I have a single EC2 instance. I identify users with a session key that is stored in a redis cache. When I autoscale to more EC2 instances, how will I be able to access the session keys?

For example, let's say I have EC2 instances `A` and `B`. My user, Lexy, logs signs in via the instance `A`, but then sends a request to instance `B` that requires that she's logged in. So instance `B` checks its redis cache for the session key in the header of Lexy's request and finds that it's not in instance `B`'s cache. How shoud I make sure Lexy appears logged in on all EC2 instances?