Definition of Isolation Levels - Amazon Neptune
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Definition of Isolation Levels

The "I" in ACID stands for isolation. The degree of isolation of a transaction determines how much or little other concurrent transactions can affect the data that it operates on.

The SQL:1992 Standard created a vocabulary for describing isolation levels. It defines three types of interactions (that it calls phenomena) that can occur between two concurrent transactions, Tx1 and Tx2:

  • Dirty read – This occurs when Tx1 modifies an item, and then Tx2 reads that item before Tx1 has committed the change. Then, if Tx1 never succeeds in committing the change, or rolls it back, Tx2 has read a value that never made it into the database.

  • Non-repeatable read – This happens when Tx1 reads an item, then Tx2 modifies or deletes that item and commits the change, and then Tx1 tries to reread the item. Tx1 now reads a different value than before, or finds that the item no longer exists.

  • Phantom read – This happens when Tx1 reads a set of items that satisfy a search criterion, and then Tx2 adds a new item that satisfies the search criterion, and then Tx1 repeats the search. Tx1 now obtains a different set of items than it did before.

Each of these three types of interaction can cause inconsistencies in the resulting data in a database.

The SQL:1992 standard defined four isolation levels that have different guarantees in terms of the three types of interaction and the inconsistencies that they can produce. At all four levels, a transaction can be guaranteed to execute completely or not at all:

  • READ UNCOMMITTED – Allows all three kinds of interaction (that is, dirty reads, non-repeatable reads, and phantom reads).

  • READ COMMITTED – Dirty reads are not possible, but nonrepeatable and phantom reads are.

  • REPEATABLE READ – Neither dirty reads nor nonrepeatable reads are possible, but phantom reads still are.

  • SERIALIZABLE – None of the three types of interaction phenomena can occur.

Multiversion concurrency control (MVCC) allows one other kind of isolation, namely SNAPSHOT isolation. This guarantees that a transaction operates on a snapshot of data as it exists when the transaction begins, and that no other transaction can change that snapshot.