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> The library ships with an in-memory backend (for development and testing) and a postgresql-ocaml[0] based backend.

How about signed cookies as a storage backend?
Python Flask and Django can use it to store session entirely in the cookies with an hmac signature and expiration time, so your server can be entirely stateless.
As long as the amount of data in your session is small, and all you need is authenticated data (and not secret data) I think its quite an elegant solution,
and more fitting with a functional style.

Now of course comes the question Cryptokit or nocrypto :)