ACID Properties in DBMS

A transaction represents a single, unified operation that retrieves and/or modifies data in a database. The access to data occurs through read and write actions. To preserve consistency in the database, the transaction follows specific rules known as ACID properties, both before and after execution.

Atomicity is a property of database transactions that ensures that a transaction is treated as a single, indivisible unit of work. In other words, if a transaction consists of multiple operations, either all of them are executed or none of them are executed. There is no intermediate state where some operations have been executed and others have not.

This property is important in ensuring the reliability and consistency of a database. If a transaction involves multiple operations that modify the contents of a database, atomicity ensures that either all changes are made or none of the changes are made. This helps to avoid partial updates that could leave the database in an inconsistent state.

For example, consider a bank transfer transaction that involves debiting an amount from one account and crediting it to another account. Atomicity ensures that if the debit operation is successful, the credit operation will also be executed, and if the debit operation fails, the credit operation will be rolled back, and the original state of the database will be preserved.

Consistency is a property of database transactions that ensures that the database remains in a consistent state, even after multiple transactions have been executed. A consistent state means that the data in the database satisfies a set of integrity constraints, such as unique keys, foreign keys, and check constraints.

Consistency ensures that data in the database is valid and accurate, and that transactions cannot leave the database in an inconsistent state. For example, if a transaction is attempting to insert a new record into a table with a unique constraint, the transaction must either insert the record and maintain the constraint, or it must be rolled back, preserving the constraint.

Ensuring consistency in a database requires proper design and implementation of transactions, as well as error handling mechanisms to ensure that transactions are rolled back in the event of an error. The use of transactions and constraints, such as those provided by relational databases, can also help to ensure consistency.

In summary, consistency is a critical property of database transactions, as it ensures that the database remains in a valid and consistent state, even after multiple transactions have been executed. This helps to avoid data corruption and other types of inconsistencies that could arise if transactions were not properly managed.

Isolation is a property of database transactions that ensures that each transaction operates independently and in isolation from other transactions. In other words, the execution of one transaction should not affect the execution of other transactions, and the result of one transaction should not be visible to other transactions until the transaction is committed.

Isolation is important in ensuring the consistency and reliability of a database, as it prevents data corruption that could occur if multiple transactions were allowed to interfere with each other. For example, if two transactions were trying to modify the same data simultaneously, isolation would ensure that one transaction completes before the other one begins, avoiding conflicts or inconsistencies.

There are different levels of isolation that can be used in database transactions, each with its own trade-offs between consistency and performance. The most common levels of isolation are:

• Read uncommitted: This level allows transactions to read data that has not yet been committed, which can result in dirty reads, where a transaction reads data that is later rolled back.

• Read committed: This level ensures that transactions only read data that has been committed, but it does not prevent other transactions from modifying the data before the current transaction has completed.

• Repeatable read: This level ensures that once a transaction has read a piece of data, no other transactions can modify that data until the first transaction is committed or rolled back.

• Serializable: This level provides the highest level of isolation, ensuring that transactions are executed as if they were executed one after the other, in a serial fashion.

The choice of isolation level depends on the specific requirements of the application and the trade-offs between consistency, performance, and complexity.

Durability is a property of database transactions that ensures that once a transaction is committed, its changes to the database persist, even in the event of system failures such as power outages, crashes, or other types of interruptions.

Durability is achieved by writing the changes made by a committed transaction to non-volatile storage, such as disk or flash memory, in a manner that ensures that the changes will persist even if the system crashes or loses power. In some databases, this is achieved through the use of write-ahead logging or other mechanisms that ensure that changes are written to storage before they are committed.

For example, consider a bank transaction where a customer withdraws money from an ATM. Once the transaction is committed, the changes to the customer’s account balance must be durable, meaning that even if the ATM machine or the underlying system crashes, the change to the customer’s account balance will still be reflected in the database.

Durability is a critical property of database transactions, as it ensures that the database remains in a consistent state, even in the event of system failures. This helps to avoid data loss and other types of inconsistencies that could arise if changes made by transactions were not durable.