This document summarizes different approaches to achieving high availability in HBase. It discusses HBase region replicas, asynchronous WAL replication, and timeline consistency introduced in HBase 1.1 to provide increased read availability. It also describes WanDisco's non-stop HBase implementation using Paxos consensus and Facebook's HydraBase which uses RAFT consensus, with HydraBase designating region replicas as active or witness. The document compares these approaches on attributes like consensus algorithm, read/write availability, strong consistency, and support for multi-datacenter deployments.