F5 to load balance MS Sql Servers

I have done this procedure in production through Cisco and F5 load balancers and you may have some issues with Connection Pooling during the transition.

 

 

If you have .NET code, or any other code that can take advantage of connection pooling, then you may have issues during the transition. Connection pooling persists connections to the database from the client and re-uses them. Without pooling, each query opens a new connection. During a failover, the persisted connection to the DB will break, but you have to ensure that a "TCP FIN" is sent back to the client, or all subsequent queries to the pool will fail. The client assumes that it can still send queries over the connection to the VIP via the same TCP/IP session, but the session paramaters have changed because the connection is now on a new back-end server. At some point, the pool will establish a new connection, but you would likely get lots of GNE's (General Network Errors) during the transition. The Cisco's were very bad at this, but some revision of their code allowed long lived connections to the DB and things were somewhat better. The F5's were less problematic, but we still received GNE's occasionally. Make sure you set your connection timeouts to "infinite" or a length of time longer than the pool timeout.