Why are multi-document operations not inherently atomic in MongoDB?

Multi-document operations in MongoDB are not inherently atomic because they do not utilize transactions by default. In MongoDB, atomicity is guaranteed at the document level, meaning that operations that modify a single document will either fully succeed or fully fail, but when it comes to operations that span multiple documents, MongoDB does not provide atomic guarantees unless transactions are explicitly used.

Transactions were introduced in MongoDB 4.0, allowing for atomic operations across multiple documents and even multiple collections. However, if transactions are not employed, changes made to multiple documents can occur independently, and it’s possible for the set of operations to be interrupted or partially completed. This is particularly relevant when designing systems that require strict data integrity across multiple documents, as operations can end up in a state where some changes are applied while others are not.

The other points do not accurately address the nature of atomicity in MongoDB. Notably, while it is true that additional processing power may be involved when transactions are utilized, the core reason for the lack of inherent atomicity in multi-document operations hinges on the absence of transactions rather than the execution environment or processing requirements.