Which isolation level prevents users from reading data when another user is writing to it?

The isolation level that prevents users from reading data while another user is writing to it is known as Serializable. This level ensures the highest degree of isolation by effectively locking the data, thereby prohibiting other transactions from accessing the data until the transaction is complete. Specifically, when a transaction is in progress, Serializable prevents the reading of any data that is being modified, ensuring that no dirty reads or non-repeatable reads occur.

In a Serializable transaction, it behaves as if all transactions are executed one after another, even though they may run concurrently. This makes it ideal for scenarios that require strict consistency and data integrity, especially in applications that handle critical operations where accuracy of data is paramount.

On the other hand, the Read Uncommitted isolation level allows users to read data that is in the process of being written, which means users can see uncommitted changes. The Read Committed level blocks readers from accessing data that is being written but will let them see committed changes. Meanwhile, the Snapshot isolation level provides a view of data as it was at a specific point in time but does not prevent writers from making changes concurrently.

Thus, Serializable is the only isolation level that fully prevents users from reading data while another transaction is actively writing to it.