F5 Field Systems Engineer Patrick Chang explains an iRules solution that supports 2 active database instances in different data centers behind a web application.  The goal is to provide the optimal user experience by load balancing requests for both the web/application tier and the database tier to a local data center whenever possible.  Since cross-data center database replication takes approximately 20 seconds, one instance is preferred for database writes, and an iRule that reads and inserts time-stamped cookies reliably directs traffic to the closest database instance when possible, and to the remote instance only when necessary to maintain session integrity.

This solution was brought to you by the iRule commands: HTTP::cookie, HTTP::uri, and HTTP::method