If you have a scenario where you have a load balancer with multiple backend servers (as we covered back in Chapter 8, Load Balancing), there can be some tricky scenarios where session tracking would be difficult to implement. While using the hash-based algorithm can ensure requests from the same IP is routed to the same backend, this doesn't always ensure a balanced distribution of requests.
One of the key features for NGINX Plus is session persistence, where requests from the same client need to be sent to the same server for the life of that session. Also known as "sticky" sessions, this can be especially important when it comes to payment systems, where the sharing of information between backend servers can be quite restrictive.
