Vive la Diff\'erence: Paxos vs. Viewstamped Replication vs. Zab

Paxos, Viewstamped Replication, and Zab are replication protocols that ensure high-availability in asynchronous environments with crash failures. Various claims have been made about similarities and differences between these protocols. But how does one determine whether two protocols are the same, and if not, how significant the differences are? We propose to address these questions using refinement mappings, where protocols are expressed as succinct specifications that are progressively refined to executable implementations. Doing so enables a principled understanding of the correctness of the different design decisions that went into implementing the various protocols. Additionally, it allowed us to identify key differences that have a significant impact on performance.