Consistency (database systems)

ConsistencyinconsistentconsistentC'''onsistencyData consistencydata inconsistencyinconsistenciesinconsistency
Consistency in database systems refers to the requirement that any given database transaction must change affected data only in allowed ways.wikipedia
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Database transaction

transactiontransactionstransactional
Consistency in database systems refers to the requirement that any given database transaction must change affected data only in allowed ways. Consistency is one of the four guarantees that define ACID transactions; however, significant ambiguity exists about the nature of this guarantee.
A database transaction, by definition, must be atomic, consistent, isolated and durable.

ACID (computer science)

ACIDACID transactionsAtomicity, consistency, isolation, durability
Consistency is one of the four guarantees that define ACID transactions; however, significant ambiguity exists about the nature of this guarantee.
In computer science, ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties of database transactions intended to guarantee validity even in the event of errors, power failures, etc. In the context of databases, a sequence of database operations that satisfies the ACID properties (and these can be perceived as a single logical operation on the data) is called a transaction.

Database

database management systemdatabasesdatabase systems
Consistency in database systems refers to the requirement that any given database transaction must change affected data only in allowed ways.
The acronym ACID describes some ideal properties of a database transaction: atomicity, consistency, isolation, and durability.

CAP theorem

AP systemavailability under partitionCAP
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."
Consistency: Every read receives the most recent write or an error

Data integrity

integrityintegrity constraintsconstraint
Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof.

Rollback (data management)

rollbackrolled backroll back
Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof.

Database trigger

triggerstriggerprocedural triggers
Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof.

Relational database management system

RDBMSrelational database management systemsrelational database
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.

Relational database

relational databasesrelationaldatabase constraints
The guarantee that database constraints are not violated, particularly once a transaction commits

NoSQL

structured storagekey/value storenon-relational
Many NoSQL stores compromise consistency (in the sense of the CAP theorem) in favor of availability, partition tolerance, and speed.

Atomicity (database systems)

atomicityatomicatomic transaction
In database systems, atomicity (from ) is one of the ACID (Atomicity, Consistency, Isolation, Durability) transaction properties.

Multiversion concurrency control

MVCCMVCC architectureMulti Version Concurrency Control (MVCC)
Without concurrency control, if someone is reading from a database at the same time as someone else is writing to it, it is possible that the reader will see a half-written or inconsistent piece of data.

Transactional memory

Transactional Executiontransactions
The goal of transactional memory systems is to transparently support regions of code marked as transactions by enforcing atomicity, consistency and isolation.

Distributed data store

distributed data storagedata storedatabases
But the high-speed read/write access results in reduced consistency, as it is not possible to have both consistency, availability, and partition tolerance of the network, as it has been proven by the CAP theorem.

Transaction log

journalloglogging
If, after a start, the database is found in an inconsistent state or not been shut down properly, the database management system reviews the database logs for uncommitted transactions and rolls back the changes made by these transactions.

Apache Cassandra

Cassandra
Cassandra is typically classified as an AP system, meaning that availability and partition tolerance are generally considered to be more important than consistency in Cassandra, Writes and reads offer a tunable level of consistency, all the way from "writes never fail" to "block for all replicas to be readable", with the quorum level in the middle.

Master data management

master dataMaster data storereference domain
At a basic level, master data management seeks to ensure that an organization does not use multiple (potentially inconsistent) versions of the same master data in different parts of its operations, which can occur in large organizations.

Pervasive PSQL

Pervasive.SQL or PSQLrelational database engines
The MKDE operates in complete database transactions and guarantees full ACID (Atomicity, Consistency, Isolation, Durability).

Concurrency control

concurrencyconcurrent accessglobal concurrency control
Consistency - Every transaction must leave the database in a consistent (correct) state, i.e., maintain the predetermined integrity rules of the database (constraints upon and among the database's objects). A transaction must transform a database from one consistent state to another consistent state (however, it is the responsibility of the transaction's programmer to make sure that the transaction itself is correct, i.e., performs correctly what it intends to perform (from the application's point of view) while the predefined integrity rules are enforced by the DBMS). Thus since a database can be normally changed only by transactions, all the database's states are consistent.

Conflict-free replicated data type

CRDTCRDTs
Concurrent updates to multiple replicas of the same data, without coordination between the computers hosting the replicas, can result in inconsistencies between the replicas, which in the general case may not be resolvable.

Atomic commit

atomic actatomic commitmentatomic commitment protocol
Atomic commits in database systems fulfil two of the key properties of ACID, atomicity and consistency.

Couchbase Server

CouchbaseMembase
In the parlance of Eric Brewer’s CAP theorem, Couchbase is normally a CP type system meaning it provides consistency and partition tolerance, or it can be set up as an AP system with multiple clusters.