ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that ensure reliable transactions in database systems. These properties are essential for maintaining data integrity and preventing data corruption. In this blog post, we will delve into each ACID property and understand its significance.
Atomicity
Atomicity guarantees that a transaction is treated as a single, indivisible unit of work. Either all the changes made by the transaction are committed to the database, or none of them are. If any part of the transaction fails, the entire transaction is rolled back, and the database is left unchanged.
The atomicity property prevents partial updates to the database, ensuring that data remains consistent. For example, if a bank transfer involves deducting money from one account and adding it to another, both operations will be treated as a single atomic transaction. If either deduction or addition fails, the entire transaction will be rolled back, avoiding any inconsistencies in the account balances.
Consistency
Consistency refers to maintaining the integrity and correctness of data during and after a transaction. A consistent database ensures that all data and constraints are valid according to predefined rules. When a transaction is executed, it should not violate any integrity constraints specified in the database schema.
For instance, consider a database with a constraint that every student must have a unique student ID. If a transaction attempts to insert a new student with a duplicate ID, the consistency property ensures that the transaction is rolled back, and the database remains in a consistent state.
Isolation
Isolation ensures that concurrent transactions do not interfere with each other. Each transaction should be executed in isolation, as if it is the only transaction running on the database. Any changes made by a transaction should be invisible to other transactions until the changes are committed.
Isolation prevents various concurrency problems, such as dirty reads, non-repeatable reads, and phantom reads. Dirty reads occur when a transaction reads uncommitted changes made by another transaction, potentially leading to inconsistent data. Non-repeatable reads happen when a transaction reads the same data multiple times and gets different results because another transaction modified it between the reads. Phantom reads occur when a transaction reads a set of records multiple times and gets different records each time, due to other transactions inserting or deleting records.
Durability
Durability ensures that once a transaction is committed, its effects are permanent and survive any subsequent failures. Once the database acknowledges the successful completion of a transaction, the changes made by that transaction become permanent and cannot be undone.
The durability property guarantees that even in the event of a system crash or power outage, the committed data will not be lost or corrupted. This is typically achieved by logging changes made by transactions to a transaction log or using other mechanisms to persistently store the data.
Conclusion
Understanding the ACID properties is essential for designing and implementing reliable database systems. These properties provide a robust foundation for managing transactions, ensuring data integrity, and maintaining consistency in databases. By adhering to the ACID principles, database systems can provide reliability and durability in the face of various failures and concurrent transactions.
本文来自极简博客，作者：星辰守望者，转载请注明原文链接：Understanding ACID Properties in Database Systems
