Tackling with different problems, using multiple programming languages and multiple database types in large scale applications, is not something strange, and based on needs there could be more than two or three languages in such an application. Although in a simple web application, there is a server-side programming language and a client-side. The term Polyglot Persistence refers to the applications with multiple data storage technologies, used by an application that could be components of a single application as well.
A large-scale application such as Google, Facebook, Instagram, etc. which handles different types of data such as image, video, and text in different client applications throughout the world, has to use different data storage and different sort of management with different sorts of programming languages. So the idea of writing such an application by mixing different programming languages and different databases is Polyglot Persistence.
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