In
database systems
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spa ...
, consistency (or correctness) refers to the requirement that any given
database transaction
A database transaction symbolizes a unit of work, performed within a database management system (or similar system) against a database, that is treated in a coherent and reliable way independent of other transactions. A transaction generally repr ...
must change affected data only in allowed ways. 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 database constraints.
[C. J. Date, "SQL and Relational Theory: How to Write Accurate SQL Code 2nd edition", ''O'reilly Media, Inc.'', 2012, pg. 180.]
Consistency can also be understood as after a successful write, update or delete of a Record, any read request immediately receives the latest value of the Record.
As an ACID guarantee
Consistency is one of the four guarantees that define
ACID transactions; however, significant ambiguity exists about the nature of this guarantee. It is defined variously as:
* The guarantee that
database constraints
A relational database is a (most commonly digital) database based on the relational model of data, as proposed by E. F. Codd in 1970. A system used to maintain relational databases is a relational database management system (RDBMS). Many relatio ...
are not violated, particularly once a transaction commits.
* The guarantee that any transactions started in the future necessarily see the effects of other transactions committed in the past.
As these various definitions are not mutually exclusive, it is possible to design a system that guarantees "consistency" in every sense of the word, as most
relational database management systems in common use today arguably do.
As a CAP trade-off
The
CAP theorem is based on three trade-offs, one of which is "atomic consistency" (shortened to "consistency" for the acronym), about which the authors note, "Discussing atomic consistency is somewhat different than talking about an ACID database, as database consistency refers to transactions, while atomic consistency refers only to a property of a single request/response operation sequence. And it has a different meaning than the Atomic in ACID, as it subsumes the database notions of both Atomic and Consistent."
In the CAP theorem, you can only have two of the following three properties: consistency, availability, or partition tolerance. Therefore, consistency may have to be traded off in some database systems.
See also
*
Consistency model
In computer science, a consistency model specifies a contract between the programmer and a system, wherein the system guarantees that if the programmer follows the rules for operations on memory, memory will be consistent and the results of read ...
*
CAP theorem
*
Referential integrity
Referential integrity is a property of data stating that all its references are valid. In the context of relational databases, it requires that if a value of one attribute (column) of a relation (table) references a value of another attribute (e ...
*
Eventual consistency
Eventual consistency is a consistency model used in distributed computing to achieve high availability that informally guarantees that, if no new updates are made to a given data item, eventually all accesses to that item will return the last up ...
References
{{DEFAULTSORT:Consistency (Database Systems)
Data management
Transaction processing