Snapshot isolation in MSSQL is important because it allows transactions to work in a consistent state by providing a version of the data as it was when the transaction started. This helps to prevent issues such as dirty reads, non-repeatable reads, and phantom reads without the need for locking rows or pages, which can improve both concurrency and performance.
Snapshot Isolation is a transaction isolation level in MSSQL that utilizes Multi-Version Concurrency Control (MVCC). This allows transactions to operate on consistent snapshots of data, preventing blocking between read and write operations, which is crucial for high-concurrency applications.
By enabling Snapshot Isolation, miniOrange can enhance performance and data consistency, as reads do not block writes. Each transaction accesses a consistent snapshot of the data as it was at the start of the transaction.
To enable MVCC behavior in MSSQL, you need to configure Snapshot Isolation for your database. This involves running two specific commands to enable and configure Snapshot Isolation and Read Committed Snapshot Isolation.
Enable Snapshot Isolation for the database to allow transactions to use the SNAPSHOT isolation level:
ALTER DATABASE YourDatabaseName
SET ALLOW_SNAPSHOT_ISOLATION ON;
Enable Read Committed Snapshot Isolation, which changes the default READ COMMITTED isolation level to use row versioning, enabling MVCC-like behavior for non-blocking reads:
ALTER DATABASE YourDatabaseName
SET READ_COMMITTED_SNAPSHOT ON;
These two commands serve different but complementary purposes:
The ALLOW_SNAPSHOT_ISOLATION command enables the SNAPSHOT isolation level, providing the highest level of consistency and ensuring that each transaction has its own data snapshot.
The READ_COMMITTED_SNAPSHOT command configures the default READ COMMITTED isolation level to use row versioning, which helps reduce blocking in typical read operations without requiring explicit SNAPSHOT isolation.
After running these commands, you can verify if Snapshot Isolation is enabled by querying the database properties:
SELECT
snapshot_isolation_state_desc,
is_read_committed_snapshot_on
FROM sys.databases
WHERE name = 'YourDatabaseName';
