Highly Scalable LDAP, Alternatives

Folks, Any experience related to implementing a highly scalable (let's say directory has billion records), high performance( in terms of read latency) LDAP server? What other alternatives should I be thinking of?

Hi, I do not have a use case as such. I have used LDAP years ago and was curious if there are any better products now in the market and what the challenges with LDAP servers are.