What is a transaction isolation level and why is it important?

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The main concept is that a transaction isolation level controls how visible the changes made by one transaction are to other concurrent transactions. It sets the rules for reading data while other transactions are in progress, balancing data consistency with performance and throughput. The levels progress from Read Uncommitted to Serializable, and each step up reduces the chances of anomalies such as reading uncommitted or changing data, but at the cost of more coordination and potential slower performance. At lower levels, you might encounter dirty reads, non-repeatable reads, or phantom reads; at the highest level, you get strict, serial-like behavior where transactions appear to run one after another. This choice matters because it determines whether a transaction can see uncommitted updates, whether repeated reads within the same transaction are stable, and how much overhead is required to coordinate access across concurrent transactions. The other options describe encryption during storage, a cap on concurrent transactions, or the ordering of commits for performance—none capture the way visibility and consistency are controlled across transactions.