What does schema-less mean for a document database?

Document databases are sometimes called schema-less, but that’s misleading, as the code that reads the data usually assumes some kind of structure—i.e., there is an implicit schema, but it is not enforced by the database.

A more accurate term is schema-on-read (the structure of the data is implicit, and only interpreted when the data is read), in contrast with schema-on-write (the traditional approach of relational databases, where the schema is explicit and the database ensures all written data conforms to it).

Schema-on-read is similar to dynamic (run-time) type checking in programming languages, whereas schema-on-write is similar to static (compile-time) type checking.

Just as the advocates of static and dynamic type checking have big debates about their relative merits, enforcement of schemas in database is a contentious topic, and in general there’s no right or wrong answer.

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Completely agree with the above the statements beautifully put by the author. An interesting excerpt from the book "Designing Data-Intensive Applications" by Martin Kleppmann.

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