Caching strategies
- Caching Strategies for MediaWiki 1.40
This article details various caching strategies available for optimizing the performance of a MediaWiki 1.40 installation. Proper caching is crucial for handling high traffic and ensuring a responsive user experience. We will cover different levels of caching, from the application level to external caching systems. This guide assumes a basic understanding of server administration and PHP.
1. Introduction to Caching
Caching is the process of storing copies of frequently accessed data in a faster storage location. When the same data is requested again, it can be retrieved from the cache instead of the original source, significantly reducing response times. In a MediaWiki environment, caching applies to various components, including database queries, parsed wikitext, and rendered HTML. Effective caching strategies minimize the load on the database server and improve overall site performance.
2. MediaWiki's Internal Caching Mechanisms
MediaWiki includes several built-in caching mechanisms. Understanding these is the first step toward optimizing your installation.
2.1. Parser Cache
The parser cache stores the output of the parser. When a page is viewed, MediaWiki parses the wikitext into HTML. This process is resource-intensive. The parser cache stores the parsed HTML, so subsequent requests for the same page can bypass parsing. This cache is configured via `$wgParserCacheType` in `LocalSettings.php`. Options include 'redis', 'memcached', and 'database'.
2.2. Query Cache
The query cache stores the results of database queries. Frequently executed queries, such as those retrieving page titles or categories, can be cached to reduce database load. This cache is managed by the `$wgQueryCacheType` setting in `LocalSettings.php` and supports similar backends as the parser cache.
2.3. Object Cache
The object cache stores various objects used by MediaWiki, like User objects and Title objects. This reduces the need to repeatedly create these objects from database queries. Configured via `$wgObjectCacheType` in `LocalSettings.php`.
Below is a table summarizing the internal cache types:
| Cache Type | Description | Configuration Setting | Recommended Backend |
|---|---|---|---|
| Parser Cache | Stores parsed HTML output. | `$wgParserCacheType` | Redis or Memcached |
| Query Cache | Stores database query results. | `$wgQueryCacheType` | Redis or Memcached |
| Object Cache | Stores frequently used objects. | `$wgObjectCacheType` | Redis or Memcached |
3. External Caching Systems
While MediaWiki's internal caches are effective, integrating with external caching systems can provide further performance improvements.
3.1. Memcached
Memcached is a high-performance, distributed memory object caching system. It's widely used for caching dynamic web application data. Configuring MediaWiki to use Memcached requires installing the Memcached PHP extension and setting `$wgMemCachedServers` in `LocalSettings.php`.
3.2. Redis
Redis is another popular in-memory data structure store. Redis offers more advanced data structures and features compared to Memcached. Configuring MediaWiki for Redis requires the Redis PHP extension and setting `$wgRedisServers` in `LocalSettings.php`. Redis is generally preferred over Memcached for larger deployments.
3.3. Varnish Cache
Varnish Cache is a powerful HTTP accelerator. It sits in front of your web server (e.g., Apache or Nginx) and caches entire HTTP responses. This can significantly reduce the load on your MediaWiki server, especially for anonymous users. Configuring Varnish requires setting up Varnish and configuring it to proxy requests to your MediaWiki server. A typical Varnish configuration will cache pages based on URL and HTTP headers.
Here's a comparison of external caching systems:
| Caching System | Type | Complexity | Scalability |
|---|---|---|---|
| Memcached | Distributed Memory Object Cache | Low | Moderate |
| Redis | In-Memory Data Structure Store | Moderate | High |
| Varnish Cache | HTTP Accelerator | Moderate | High |
4. Database Query Optimization and Caching
Even with robust caching layers, optimizing database queries is essential.
4.1. Indexing
Ensure that your database tables are properly indexed. Indexes speed up query execution by allowing the database to quickly locate specific data. Pay close attention to indexes on frequently queried columns, such as `page.title` and `category.category_title`.
4.2. Query Analysis
Use database profiling tools to identify slow-running queries. Analyze these queries and optimize them by rewriting them, adding indexes, or using more efficient database features. The MySQL `EXPLAIN` statement is a valuable tool for query analysis.
4.3. Database Cache Tuning
Tune the database server's internal cache settings. For example, in MySQL, adjust the `innodb_buffer_pool_size` to allocate sufficient memory for caching data.
Here's a table detailing common database optimization techniques:
| Optimization Technique | Description | Impact |
|---|---|---|
| Indexing | Adding indexes to frequently queried columns. | High |
| Query Rewriting | Optimizing slow-running queries. | Medium to High |
| Database Tuning | Adjusting database server settings. | Medium |
5. Monitoring and Maintenance
Regular monitoring and maintenance are crucial for ensuring that your caching strategies are effective.
5.1. Cache Hit Ratio
Monitor the cache hit ratio for each caching layer. A low hit ratio indicates that the cache is not being utilized effectively. Adjust cache settings or consider using a larger cache size.
5.2. Cache Eviction
Understand how your caching systems evict data. Some systems use Least Recently Used (LRU) eviction, while others use other algorithms. Configure eviction settings to optimize cache performance.
5.3. Regular Purging
Periodically purge the cache to ensure that users are seeing the latest content. This is particularly important after making changes to templates or configuration files. You can use the Special:Purge page for individual pages.
6. Conclusion
Implementing a comprehensive caching strategy is vital for maintaining a fast and responsive MediaWiki installation. By leveraging MediaWiki's internal caches, integrating with external caching systems like Memcached, Redis, and Varnish, and optimizing database queries, you can significantly improve site performance and handle high traffic volumes. Remember to regularly monitor and maintain your caching infrastructure to ensure continued effectiveness.
Help:Contents Manual:Configuration Manual:Caching Extension:CacheHitCount Special:Purge PHP MySQL Apache Nginx Redis Memcached Varnish Cache Database Server User Title
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