What is a primary database?

There is no formal definition of what a primary database is and what requirements it should have; the interpretation of what features a primary database should support largely depends on the use case the data store fulfills. However, by looking at how traditional DBMSs are used as primary and often unique solutions in the data layer, we can sketch a few traits:

• A database can be considered primary when it is the authoritative source of data and stores the most recent copy of data. Secondary databases instead serve read-only workloads, either using the same technology as the primary database (using proprietary master-replica protocols) or as in-memory caches (using methods such as change data capture).
• A primary database serves mixed online transactional processing (OLTP) workloads, such as searches or scans and lookups, and may have analytical processing capabilities.
• A primary database is reliable and can stand single software or hardware...