Isolation levels

Transaction Isolation

Isolation is one of the common ACID properties: Atomicity, Consistency, Isolation, and Durability. Isolation describes how changes applied by concurrent transactions are visible to each other.

Each isolation level prevents zero or more concurrency side effects on a transaction:

Dirty read: read the uncommitted change of a concurrent transaction Nonrepeatable read: get different value on re-read of a row if a concurrent transaction updates the same row and commits Phantom read: get different rows after re-execution of a range query if another transaction adds or removes some rows in the range and commits We can set the isolation level of a transaction by @Transactional::isolation. It has these five enumerations in Spring: DEFAULT, READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE.

levels

Read uncommitted

Given two transactions over the same row at the same time: T1- is performing an insert/update T2- is reading

T1 blocks the rows, but allows read the row T2 can read the row, but it can't perform an insert/update

Read commited

Given two transactions over the same row at the same time: T1- is performing an insert/update T2- is reading

T1 blocks the rows, and doesn't allow access the row by T2. T2 neither can't read nor perform an insert/update

Repeatable read

Given two transactions over the same row at the same time: T1- is performing a reading, and it's blocking the row T2- is reading or performing an insert/update

T1 blocks the rows, and doesn't allow access the row by T2. T2 neither can't read nor perform an insert/update

Serializable

Given two transactions over the same row at the same time: T1- is reading or performing an insert/update, it's blocking the table T2- is reading or performing an insert/update

T1 blocks the table, and doesn't allow access the table by T2. T2 neither can't read nor perform an insert/update