In large scale applications, using multiple programming languages and multiple database types in order to tackle with different problems 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 which 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 through the world, has to use different data storage and different sort of management with different sort of programming languages. So the idea of the writing such an application by mixing different programming languages and different databases is Polyglot Persistence.
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