ADFS servers require a database, and can be configured to use either the WindowsInternalDatabase (WID) or full SQLServer. If WID is used, then ADFS servers in a farm are configured as primary or secondary. A primaryfederationserver is initially the first federationserver in the farm, and has a read/write copy of the ADFS configuration database. All other federation servers created in the farm (the secondary servers) regularly poll the primaryserver and synchronize any changes to a read-only copy of the ADFS configuration database stored locally.