There are a few questions about configuring the system.
first, I am trying to configure two ignition servers up to redundancy. Is it possible to link two servers with one database?
second, I am trying to configure the database as a separate hardware. Is it possible? (It is not a database inside the server.) If possible, is there a set model?
third, I can purchase the SQL Bridge module on the server, but what do you need to prepare for the database?
Yes, and recommended for redundancy. The DB itself should handle its own redundancy/high availability functionality.
Yes, and highly recommended for production.
Nothing beyond the normal permissions (including table creation) for the user account Ignition connects with. The SQL Bridge module can create/alter tables for you within the transaction group operation.