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Transaction Isolation Levels and Hints
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A developer or administrator can change the default behavior of SQL Server using transaction isolation levels or lock hints. Transaction isolation levels set locking on the connection level, and lock hints on the statement level. SQL Server can work on four different transaction isolation levels:
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Serializable is the highest level in which transactions are completely isolated. The system behaves as though the transactions are occurring one after another. This level of isolation may lead to some performance issues. Repeatable Read level forces SQL Server to place shared locks and hold them until the transaction is completed. Read Committed is the default level in SQL Server. SQL Server places shared locks when reading. Read Uncommitted is the lowest level of isolation in SQL Server. It ensures that physically corrupt data is not read. SQL Server will not place shared locks, and it will ignore exclusive locks. You will have the fewest performance issues when using this level, but you will also likely have many data integrity problems.
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Isolation level is specified in the Set statement. For example
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Set Transaction Isolation Level Repeatable Read
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Locking hints change the behavior of the locking manager as it processes a single Transact-SQL statement. They overwrite behavior set by the transaction isolation level. The following table describes hints that can be used to control locking:
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Hints Description
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Holdlock or Serializable
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Holds a shared lock until a transaction is completed. The lock will not be released when the resource is no longer needed, but when the transaction is completed. This hint applies only to Select statements. SQL Server will not place shared locks, and it will ignore exclusive locks. Uses update instead of shared locks while reading a table. Specifies the granularity of locks at the row level. Specifies the granularity of locks at the page level. Specifies the granularity of locks at the table level. Specifies the granularity of locks at the table level and the type of lock to be exclusive. Equivalent to the default isolation level (Read Committed).
Nolock
Updlock Rowlock Paglock Tablock Tablockx
Readcommitted
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Composite Transact-SQL Constructs Batches, Scripts, and Transactions
Readpast
This hint is applicable only in Select statements working under the Read Committed isolation level. Resultsets created with this hint will not contain records locked by other transactions. Equivalent to Read Uncommitted isolation level. Equivalent to Repeatable Read isolation level.
Readuncommitted Repeatableread
Locking hints can be used in Select, Insert, Update, or Delete statements. They are part of the From clause. For example, the following command will hold a lock until the transaction is completed:
Select * From Inventory With (HOLDLOCK) Where InventoryId = @intInventoryId
The next example demonstrates the use of hints in an Update statement and the use of more than one hint in a statement:
Update Inventory With (TABLOCKX, HOLDLOCK) Set StatusId = 4 Where StatusId = @intStatusId
Distributed Transactions
Microsoft Distributed Transaction Coordinator (MS DTC) is a component that allows users to span transactions over two or more servers. Servers in this scenario are called resource managers, and MS DTC performs the function of transaction coordinator. In fact, all those resource managers do not even have to be Microsoft servers. They just have to be compatible with MS DTC. For example, it is possible
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to execute a single transaction against databases on Microsoft SQL Server and Oracle. When transactions are distributed over different resource managers, different mechanisms have to be applied by the transaction coordinator to compensate for problems that might occur in such an environment. A typical problem is network failure. For example, everything might be executed properly by individual resource managers, but if the transaction coordinator is not informed, the result is the same as if one of the resource managers had failed. The transaction will be rolled back. One mechanism for dealing with such problems is called two-phase commit (2PC). As the name implies, it consists of two phases:
Prepare phase Commit phase
The prepare phase starts when a transaction manager receives a request to execute a transaction. It notifies the resource managers and informs them of the work that needs to be done. The resource managers perform all changes and even write everything from the transaction log in memory to the disk. When everything is completed, each resource manager sends a message indicating success or failure status to the transaction coordinator. The commit phase starts when the transaction coordinator receives messages from resource managers. If the resource managers successfully complete the preparation phase, the transaction coordinator sends a Commit command to the resource managers. Each of them makes the changes permanently to the database and reports the success of the operation to the transaction coordinator. If any of the resource managers reports failure during the preparation phase, the transaction coordinator will send a Rollback command to all resource managers. From a developer s point of view, distributed transactions are very similar to regular transactions. The major difference is that the developer needs to use the following statement to start the transaction:
Begin Distributed Tran[saction] [transaction_name]
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