Redis was used as a metadata store and pub/sub broker in Ray until version 1.11.0, but starting from Ray 1.11, Redis is no longer the default. The changes allow Ray to focus on adding better support for fault tolerance and high availability in the future. In the pre-1.0 Ray architecture, Redis was used as a backing store for key-value storage and message pub/sub. However, this approach had several shortcomings, including finite CPU and memory limitations, inefficient use of Redis pub/sub, and increased coupling between components with direct access to Redis. The new architecture in Ray 1.11 eliminates these issues by storing cluster metadata inside the Global Control Store (GCS) instead of Redis and using internal broadcast/pub/sub implementations for actor/node/worker state updates. Although Redis is no longer stored as the default, it can still be used as an external metadata store. The updated configuration process also changes to use the RAY_REDIS_ADDRESS environment variable.