Manual:Configuration settings/cache settings

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  1. Manual:Configuration settings/cache settings

This article details the cache settings available in MediaWiki, explaining their function, impact on performance, and how to configure them effectively. It is aimed at MediaWiki administrators and those responsible for maintaining a wiki's performance. Understanding these settings is crucial for ensuring a responsive and efficient wiki experience for all users.

Introduction to Caching in MediaWiki

Caching is a fundamental technique for improving the performance of any web application, and MediaWiki is no exception. In essence, caching involves storing frequently accessed data in a temporary storage location (the "cache") so that future requests for that data can be served faster. Instead of re-generating the content from scratch each time, MediaWiki can retrieve it from the cache, saving significant server resources and reducing response times.

Without caching, every page view would require MediaWiki to:

  • Query the database for the page's content.
  • Parse the wikitext.
  • Render the HTML.
  • Apply any extensions or modifications.

This process is resource-intensive, especially for popular pages. Caching minimizes these operations, resulting in a much faster and more scalable wiki. MediaWiki offers several layers of caching, each targeting different aspects of the system. These include:

  • **Parser Cache:** Stores the output of the parser, so the same wikitext doesn't need to be re-parsed repeatedly.
  • **Object Cache:** Stores database query results and other frequently used objects, reducing database load.
  • **Output Cache:** Stores the fully rendered HTML of pages, allowing them to be served directly without any processing.
  • **Query Cache:** Caches the results of simple SELECT queries.

Configuring Cache Settings

The cache settings are primarily configured in the `LocalSettings.php` file. Here's a detailed breakdown of the important settings:

$wgCacheDirectory

This setting defines the directory where MediaWiki stores its cache files. This directory *must* be writable by the web server user. A common location is `/var/cache/mediawiki`.

```php $wgCacheDirectory = "/var/cache/mediawiki"; ```

Incorrect permissions on this directory are a frequent source of caching problems. Ensure the web server user (e.g., `www-data` on Debian/Ubuntu systems) has full read/write/execute permissions. Consider using a dedicated user for the web server process and granting that user ownership of the cache directory. Protecting this directory from direct web access is also vital for security.

$wgParserCacheType

This setting determines the type of cache used for the parser output. Available options include:

  • `'redis'`: Uses Redis, a popular in-memory data store, for caching. This is generally the fastest option. Requires the Redis PHP extension. Manual:Configuration settings/backend support provides more information on setting up backend support.
  • `'memcached'`: Uses Memcached, another in-memory data store. Similar to Redis in performance, but may have different operational characteristics. Requires the Memcached PHP extension.
  • `'apc'`: Uses APC (Alternative PHP Cache), a PHP extension that caches opcode and data. Suitable for smaller wikis, but may not scale as well as Redis or Memcached.
  • `'database'`: Stores the parser cache in the database. This is the slowest option, but it doesn't require any additional software. Use this only as a last resort.
  • `'null'`: Disables the parser cache. Useful for debugging, but severely impacts performance.

```php $wgParserCacheType = 'redis'; ```

Choosing the right parser cache type depends on your server infrastructure and wiki size. Redis and Memcached are highly recommended for larger wikis.

$wgMemCachedServers

If you're using `memcached` as your parser cache type, this setting specifies the Memcached server(s) to connect to. It's an array of server addresses.

```php $wgMemCachedServers = array(

 '127.0.0.1:11211'

); ```

You can specify multiple servers for redundancy and increased capacity.

$wgRedisServers

If you're using `redis` as your parser cache type, this setting specifies the Redis server(s) to connect to. It’s an array of server addresses.

```php $wgRedisServers = array(

 '127.0.0.1:6379'

); ```

Similar to `$wgMemCachedServers`, you can specify multiple servers for high availability.

$wgObjectCacheEnabled

This setting enables or disables the object cache.

```php $wgObjectCacheEnabled = true; ```

Disabling the object cache will significantly increase database load.

$wgObjectCacheType

This setting determines the type of cache used for the object cache. Options are the same as for `$wgParserCacheType`: `'redis'`, `'memcached'`, `'apc'`, and `'database'`.

```php $wgObjectCacheType = 'redis'; ```

Like the parser cache, Redis or Memcached are the best choices for the object cache.

$wgOutputCacheTTL

This setting defines the Time To Live (TTL) for the output cache, in seconds. This determines how long a rendered page will be stored in the cache before it's considered stale and regenerated. A longer TTL reduces server load but may result in users seeing outdated content. A shorter TTL ensures fresher content but increases server load.

```php $wgOutputCacheTTL = 3600; // 1 hour ```

Consider the frequency of updates to your wiki when setting this value. For wikis with infrequent updates, a longer TTL is appropriate. For wikis with frequent updates, a shorter TTL is necessary. You can also set this on a per-page basis using the `Template:Cachedin` parser function. Manual:Parser functions provides more details on this.

$wgQueryCacheTTL

This setting defines the TTL for the query cache, in seconds. This is similar to `$wgOutputCacheTTL` but applies to simple SELECT query results.

```php $wgQueryCacheTTL = 300; // 5 minutes ```

$wgCachePages

This setting controls whether or not MediaWiki caches the output of pages.

```php $wgCachePages = true; ```

Disabling this will prevent MediaWiki from caching pages, effectively negating the benefits of the output cache.

$wgUsePersistentCache

This setting determines whether MediaWiki uses a persistent cache or a volatile cache. A persistent cache stores cache data on disk, so it survives server restarts. A volatile cache stores cache data in memory, which is lost when the server restarts.

```php $wgUsePersistentCache = true; ```

Using a persistent cache is generally recommended, as it reduces the time it takes to warm up the cache after a server restart.

$wgCacheEpochs

This setting controls how often MediaWiki checks for changes to cached pages. Increasing this value reduces the frequency of cache invalidation, but may result in users seeing slightly outdated content.

```php $wgCacheEpochs = 3; ```

Advanced Cache Configuration

Beyond these core settings, there are several other options available for fine-tuning the cache:

  • **$wgMainCacheType:** Allows you to specify a global cache backend, which will be used for all caches if a specific type isn't defined.
  • **$wgCacheBaseDirectory:** Allows you to change the base directory for all cache files.
  • **$wgCacheCompression:** Enables compression of cache files to save disk space.

Monitoring and Troubleshooting Cache Issues

Monitoring your cache performance is crucial for identifying potential issues. Here are some things to look for:

  • **Cache Hit Rate:** This measures the percentage of requests that are served from the cache. A low cache hit rate indicates that the cache isn't being used effectively.
  • **Cache Miss Rate:** The opposite of the hit rate. A high miss rate suggests the cache isn't effective.
  • **Cache Evictions:** This measures how often the cache is forced to discard entries to make room for new ones. High eviction rates may indicate that the cache is too small.
  • **Server Resource Usage:** Monitor CPU, memory, and disk I/O to identify potential bottlenecks.

Tools like `redis-cli` or `memcached-tool` can be used to monitor Redis and Memcached servers, respectively. MediaWiki also provides some basic caching statistics through the Special:Statistics page. Special:Statistics provides details on wiki statistics.

Common cache-related issues include:

  • **Incorrect Permissions:** The web server user doesn't have write access to the cache directory.
  • **Cache Server Downtime:** The Redis or Memcached server is unavailable.
  • **Configuration Errors:** Incorrect settings in `LocalSettings.php`.
  • **Cache Invalidation Issues:** Pages aren't being updated in the cache when they should be. This can be caused by incorrect `$wgCacheEpochs` settings or bugs in extensions.
  • **Insufficient Cache Size:** The cache is too small to hold all the frequently accessed data. Try increasing the memory allocated to Redis or Memcached.

Caching and Extensions

Many extensions also utilize caching to improve performance. It's important to understand how these extensions interact with the core MediaWiki cache and to configure them appropriately. Refer to the documentation for each extension for specific caching instructions. Extension:Semantic MediaWiki is an example of an extension that benefits greatly from proper caching.

Caching Strategies and Technical Analysis

Understanding caching isn't just about configuration; it’s about employing effective strategies. Here's a brief overview of related concepts:

  • **Content Delivery Networks (CDNs):** Distribute your wiki's content across multiple servers geographically, reducing latency for users around the world.
  • **Database Indexing:** Optimizing database queries with appropriate indexes reduces database load and improves caching efficiency. Database optimization is key.
  • **Load Balancing:** Distribute traffic across multiple web servers to prevent overload.
  • **HTTP Caching:** Leverage browser caching and server-side HTTP caching mechanisms (e.g., Varnish) to further reduce server load.
  • **Cache Warming:** Pre-populate the cache with frequently accessed pages to improve initial performance after a server restart.
  • **Cache Invalidation Strategies:** Implement robust mechanisms for invalidating the cache when content changes.
  • **Trend Analysis:** Identifying frequently accessed pages allows you to prioritize caching efforts.
  • **Performance Monitoring Indicators:** Key metrics such as page load time, server response time, and cache hit rate provide valuable insights into caching performance.
  • **Root Cause Analysis:** When performance issues arise, systematically investigate the underlying causes to identify caching-related problems.
  • **A/B Testing:** Experiment with different caching configurations to determine the optimal settings for your wiki.
  • **Predictive Caching:** Anticipating future requests and pre-caching relevant data.
  • **Adaptive Caching:** Dynamically adjusting caching parameters based on real-time traffic patterns.
  • **Data Compression Techniques:** Compressing cache data reduces storage space and improves transfer speeds.
  • **Cache Partitioning:** Dividing the cache into smaller partitions to improve scalability.
  • **Cache Coherence Protocols:** Ensuring that multiple cache instances maintain consistent data.
  • **Cache Eviction Algorithms:** Strategies for deciding which cache entries to discard when the cache is full (e.g., Least Recently Used (LRU), Least Frequently Used (LFU)).
  • **Cache Hierarchy:** Using multiple levels of caching (e.g., browser cache, server-side cache, database cache) to optimize performance.
  • **Cache Tagging:** Associating tags with cache entries to enable more granular cache invalidation.
  • **Cache Debugging Tools:** Tools for inspecting the contents of the cache and identifying caching-related issues.
  • **Cache Cluster Management:** Managing a cluster of cache servers for scalability and high availability.
  • **Anomaly Detection:** Identifying unusual patterns in cache behavior that may indicate problems.
  • **Capacity Planning:** Estimating the required cache size based on traffic patterns and data volume.
  • **Performance Benchmarking:** Measuring the performance of the cache under different workloads.
  • **Scalability Testing:** Evaluating the cache's ability to handle increasing traffic loads.
  • **Cache Security Considerations:** Protecting the cache from unauthorized access and modification.


Conclusion

Properly configuring and monitoring the cache settings in MediaWiki is essential for delivering a fast and responsive wiki experience. By understanding the different caching layers, choosing the right cache backend, and optimizing the configuration settings, you can significantly improve your wiki's performance and scalability. Regular monitoring and troubleshooting are also crucial for ensuring that the cache is functioning optimally. Remember to always test changes in a staging environment before deploying them to production. Manual:How to debug will help with testing and debugging.


Manual:Configuration settings Manual:Extensions Manual:Database setup Manual:Performance Special:Statistics Manual:Parser functions Extension:Semantic MediaWiki Database optimization Manual:How to debug Manual:Configuration settings/backend support

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