Consensus algorithms are foundational to distributed systems. Choosing among Paxos and its many variants ultimately determines the performance and fault-tolerance of the underlying system. Boaz, Jason, and Yannick will discuss the basic mechanics of quorum-based consensus algorithms as well as their tradeoffs compared to the primary-backup approach – both of which are used by Elasticsearch. They will show how these two layers work together to facilitate cluster state changes and data replication while guaranteeing speed and safety. They will finish with a deep dive into the data replication layer and the recent addition of sequence numbers, which are the foundation of faster operation-based recoveries and cross-data-center replication.