When working with databases, especially relational database management systems (RDBMS), you will often hear the term "ACID". ACID is a set of properties that ensure that database transactions are processed reliably. These four properties include:
Let's explore each of these properties in detail and see how important they are.
Atomicity ensures that a transaction is treated as an indivisible unit of work. This means that either all operations in the transaction succeed, or none of them are performed. If any part of the transaction fails, the entire transaction is aborted and the database is restored to the state it was in before the transaction began.
For example:
Suppose you make a transaction to transfer money from account A to account B. This transaction involves two operations:
If a debit from account A succeeds, but a credit to account B fails (e.g. due to a power outage), atomicity ensures that the amount deducted from account A is returned, and the transaction is aborted. This ensures that no funds are “lost” during the transaction.
Consistency ensures that a transaction can only move the database from one valid state to another. It ensures that the data in the database always complies with defined constraints and rules.
For example:
Suppose you have a database that manages information about students. The database has a constraint that the age of a student must be greater than 17. If a transaction tries to add a student whose age is 16, consistency will block this transaction, because it violates the age constraint. The database will not allow this transaction to change its state, because it will result in an invalid state.
Isolation ensures that concurrent transactions execute independently of each other. This means that one transaction should not see the unfinished changes of another transaction. Each transaction should feel like it is operating on a separate database.
For example:
Suppose there are two transactions accessing and modifying the same record. Transaction 1 is in the process of increasing the price of a product by 10%, but it is not yet completed (not yet committed). Transaction 2 should not see the new price (10% increase) until transaction 1 completes and commits those changes. This ensures that transaction 2 operates on consistent data and is not affected by the incomplete changes of transaction 1.
There are different levels of isolation, and choosing the right isolation level is a trade-off between consistency and performance. Higher isolation levels (e.g. Serializable) ensure better consistency, but can reduce performance due to more locking. Lower isolation levels (e.g. Read Uncommitted) allow better performance, but can lead to problems like dirty reads.
Durability ensures that once a transaction has been committed (completed), the changes it made are stored permanently and are not lost even if there is a system failure (e.g. power failure, hardware failure). This is usually done by writing changes to the transaction log and backing up the data periodically.
For example:
Let's say you make an online purchase and successfully pay. Even if the server crashes right after you complete the transaction, persistence ensures that your order information is stored securely and not lost. When the system is restored, your order will still be processed normally.
The four ACID properties are the foundation for building reliable database systems. They ensure that data is managed consistently, accurately, and sustainably. Understanding ACID is important for anyone working with databases, from application developers to database administrators.
Hopefully this article has provided you with a comprehensive and easy-to-understand overview of ACID properties. Good luck in applying ACID to your database projects!