What are Redis SELECT databases?

Redis supports multiple logical databases within a single server instance, numbered 0 through 15 by default (configurable with databases in redis.conf). The SELECT index command switches the current connection to the specified database. Each database is completely isolated — keys in database 0 are not visible in database 1. By default, connections start on database 0. However, Redis Cluster does not support multiple databases — it only uses database 0. In practice, using multiple databases to separate concerns (e.g., cache in DB 0, sessions in DB 1) is generally discouraged in modern usage; running separate Redis instances or using key prefixes (e.g., cache:, session:) is preferred for clarity and compatibility with Cluster mode.