Redis should be the primary database for three key reasons: 1. It is extremely fast and scales linearly even as more nodes are added. Performance remains highly consistent across scaling tests. 2. It is highly available with techniques like quorum-based replication across nodes rather than shards, in-memory replication for faster performance, and watchdogs to monitor the cluster. 3. It uses CRDTs to provide strong eventual consistency for active-active replication across replicas in under 1 millisecond, solving the conflicts that arise much faster than other approaches.