Caching mechanisms

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  1. Caching Mechanisms in MediaWiki 1.40

This article details the various caching mechanisms available in MediaWiki 1.40, and how to configure them for optimal performance. Effective caching is crucial for a responsive and scalable wiki, reducing database load and improving user experience. This guide is intended for system administrators and experienced wiki operators.

Understanding Caching in MediaWiki

MediaWiki employs multiple layers of caching to minimize the need to repeatedly generate content from the underlying database. These layers range from object caching to page caching, each serving a specific purpose. Properly configuring these caches can significantly improve wiki performance, especially under high load. Before diving into the specifics, understand the general principle: caching stores the results of expensive operations (like database queries or complex template parsing) so that future requests for the same data can be served from the cache, which is much faster.

Object Caching

Object caching stores individual PHP objects, such as database query results, parser cache entries, and other intermediate data. This is the first line of defense against database load. MediaWiki supports several backends for object caching:

  • Memcached
  • Redis
  • APC (Advanced PHP Cache) - *Deprecated in newer PHP versions, consider alternatives.*

The choice of backend depends on your server infrastructure and performance requirements.

Object Cache Configuration

The `$wgMemCachedServers` configuration variable in `LocalSettings.php` defines the Memcached servers. For Redis, use `$wgRedisServers`. For APC, the configuration is largely automatic.

Here's an example configuration using Memcached:

```wiki $wgMemCachedServers = array( 

 '127.0.0.1:11211'

); ```

Here's an example using Redis:

```wiki $wgRedisServers = array( 

   '127.0.0.1:6379'

); ```

For optimal performance, ensure your object cache has sufficient memory allocated. Monitor cache statistics using tools provided by your chosen backend.

Object Cache Statistics

You can view object cache statistics via the Special:Statistics page. This page provides information such as hit rate, miss rate, and memory usage.

Page Caching

Page caching stores the fully rendered HTML output of wiki pages. This drastically reduces the load on the application server, as it avoids re-rendering the page for each request. MediaWiki offers two primary page caching methods:

  • Varnish (Reverse Proxy Cache)
  • Internal Page Cache

Varnish Configuration

Varnish is a powerful reverse proxy cache that sits in front of your MediaWiki server. It caches static content and frequently accessed pages, serving them directly to users without involving the MediaWiki application server. Configuring Varnish requires expertise in Varnish Configuration Language (VCL).

Here's a simplified example VCL snippet:

```wiki vcl 4.0;

backend default { 

   .host = "127.0.0.1";
   .port = "80";

}

sub vcl_recv { 

   if (req.http.Authorization || req.http.Cookie) {
       return (pass);
   }
   if (req.url ~ "\.(png|jpg|jpeg|gif|css|js)$") {
       return (hash);
   }
   return (hash);

}

sub vcl_backend_response { 

   set beresp.ttl = 1h;
   return (deliver);

} ```

This VCL configuration caches static files (images, CSS, JavaScript) and pages for one hour. Adjust the TTL (Time To Live) based on your content update frequency.

Internal Page Cache Configuration

MediaWiki's internal page cache stores rendered pages in the filesystem. This is a simpler alternative to Varnish, but less flexible and typically less performant.

The `$wgMainCacheType` configuration variable in `LocalSettings.php` controls the internal page cache type.

Cache Type Description
`default` Uses the default filesystem cache.
`database` Stores cached pages in the database (not recommended for performance).
`memcached` Stores cached pages in Memcached.

The `$wgMainCacheDirectory` variable defines the directory where cached pages are stored. Ensure this directory is writable by the web server user.

Database Caching

While not a direct cache, optimizing database queries is critical for performance. Database replication and careful index creation can improve query speeds, effectively acting as a form of caching. Using a database like MySQL or PostgreSQL efficiently is paramount.

Database Query Cache

The database server itself often has a query cache. Ensure this is enabled and appropriately sized for your workload. However, be aware that the query cache can be invalidated frequently by write operations, so monitor its effectiveness.

Transformer Caching

MediaWiki uses transformers to modify content, such as syntax highlighting or image resizing. These transformations can be computationally expensive. Caching the results of these transformations avoids redundant processing. Configuration is generally handled automatically, but monitoring the Special:TransformerStats page is helpful.

Configuration Summary Table

Configuration Variable Location Description
`$wgMemCachedServers` `LocalSettings.php` Specifies the Memcached servers.
`$wgRedisServers` `LocalSettings.php` Specifies the Redis servers.
`$wgMainCacheType` `LocalSettings.php` Specifies the internal page cache type.
`$wgMainCacheDirectory` `LocalSettings.php` Specifies the directory for the internal page cache.
VCL Configuration Varnish Configuration Files Configures the Varnish reverse proxy cache.

Monitoring and Tuning

Regularly monitor cache statistics using the tools provided by your chosen caching backends and the Special:Statistics page in MediaWiki. Adjust cache sizes and TTLs based on your wiki's traffic patterns and content update frequency. Use a profiler to identify performance bottlenecks and optimize database queries.

Further Reading


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