Regarding Zookeeper: We actually build a locking system into Titan that uses quorum reads/writes with time-outs and cleanup to ensure consistency for certain edge/property types as defined inside Titan. This gives you consistency guarantees out of the box without having to introduce another component (like Zookeeper) into your deployment. For infrequent lock usage (which I strongly encourage ;-) this should be sufficient. For frequent locking, something like Zookeeper is far superior.