with cached content. When data associated with a tag changes, all cached content tagged with that tag is invalidated. This is more granular than time-based expiration and event-based invalidation.
  • **Purging:** Manually clearing the cache using the Special:Purge page or the API. This is useful for troubleshooting or when you know that the cache contains incorrect data.
    1. Monitoring and Tuning Cache Performance

Regularly monitoring cache performance is crucial to ensure that your caching strategy is effective. Key metrics to track include:

  • **Cache Hit Rate:** The percentage of requests served from the cache. A higher hit rate indicates better caching performance.
  • **Cache Miss Rate:** The percentage of requests that require a database lookup. A lower miss rate is desirable.
  • **Cache Size:** The amount of memory used by the cache. Monitor this to ensure that the cache doesn't exceed available resources.
  • **Page Load Times:** Track page load times to measure the impact of caching on user experience.

Tools like `redis-cli` (for Redis) and `memcached-tool` (for Memcached) can be used to monitor cache statistics. Web server logs can also provide valuable insights into caching performance.

    1. Advanced Caching Techniques
  • **Vary Headers:** Using the `Vary` HTTP header to cache different versions of a page based on request headers (e.g., `Accept-Encoding`).
  • **Fragment Caching:** Caching specific parts of a page, rather than the entire page. This can be useful for dynamic content that changes frequently.
  • **CDN Integration:** Leveraging a Content Delivery Network (CDN) to cache and deliver content from geographically distributed servers. [Cloudflare](https://www.cloudflare.com/), [Amazon CloudFront](https://aws.amazon.com/cloudfront/), and [Akamai](https://www.akamai.com/) are popular CDN providers.
    1. Common Pitfalls
  • **Incorrect Permissions:** Ensure that the cache directory has the correct permissions for the web server user.
  • **Insufficient Memory:** Allocate enough memory to the cache to accommodate your workload.
  • **Overly Aggressive Caching:** Caching content for too long can lead to stale data.
  • **Ignoring Cache Invalidation:** Failing to invalidate the cache when data changes can result in incorrect information being displayed.
  • **Lack of Monitoring:** Not monitoring cache performance can prevent you from identifying and addressing issues. [Performance monitoring tools](https://www.newrelic.com/) can be helpful.
    1. Resources



Configuration, Performance, Database, PHP, Web server

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