ACID is a fundamental concept in database systems, standing for Atomicity, Consistency, Isolation, and Durability. These properties ensure reliable processing of database transactions.
ACID Properties of Database Systems
Atomicity
Atomicity ensures that a transaction is treated as a single, indivisible unit of work, which either succeeds completely or fails completely. For instance, in a financial transaction, the transfer of funds from one account to another involves debiting one account and crediting another. Atomicity ensures that both these operations either succeed or fail as a unit, preventing any discrepancies.
Many modern databases achieve atomicity through a combination of logging and locking mechanisms. For instance, databases like MySQL and PostgreSQL use a transaction log that records all changes made within a transaction. If a failure occurs, the database can use the log to roll back the transaction and maintain atomicity.
Consistency
Consistency guarantees that a transaction brings the database from one valid state to another, adhering to certain integrity constraints. For example, if a rule states that the balance of a bank account should never be negative, the consistency property will prevent any transaction that results in a negative balance. Another rule could state that two accounts can never have the same account number.
Consistency is often achieved through the use of database triggers, constraints, and rules. For example, in PostgreSQL, you can define constraints on tables that must be satisfied for a transaction to commit.
Isolation
Isolation ensures that concurrent execution of transactions results in a system state that would be obtained if transactions were executed sequentially. This means that the execution of one transaction is isolated from the execution of other transactions. For example, if two people try to withdraw the last $100 from a bank account at the same time, isolation ensures that only one transaction will be successful.
This is typically achieved through concurrency control techniques like locking and multiversion concurrency control (MVCC). For instance, PostgreSQL uses MVCC to allow multiple transactions to access the same data without interfering with each other.
Durability
Durability guarantees that once a transaction has been committed, it will remain so, even in the event of a system failure. This means that the results of the transaction are permanent and persistent.
This is often achieved through write-ahead logging (WAL), where changes are logged before they are applied to the database. Both PostgreSQL and MySQL use WAL to ensure durability.
ACID support in SQL and NoSQL databases
- MySQL and PostgreSQL: These relational databases fully support ACID properties. They provide features like transaction logs, table constraints, MVCC, and WAL to ensure these properties.
- MongoDB: This NoSQL database supports ACID properties at the document level. As of version 4.0, it also supports multi-document transactions.
- Cassandra: This NoSQL database supports atomicity and isolation at the level of a single partition. However, it doesn’t fully support ACID properties. It uses a “last write wins” conflict resolution strategy to ensure atomicity at the partition level. ACID support is slated to be made available for Cassandra soon.
- Oracle Database and SQL Server: These relational databases fully support ACID properties. They provide features like transaction logs, constraints, locking mechanisms, and WAL.
- CouchDB: This NoSQL database provides ACID semantics. It uses an append-only storage model to ensure atomicity and durability.
- DynamoDB: This NoSQL database provided by Amazon Web Services supports atomicity and isolation for individual operations but does not support multi-document transactions.
- Riak: This NoSQL database is eventually consistent by default but can be configured to provide strong consistency.
Please note that the level of ACID compliance can vary based on the specific configurations and versions of the database systems.