Symmetrical Session and Sticky Session

Distributed Sessions

Distributed sessions are intrinsically more complicated than single-server sessions. Single-server session can be implemented as a simple memory-based Hashtable. Distributed sessions must communicate between machines to ensure the session state remains consistent.

Load balancing with multiple machines either uses sticky sessions or symmetrical sessions. Sticky sessions put more intelligence on the load balancer, and symmetrical sessions puts more intelligence on the JVMs. The choice of which to use depends on what kind of hardware you have, how many machines you're using and how you use sessions.

Distributed sessions can use a database as a backing store, or they can distribute the backup among all the servers using TCP.
Symmetrical Sessions

Symmetrical sessions happen with dumb load balancers like DNS round-robin. A single session may bounce from machine A to machine B and back to machine B. For JDBC sessions, the symmetrical session case needs the always-load-session attribute described below. Each request must load the most up-to-date version of the session.

Distributed sessions in a symmetrical environment are required to make sessions work at all. Otherwise the state will end up spread across the JVMs. However, because each request must update its session information, it is less efficient than sticky sessions.
Sticky Sessions

Sticky sessions require more intelligence on the load-balancer, but are easier for the JVM. Once a session starts, the load-balancer will always send it to the same JVM. Resin's load balancing, for example, encodes the session id as 'aaaXXX' and 'baaXXX'. The 'aaa' session will always go to JVM-a and 'baa' will always go to JVM-b.

Distributed sessions with a sticky session environment add reliability. If JVM-a goes down, JVM-b can pick up the session without the user noticing any change. In addition, distributed sticky sessions are more efficient. The distributor only needs to update sessions when they change. So if you update the session once when the user logs in, the distributed sessions can be very efficient.
always-load-session

Symmetrical sessions must use the 'always-load-session' flag to update each session data on each request. always-load-session is only needed for jdbc-store sessions. tcp-store sessions use a more-sophisticated protocol that eliminates the need for always-load-session, so tcp-store ignores the always-load-session flag.

The always-load-session attribute forces sessions to check the store for each request. By default, sessions are only loaded from persistent store when they are created. In a configuration with multiple symmetric web servers, sessions can be loaded on each request to ensure consistency.
always-save-session