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ACID Properties of Transactions

Most of what we’re calling transactional locking relates to the ability of a database management system (DBMS) to ensure reliable transactions that adhere to these ACID properties. ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. Each of these properties is described in more detail below. However, these properties are related and must be considered together. They are more like different views of the same object than independent things.

Atomicity

Atomicity means all or nothing. Transactions often contain multiple separate actions. For example, a transaction may insert data into one table, delete from another table, and update a third table. Atomicity ensures that either all actions occur or none occur.

Consistency

Consistency means that transactions always take the database from one consistent state to another. So, if a transaction violates the databases consistency rules, then the entire transaction will be rolled back.

Isolation

Isolation means that concurrent transactions, and the changes made within them, are not visible to each other until they complete. This avoids many problems, including those that could lead to violation of other properties. The implementation of isolation is quite different in different DBMS'. This is also the property most often related to locking problems.

Durability

Durability means that committed transactions will not be lost, even in the event of abnormal termination. That is, once a user or program has been notified that a transaction was committed, they can be certain that the data will not be lost.