SQL Isolation

Isolation is one of the four ACID (Atomicity, Consistency, Isolation, Durability) properties of database transactions.

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1. Overview

Isolation in the context of SQL transactions refers to the property that ensures transactions are executed independently of each other, even when multiple transactions are executed concurrently.

Isolation prevents interference or interaction between transactions, ensuring that each transaction sees a consistent snapshot of the database, as if it were the only transaction executing at that time.

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2. Isolation Levels

SQL databases offer different isolation levels to control the degree of isolation between transactions. Common isolation levels include:

• Read Uncommitted: Allows transactions to read data that has been modified but not yet committed by other transactions.
• Read Committed: Ensures that transactions only read data that has been committed by other transactions, preventing dirty reads.
• Repeatable Read: Ensures that a transaction can re-read the same data multiple times within the same transaction, and the data remains consistent.
• Serializable: Provides the highest level of isolation, ensuring that transactions are executed serially, as if one transaction completes before the next one starts.

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3. Example

Consider a scenario where two transactions are concurrently updating the balance of a bank account. The isolation property ensures that each transaction operates independently without interference from the other, preventing inconsistencies in the account balance.

Example:

// Example of concurrent transactions with isolation
-- Transaction 1: Update balance
START TRANSACTION;
UPDATE accounts SET balance = balance + 100 WHERE account_id = 123;
COMMIT;

-- Transaction 2: Update balance
START TRANSACTION;
UPDATE accounts SET balance = balance - 50 WHERE account_id = 123;
COMMIT;

In this example, both transactions update the balance of the account with ID 123 concurrently. The isolation property ensures that each transaction sees a consistent snapshot of the database and operates independently, preventing interference between transactions.

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4. Importance

Isolation is essential for maintaining the integrity and consistency of the database, especially in environments with concurrent transactions. It ensures that transactions are executed safely and reliably, preventing data corruption or inconsistency.

By providing different isolation levels, SQL databases allow developers to choose the appropriate level of isolation based on the requirements of their applications, balancing concurrency and consistency.

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5. Conclusion

SQL isolation is a critical property of transactions that ensures transactions are executed independently and reliably, even in concurrent environments. By providing different isolation levels, SQL databases offer flexibility in managing the trade-off between concurrency and consistency, allowing developers to tailor transaction behavior to their specific needs.