Wait, you can't shard on session ID?

And this is an ephemeral key-value store here, which is basically a best-case scenario from a performance standpoint. It's basically the last thing you're going to need to think about sharding, which is why session stores traditionally cohabitate(d) with web servers.

No, session storage doesn't get expensive fast. It's extraordinarily cheap unless you screw up the configuration very badly indeed (Apparently PHP still defaults to writing session data to disk?!)