Implementing optimistic concurrency control with ACID properties in SQL databases

31 Oct 2023

In a multi-user environment, concurrent access to a database can lead to data conflicts and inconsistencies. Optimistic Concurrency Control (OCC) is a technique that allows multiple users to access and modify the same data simultaneously while ensuring data integrity and maintaining ACID (Atomicity, Consistency, Isolation, Durability) properties.

What is Optimistic Concurrency Control?

Optimistic Concurrency Control is a strategy that assumes conflicts between transactions are rare. It allows concurrent transactions to proceed without acquiring locks on the data they access. Instead, it detects conflicts at the time of commit and resolves them.

The key idea behind OCC is to use a versioning mechanism to track modifications to the data. Each transaction operates on a specific version of the data independently, and conflicts are detected by checking if the version it initially read has changed before committing.

Steps to implement Optimistic Concurrency Control in SQL Databases

Step 1: Add a version column to the table

In order to track the modifications to the data, a version column needs to be added to the table to store the version number of each row.

ALTER TABLE table_name ADD COLUMN version INT DEFAULT 0;

Step 2: Read the version while reading the data

When reading data for modification, we also need to retrieve the current version of the record. This can be done using a simple SELECT statement.

SELECT column1, column2, ..., version
FROM table_name
WHERE ...

Step 3: Perform modifications and increment the version number

Once the data is retrieved, perform the necessary modifications in the transaction. After modifying the data, increment the version number.

UPDATE table_name
SET column1 = new_value1, column2 = new_value2, ..., version = version + 1
WHERE ...

Step 4: Check for conflicts before committing

Before committing the transaction, check if the original version of the data has changed. If the version has changed, it means there was a conflict with another transaction and the changes cannot be applied.

SELECT COUNT(*)
FROM table_name
WHERE original_version != new_version

If the count is greater than 0, there is a conflict, and appropriate actions can be taken to handle it.

Step 5: Commit the transaction

If there are no conflicts, commit the transaction and persist the changes to the database.

COMMIT;

Benefits of Optimistic Concurrency Control

Improved concurrency: Multiple transactions can run simultaneously without acquiring locks, allowing for better performance in high-concurrency environments.
Reduced contention: Transactions don’t block each other, reducing the likelihood of conflicts and contention for resources.
Easy conflict resolution: Conflicts are detected at commit time, making it easier to resolve conflicts and retry transactions if necessary.

Conclusion

Implementing Optimistic Concurrency Control in SQL databases offers a way to handle concurrent modifications in a multi-user environment while maintaining ACID properties. By using a versioning mechanism and detecting conflicts at commit time, OCC allows for improved concurrency and reduced contention among transactions.