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I think it would be more accurate to say you could build a basic Zookeeper on top of FoundationDB rather than use it as a replacement.

It depends on what level you're using Zookeeper at. Because it's fairly trival to slap together a consistent store on top of FundationDB because, well that's what it is.

But if you need ephemeral nodes tired to heartbeated sessions and sequentially consistent set of watches to be triggered... Well that's quite a bit harder.

I'm less familiar with etcd, but I suspect the devil is in the details.

Apache's CouchDB is currently planning their v4 release to be based on FoundationDB

Wavefront is another public user. If you poke around on the forums and Google peoples' names you can see where they work.

There are a number of large companies you have definitely heard of in a variety of domains including ad-tech, analytics, gaming, and logistics who are evaluating or using FoundationDB in production.

FoundationDB can replace ZooKeeper or etcd. Not literally of course, given their APIs vary wildly, but you can achieve the same semantics. Plus you get arbitrary conversational transactions instead of just things CAS.