ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. It is a set of properties that ensure reliability and integrity in database transactions. ACID is used to guarantee that data is processed correctly and that transactions are completed successfully.
Atomicity
The atomicity property of ACID guarantees that a transaction is treated as a single, indivisible unit of work. In other words, either all the changes made to the data during the transaction are committed or none of them are. If a transaction is interrupted or fails, all changes made up to that point are rolled back or undone, so that the database remains in a consistent state.
For example, consider a bank transaction that involves transferring money from one account to another. If the transfer fails midway due to a system error, it would be unacceptable to leave the database in a state where the money has been deducted from the source account but has not been credited to the destination account. Atomicity guarantees that this scenario will not happen, as the entire transaction will be rolled back in case of failure.
Consistency
The consistency property of ACID ensures that a transaction brings the database from one valid state to another. This means that the transaction must preserve the integrity of the data and any constraints imposed on it.
For example, suppose we have a database of students’ grades, with a constraint that each student’s grade must be between 0 and 100. Now, suppose a transaction attempts to update a student’s grade to 120. Since this value violates the constraint, the transaction will be rejected and rolled back, and the database will remain in its original state.
Consistency also means that the database must satisfy any business rules or policies that are defined for it. For instance, a bank may have a policy that a customer cannot withdraw more money than their account balance. If a transaction tries to make such a withdrawal, it will be rejected and rolled back, as it violates the policy.
Isolation
The isolation property of ACID ensures that concurrent transactions do not interfere with each other. This means that each transaction must be executed as if it were the only transaction running on the system. In other words, the results of a transaction must be isolated from the results of other transactions running concurrently.
For example, consider two transactions that update the same record in a database. If the transactions are executed concurrently without isolation, one transaction may overwrite the changes made by the other, leading to inconsistent data. Isolation ensures that this scenario will not happen, as the database system will lock the record until the first transaction is completed, preventing the second transaction from accessing it.
Durability
The durability property of ACID ensures that once a transaction is committed, its changes are permanent and will survive any subsequent system failures. This means that the changes must be stored on a durable storage medium such as a hard disk, so that they can be recovered even if the system crashes.
For example, consider a database server that experiences a power outage. If the database is not durable, any changes made to it during the current session will be lost, as they were stored in volatile memory. Durability ensures that this scenario will not happen, as the changes made during the session will be recovered from the durable storage medium when the system restarts.
In summary, the ACID properties are a set of fundamental characteristics that ensure reliable and consistent data processing in database systems. Atomicity guarantees that a transaction is treated as a single unit of work, Consistency ensures that the transaction brings the database to a valid state, Isolation prevents concurrent transactions from interfering with each other, and Durability ensures that the transaction changes are permanent and survive any subsequent system failures.
Responsibility for maintaining properties
The responsibility for maintaining the ACID properties of a database system typically falls on the database management system (DBMS) and the application developers who use the system.
The DBMS is responsible for implementing the ACID properties by providing mechanisms for managing transactions, enforcing data consistency and integrity, ensuring isolation, and providing durable storage. For example, a DBMS may provide features such as logging, transaction management, locking, and backup and recovery mechanisms to ensure that the ACID properties are maintained.
Application developers, on the other hand, are responsible for designing and implementing transactions that maintain the ACID properties. This involves identifying the boundaries of a transaction, ensuring that the transaction is atomic, defining and enforcing data constraints, and handling errors that may occur during a transaction. Application developers must also be aware of the isolation level of transactions and design their applications accordingly to ensure that concurrency issues do not arise.
