What are transaction isolation levels?

Transaction isolation levels determine the degree to which data modifications made by one transaction become visible to other concurrently executing transactions. They help manage concurrency issues such as "dirty reads," "non-repeatable reads," and "phantom reads."

The main isolation levels defined by the SQL standard and supported by most relational databases:

• READ UNCOMMITTED:
  • The lowest isolation level.
  • Allows "dirty reads": a transaction can see data modified by another transaction that has not yet committed.
  • No read locks are used.
• READ COMMITTED:
  • Protects against "dirty reads."
  • Reads only committed data.
  • Can result in "non-repeatable reads": when reading the same data within a transaction, its value may change because another transaction has committed its changes.
  • Can cause "phantom reads": when re-executing a query with a WHERE condition, a row added or removed by another committed transaction may appear or disappear.
• REPEATABLE READ:
  • Protects against "dirty reads" and "non-repeatable reads."
  • When data is read for the first time, the transaction places a lock on it, preventing other transactions from modifying it until the current transaction completes.
  • Can result in "phantom reads."
• SERIALIZABLE:
  • The highest isolation level.
  • Fully prevents "dirty reads," "non-repeatable reads," and "phantom reads."
  • Transactions are executed sequentially as if they were fully isolated.
  • Uses stricter locking, which may reduce performance under high concurrency.

Comparison of isolation levels and concurrency issues:

Isolation Level	Dirty Read	Non-repeatable Read	Phantom Read
READ UNCOMMITTED	Yes	Yes	Yes
READ COMMITTED	No	Yes	Yes
REPEATABLE READ	No	No	Yes
SERIALIZABLE	No	No	No

Choosing an isolation level depends on the application's requirements for data consistency and performance. Lower levels of isolation offer better performance at the potential cost of concurrency issues, while higher levels guarantee greater consistency but may increase overhead.

Example of setting an isolation level in Go using database/sql:

// Assuming we have a *sql.DB connection
 tx, err := conn.BeginTx(context.Background(), &sql.TxOptions{Isolation: sql.LevelRepeatableRead})
if err != nil {
 // Handle error
}
defer tx.Rollback() // Rollback the transaction if not committed

// Perform operations within the transaction
// ...

err = tx.Commit()
if err != nil {
 // Handle error
}