Atomicity, consistency, isolation, durability

In computer science, ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties of database transactions. In the context of databases, a sequence of database operations that satisfies the ACID properties and, thus, can be perceived as single logical operation on the data, is called a transaction. For example, a transfer of funds from one bank account to another, even involving multiple changes such as debiting one account and crediting another, is a single transaction.

In 1983,Andreas Reuter and Theo Härder coined the acronym ACID as shorthand for Atomicity, Consistency, Isolation, and Durability, building on earlier work by Jim Gray who enumerated Atomicity, Consistency, and Durability but left out Isolation when characterizing the transaction concept. These four properties describe the major guarantees of the transaction paradigm, which has influenced many aspects of development in database systems.

According to Gray and Reuter, IMS supported ACID transactions in 1973 already (although the term ACID came later).

The characteristics of these four properties as defined by Reuter and Härder are as follows:

Atomicity requires that each transaction be "all or nothing": if one part of the transaction fails, then the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors and crashes. To the outside world, a committed transaction appears (by its effects on the database) to be indivisible ("atomic"), and an aborted transaction does not happen.

The consistency property ensures that any transaction will bring the database from one valid state to another. Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof. This does not guarantee correctness of the transaction in all ways the application programmer might have wanted (that is the responsibility of application-level code), but merely that any programming errors cannot result in the violation of any defined rules.

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