Session persistence (stickiness)
termnetworkingcloud
Sending a returning client back to the same backend server, because that server holds state the others do not have.
It is best understood as an admission that the application is not stateless, and every method carries a cost. Source-address persistence concentrates whole carrier-NAT populations onto one server. Cookie persistence requires terminating the connection. The operational consequence people miss is that persistence and capacity work against each other: adding servers during a peak leaves them idle, because the load you wanted to move is pinned where it already is. Moving state to a shared store is the fix; persistence is what you run until then.
Also known as: stickiness, affinity, source ip persistence, cookie persistence