When a "write" operation occurs in MongoDB, where is the data first saved?

In MongoDB, when a write operation occurs, the data is first saved to the primary node. This is because the primary node is responsible for handling all write operations in a replica set. When a write request is issued, the primary node processes the operation and confirms the change locally before it is replicated to the secondary nodes. This ensures that there is a single point for write operations, maintaining consistency and avoiding conflicts that could arise if multiple nodes accepted writes simultaneously.

The primary node serves as the leading authority for data changes, allowing it to manage and structure the write operations efficiently. Once the write is successful on the primary node, the data is then replicated to secondary nodes in the background. Hence, the correct choice reflects the fundamental architecture of MongoDB’s write operations and their handling within a distributed database setup.