Redis
ProtonAuctions is in public beta. See Beta Phase for what is stable and what is still maturing.
On a network where several servers share one database, Redis is what keeps their auction views in step. When a listing appears, sells, or ends on one server, that server sends a short signal over Redis and the others update at once, so nobody is left looking at a stale list.
Redis is a live-signal layer, not a data store. The shared Database is still where every listing, bid and sale actually lives, and Redis only carries small "this auction changed" messages on top of it. It is off by default and only matters on a multi-server network. A single server has nothing to sync with and does not need it.
Redis is one half of a network setup. The shared database is the other half, and Cross-Server Sync covers how the two fit together.
Redis 6 or newer is required. ProtonAuctions publishes and subscribes on one connection, which older Redis does not allow.