Oracle enforces ACID by means of Undo Segments & Redo Logs.
Undo Segments help enforce-- atomicity and consistency .
Isolation requires undo segments & locks.
Durability is enforced with redo logs.
Oracle provides the following transaction isolation levels.
Read committed
Default transaction level is Read Committed.
Each query executed will only see the committed data. In other words an Oracle query will never read uncommitted data.
Serializable
Serializable transactions can only see those changes that were committed at the time the transaction began and those changes that are being made by the transaction itself.
Read-only
Read-only transactions see only those changes that were committed at the time the transaction began
Isolation levels can be set at the begining of the transaction and at session level.
Commands for transaction level setting of isolation.
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET TRANSACTION READ ONLY;
Commands for session level setting;
ALTER SESSION SET ISOLATION_LEVEL SERIALIZABLE;
ALTER SESSION SET ISOLATION_LEVEL READ COMMITTED;
