Zab is a crash-recovery atomic broadcast algorithm we designed for the ZooKeeper coordination service. ZooKeeper implements a primary-backup scheme in which a primary process executes clients operations and uses Zab to propagate the corresponding incremental state changes to backup processes1. Due the dependence of an incremental state change on the sequence of changes previously generated, Zab must guarantee that if it delivers a given state change, then all other changes it depends upon must be delivered first. Since primaries may crash, Zab must satisfy this requirement despite crashes of primaries. Applications using ZooKeeper demand high-performance from the service, and consequently, one important goal is the ability of having multiple outstanding client operations at a time. Zab enables multiple outstanding state changes by guaranteeing that at most one primary is able to broadcast state changes and have them incorporated into the state, and by using a synchronization phase while establishing a new primary. Before this synchronization phase completes, a new primary does not broadcast new state changes. Finally, Zab uses an identification scheme for state changes that enables a process to easily identify missing changes. This feature is key for efficient recovery. Experiments and experience so far in production show that our design enables an implementation that meets the performance requirements of our applications. Our implementation of Zab can achieve tens of thousands of broadcasts per second, which is sufficient for demanding systems such as our Web-scale applications.