how do web application sessions work when running on more than one server?

Question

This is a general question based on how web sessions work across multiple servers, my knowledge around web sessions is not very deep but afaik a web session is typically stored directly in memory of the running web server application so when a request comes in it doesn't have to make database requests to fetch the session data. If a popular website needs multiple servers to handle the level of traffic it is receiving, when a request comes in I assume that it could get directed to any of the servers by some load balancer, but how does the server handling that request get the associated session data if the previous request was handled by a different server? do multi server sites require special session handling infrastructure, or do the load balancers know some how to route requests from the same client to the same server?

Answer 1

This question on ServerFault is the same as this question. with a good answer. in overview there are 3 common methods:

• Session information stored in cookies only
• Load balancer always directs user to the same machine
• Shared backend database or key/value store.

See link for more indepth details of each.