Use persistence to make sure that a user is connected to the same real server every time they make an HTTP, HTTPS, or SSL request that is part of the same user session. For example, if you are load balancing HTTP and HTTPS sessions to a collection of eCommerce web servers, when a user is making a purchase they will be starting multiple sessions as they navigate the eCommerce site. In most cases all of the sessions started by this user during on eCommerce session should be processed by the same real server. Typically, the HTTP protocol keeps track of these related sessions using cookies. HTTP cookie persistence makes sure that all sessions that are part of the same user session are processed by the same real server
When you configure persistence, the FortiGate unit load balances a new session to a real server according to the load balance method. If the session has an HTTP cookie or an SSL session ID, the FortiGate unit sends all subsequent sessions with the same HTTP cookie or SSL session ID to the same real server. For more information about HTTP and HTTPS persistence, see “HTTP and HTTPS persistence”.