For scalability reasons, UnifiedLogon database can be split into two databases - the primary OLTP database and the historical database.
The primary database maintains all user accounts, permissions, configuration and other data required in real-time operation.
The historical database provides data stores for events history, activity history and audit logs.
The historical database may grow to significant size for a multi-million user base (over one terabyte), while the real-time database would normally stay below 100GB for a similar user base.
Although real-time to historical database data transfer happens via staging data stores, the staging data stores stay in the real-time database to eliminate expensive cross-database transactions involving real-time tables.
Further scaling of the UnifiedLogon database is possible by allocating data store instances of the UnifiedLogon database, each hosting one or many data stores.
A single instance of the primary UnifiedLogon database may host a virtually unlimited number of data store databases.
We are planning to introduce more options for splitting the primary UnifiedLogon database. This will bring even more scalability options into system design, which relies on UnifiedLogon as a backbone for user accounts, SSO, authentication and configuration.