At Twitter, we use Apache ZooKeeper as a cornerstone for nearly all of our distributed systems. SREs run ZooKeeper clusters and write tools for diagnosing bad actors. They also extend the service to make it more reliable. During this talk, we'll go over the tools we've written (and open sourced!) to monitor our systems and debug problems seen at large scale. In addition, as part of our work to rate-limit our myriad clients, we’ve developed several protocol extensions for ZooKeeper without breaking backwards compatibility. We’ll discuss the design considerations involved when extending a service with a multitude of client library versions, as well as how it has enabled us to improve our ability to quickly experiment and iterate.