Row versioning is a technique used in Microsoft SQL Server to manage data concurrency and isolation, particularly in the context of isolation levels like Snapshot Isolation and Read Committed Snapshot Isolation. It involves creating and maintaining multiple versions of rows in the database to allow for non-blocking read operations and to support different isolation levels:

1. Purpose of Row Versioning:
   • Row versioning is designed to reduce blocking between read and write operations in high-concurrency scenarios.
   • It enables transactions to read data without locking rows, providing better concurrency and scalability.
   • Row versioning is especially useful when using isolation levels like Snapshot Isolation and Read Committed Snapshot Isolation (RCSI).
2. Version Generation:
   • When a transaction modifies a row, SQL Server creates a new version of the row with a unique version number or a timestamp.
   • The original row remains unchanged in the database, and the new version is associated with the transaction.
3. Version Cleanup:
   • To prevent the database from growing indefinitely due to the creation of versions, SQL Server employs a mechanism called version cleanup.
   • The database engine periodically identifies and removes versions that are no longer needed.
   • Versions that are older than the oldest active transaction can be safely removed.
4. Snapshot Isolation:
   • In Snapshot Isolation, each transaction sees a consistent snapshot of the data as it existed at the start of the transaction.
   • Read operations access the appropriate version of rows without blocking write operations.
   • This ensures that read operations are not blocked by write locks and provides non-blocking reads.
5. Read Committed Snapshot Isolation (RCSI):
   • Similar to Snapshot Isolation but applied at the row level.
   • Read operations do not block write operations and vice versa.
   • Each transaction reads the latest committed version of each row.
6. Versioning Overhead:
   • While row versioning improves concurrency, it comes with some overhead in terms of storage space.
   • The database must store multiple versions of rows, which can increase the size of the database.
   • However, this overhead is often considered worthwhile for the benefits it provides in high-concurrency environments.
7. Implementation Considerations:
   • Row versioning requires enabling the ALLOW_SNAPSHOT_ISOLATION and READ_COMMITTED_SNAPSHOT database options.
   • Careful consideration should be given to the application design and the isolation level chosen to make the best use of row versioning.
8. Concurrency and Scalability:
   • Row versioning enhances concurrency and scalability by reducing locking and contention.
   • It allows multiple transactions to work concurrently without waiting for locks to be released.

Row versioning is a powerful feature in SQL Server that significantly improves the ability to handle concurrent transactions efficiently. It is particularly valuable when implementing high-concurrency systems or when requiring non-blocking read operations while maintaining strong data consistency.