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what is ACID?
(Publish Date: 2008-9-1 12:36pm, Total Visits: 921, Today: 1, This Week: 3, This Month: 3)

Transactional systems often are described as being ACID compliant.

where "ACID" stands for the following properties:

  • Atomic. All the statements execute successfully or are canceled as a unit.

  • Consistent. A database that is in a consistent state when a transaction begins is left in a consistent state by the transaction.

  • Isolated. One transaction does not affect another.

  • Durable. All the changes made by a transaction that completes successfully are recorded properly in the database. Changes are not lost.

Oracle is READ COMMITTED as default. READ COMMITTED allows a transaction to see changes made by other transactions only if they've been committed. Uncommitted changes remain invisible. This isolation level allows non-repeatable reads and phantoms to occur.

And MySQL InnoDB operates by default in REPEATABLE READ mode: Each transaction sees a view of the database that consists of all changes that have been committed by the time the transaction issues its first consistent read (such as a SELECT statement), plus any changes that it makes itself. It does not see any uncommitted changes, or committed changes made by transactions that begin later than itself.