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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.

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.