Modern app architectures rely on data with different models and access patterns. Polyglot persistence, first introduced in 2011, states that each such data model should be powered by an independent database that is purpose-built for that model. The original intent was to look beyond relational/SQL databases to the emerging world of NoSQL.
Polyglot Persistence in Action at an E-Commerce App (Source: Martin Fowler)
The Messy Reality of Polyglot Persistence
Polyglot persistence is not free of costs — it leads to increased complexity across the board.