Why do we demand at least one backup server? The full answer to this question actually has its own recipe in this chapter. However, catastrophic failure is a fact of life and we must be ready for such an event. Even if the separate server is not a fully operational PostgreSQL node, it must exist and should be part of the reference design.
Likewise, we must have at least one PostgreSQL replica. Some of our designs work with either physical or logical replicas, so we won't differentiate between them here. Simply assume that every highly active PostgreSQL cluster must have at least two nodes that can fulfill the role of a primary database. Backups take time to restore, whereas replicas are generally writable in a minute or less. ...
