How can a schema be enforced in MongoDB Atlas?

In MongoDB Atlas, schema enforcement can be achieved through optional schema validation, which allows you to define rules for the structure of documents stored in a collection. This validation is implemented using JSON Schema, enabling developers to specify the required fields, types, and constraints for the documents.

While schema validation is optional, it provides a mechanism to ensure that the data being inserted or modified conforms to defined rules, helping maintain data integrity without enforcing a rigid structure. This flexibility is crucial in a document-oriented database like MongoDB, where schemas may evolve over time, allowing for adaptations without disruptions.

The other options do not align as closely with the native functionality of MongoDB Atlas for enforcing schema. Schema migrations, for instance, refer more to processes of changing a database schema over time rather than enforcing it. Mandatory fields can be part of validation rules but themselves do not represent a complete enforcement mechanism. Defining strict data types is not a feature of MongoDB in the same vein as you might find in other relational databases, as MongoDB allows for more fluidity in data types due to its NoSQL nature. The optional schema validation thus stands out as the most accurate representation of how schema enforcement can be approached within MongoDB Atlas.