How to deal with concurrency in sqlserver

SQL Server uses the following mechanisms to handle concurrency: Lock mechanism: shared lock (S lock), exclusive lock (X lock), intention lock transaction atomicity, consistency, isolation, durability Isolation level: read Uncommitted, read committed, repeatable read, serialization and other concurrency control mechanisms: optimistic concurrency control, row versioning

SQL Server Concurrency processing

SQL Server is a database management system that supports high concurrency. Concurrency refers to the ability of multiple users to access and operate on the same database at the same time. In order to handle concurrency, SQL Server uses the following mechanism:

Lock mechanism

• Shared lock (S lock): Allows multiple Users can read the same row of data at the same time, but cannot modify it.
• Exclusive lock (X lock): Allows one user to write the same row of data exclusively, and other users can only read.
• Intention lock: When a transaction prepares to acquire a row lock, it will first acquire the intention lock for the row to prevent conflicts with other transactions.

Transaction

• Atomicity: All operations in a transaction are either fully executed or rolled back.
• Consistency: After the transaction is completed, the database must be in a consistent state.
• Isolation: Transactions are not affected by other concurrent transactions.
• Persistence: Once a transaction commits, its changes are persisted permanently.

Isolation Level

SQL Server provides different isolation levels to control the concurrency and consistency of transactions:

• Read Uncommitted: Allows reading changes from other uncommitted transactions.
• Read Committed: Only the changes of committed transactions are allowed to be read.
• Repeatable Read: It is guaranteed that within a transaction, reading the same data multiple times will get the same result.
• Serialization: Concurrency is completely prohibited and all transactions are executed sequentially.

Other concurrency control mechanisms

• Optimistic concurrency control: Use version management to allow multiple users to modify the same line at the same time Data is checked for conflicts on submission.
• Row Versioning: Stores different versions of the data, allowing users to view the data at different points in time.

Through these mechanisms, SQL Server achieves high concurrency, allowing multiple users to access and operate the database at the same time, while maintaining data consistency and integrity.

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