How is schema enforced in Delta Lake?

Schema enforcement in Delta Lake is crucial for maintaining data integrity and ensuring that the data conforms to a predefined structure. The correct answer highlights that schema enforcement occurs by using constraints during write operations. When data is written to a Delta table, Delta Lake checks if the data aligns with the existing schema. If there are discrepancies, such as data types not matching or missing required fields, the write operation will fail. This prevents corrupted or non-conforming data from being added to the table, maintaining the integrity of the dataset.

The use of constraints, such as those specifying data types or whether fields are nullable, ensures that only valid data can be stored in the Delta table. This built-in schema enforcement is one of the key features that makes Delta Lake a robust solution for managing large datasets in a controlled manner.

Other approaches, such as automated data cleanup processes, user-defined functions for validation, or monitoring data for consistency, do not directly enforce the schema at write time. Instead, they may serve different purposes, such as maintaining data cleanliness or validating data post hoc. These approaches can complement schema enforcement but do not provide the foundational guarantee that is achieved through constraints during write operations.