HyperDB

It is based on code already being used on the WordPress blogging website with multiple database servers.

Installation:

WordPress MU only: add this line near the top of wp-config.php: define('WPMU', true);
Upload db.php to the /wp-content/ directory. At this point, HyperDB is active. It will use the database connection constants until you complete the final steps.
Upload db-settings.php in the directory that holds wp-config.php
Edit the db settings according to the directions in that file.
Add this line near the top of wp-config.php: require('db-settings.php');
Any value of WP_USE_MULTIPLE_DB will be ignored by HyperDB. If you wish to switch off multiple DB, remove the 'require' statement from step 5.

Features:

• Read and write servers (replication)
• Local and remote datacenters
• Private and public networks
• Different tables on different databases/hosts
• Smart post-write master reads
• Failover for downed host
• Advanced statistics for profiling

Requirements:

• WordPress 2.3 or higher