What is a transaction in a database? What are transaction isolation levels and how do they differ?

A transaction in a database is a sequence of operations that are executed as a single unit. It either completes successfully (commit) or is completely rolled back (rollback), ensuring data integrity.

Transaction isolation levels determine how changes made by one transaction are visible to other concurrently executing transactions, and help manage concurrency issues:

1. Read Uncommitted — the lowest isolation level. A transaction can see uncommitted changes made by other transactions (dirty read).

2. Read Committed — a transaction only sees data that has been committed by other transactions. It avoids dirty reads but can still encounter non-repeatable reads.

3. Repeatable Read — guarantees that data read by a transaction will not change until the transaction completes. It avoids non-repeatable reads but can still have phantom reads.

4. Serializable — the strictest level. Transactions are executed as if they were sequential, fully preventing phantom reads and providing maximum isolation.

The differences between levels involve a trade-off between performance and data integrity. The higher the isolation level, the less concurrency and the higher the system load.

In Go, when working with databases via the database/sql package, isolation levels can be set through transaction parameters if the driver supports it.