Handling state behind a balancer.
WHAT THE PROBLEM IS
A user's requests may reach different backends, and state held on one is not available on another.
WHAT SESSION PERSISTENCE DOES
Directs a user consistently to the same backend.
HOW IT IS IMPLEMENTED
A cookie set by the balancer Hashing of the client address
WHY ADDRESS HASHING IS UNRELIABLE
Many users share addresses, and addresses change.
WHAT PERSISTENCE COSTS
Uneven distribution Sessions lost when a backend fails Difficulty draining a backend for maintenance
WHAT THE BETTER ANSWER IS
Stateless backends, with state in shared storage.
WHERE TO PUT SESSION STATE
A shared store, or a signed token held by the client.
WHAT A SIGNED TOKEN PROVIDES
No server-side lookup, at the cost of not being revocable without additional mechanism.
WHAT TO PREFER FOR ANYTHING REQUIRING REVOCATION
Server-side sessions.
WHAT STATELESSNESS ENABLES
Adding and removing backends freely Draining for maintenance without disruption Recovery from a backend failure with no user impact
WHAT TO TEST
Removing a backend during active use.
