ZooKeeper does not provide strong consistency, primarily to achieve high performance and higher availability.

While all writes must go through the current leader, the dissemination lag might mean that different clients can read stale or different values at some point. The state eventually becomes consistent if new writes are not pending.

Clients that need stronger consistency might do reads only at the leader. However, if many clients start doing so, it will impact the performance for everyone.

Clients can use the sync() method to enforce updates until a point in time. Calling the sync() method synchronizes all the write operations on all the servers.

If all replicas of ZooKeeper are physically nearby, e.g, in one data center where latency between any two nodes is ~ $1\ ms$ or less, clients might not see many consistency discrepancies. However, if some replicas are far away, e.g., in a different data center on the other coast, latency will be higher and consistency issues will be more visible.

Quiz on ZooKeeper