4.4 Server affinity in Load Balancer
Server affinity enables the Load Balancer to remember which balanced server was chosen for a certain client at its initial request. Subsequent requests are then directed to the same server again.
If the affinity feature is disabled when a new TCP/IP connection is received from a client, Load Balancer chooses the right server at that moment and forwards the packet to it. If a subsequent connection comes in from the same client, Load Balancer treats it as an unrelated connection, and again chooses the most appropriate server at that moment.
Server affinity allows load balancing for those applications that need to preserve state across distinct connections from a client. Maintaining state is a requirement of many applications encountered on the Internet today, including "shopping carts," home banking, and so on.
Some options available to maintain application state based on server affinity are:
- Stickyness to source IP address
- Cross port affinity
- Passive cookie affinity
- Active cookie affinity
- URI affinity
- SSL session ID
The passive cookie, active cookie, and URI affinity options are rules-based. They depend on the content of the client requests.