create barcode labels c# Transactions and Concurrency in Visual Basic .NET

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TABLE 10-15
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Transactions and Concurrency
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Concurrent DDL Outside the SNAPSHOT Transaction
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Transaction 1
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SET TRANSACTION ISOLATION LEVEL SNAPSHOT; BEGIN TRAN SELECT TOP 10 * FROM Production.Product; -- This is the start of -- the transaction BEGIN TRAN ALTER TABLE Purchasing.Vendor ADD notes varchar(1000); COMMIT TRAN SELECT TOP 10 * FROM Production.Product; -- Succeeds -- The ALTER to a different -- table does not affect -- this transaction BEGIN TRAN ALTER TABLE Production.Product ADD LowestPrice money; COMMIT TRAN SELECT TOP 10 * FROM Production. Product; -- ERROR
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Time
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1 2 3
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Transaction 2
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For the preceding situation, in Transaction 1, the repeated SELECT statements should always return the same data from the Product table. An external ALTER TABLE on a completely different table has no effect on the snapshot transaction, but Transaction 2 then alters the Product table to add a new column. Because the metadata representing the former table structure is not versioned, Transaction 1 cannot produce the same results for the third SELECT. SQL Server generates this error:
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Msg 3961, Level 16, State 1, Line 1 Snapshot isolation transaction failed in database 'AdventureWorks2008' because the object accessed by the statement has been modified by a DDL statement in another concurrent transaction since the start of this transaction. It is disallowed because the metadata is not versioned. A concurrent update to metadata can lead to inconsistency if mixed with snapshot isolation.
In this version, any concurrent change to metadata on objects referenced by a snapshot transaction generates this error, even if there is no possibility of anomalies. For example, if Transaction 1 issues a SELECT count(*), which is not affected by the ALTER TABLE statement, SQL Server still generates error 3961.
Microsoft SQL Server 2008 Internals
Summary of Snapshot-Based Isolation Levels
SI and RCSI are similar in the sense that they are based on the versioning of rows in a database. However, there are some key differences in how these options are enabled from an administration perspective and also in how they affect your applications. I have discussed many of these differences already, but for completeness, Table 10-16 lists both the similarities and the differences between the two types of snapshot-based isolation.
TABLE 10-16
Snapshot vs. Read Committed Snapshot Isolation
Read Committed Snapshot Isolation
The database must be con gured to use RCSI, and sessions must use the default isolation level. No code changes are required. Enabling RCSI for a database requires a SHARED_ TRANSACTION_WORKSPACE lock on the database. All users must be kicked out of a database to enable this option.
Snapshot Isolation
The database must be con gured to allow SI, and the session must issue the command SET TRANSACTION ISOLATION LEVEL SNAPSHOT. Enabling SI for a database is an online operation. It allows a DBA to turn on versioning for one particular application such as one that is creating large reports. The DBA can then turn off versioning after the reporting transaction has started to prevent new snapshot transactions from starting. Turning on SI in an existing database is synchronous. When the ALTER DATABASE command is given, control does not return to the DBA until all existing update transactions that need to create versions in the current database nish. At this time, ALLOW_SNAPSHOT_ISOLATION is changed to ON. Only then can users start a snapshot transaction in that database. Turning off SI is also synchronous. There are no restrictions on active sessions in the database when this database option is enabled. If an application runs a snapshot transaction that accesses tables from two databases, the DBA must turn on ALLOW_SNAPSHOT_ ISOLATION in both databases before the application starts a snapshot transaction.
There should be no other sessions active in the database when you enable this option. RCSI is really a table-level option, so tables from two different databases, referenced in the same query, can each have their own individual setting. One table might get its data from the version store, while the other table is reading only the current versions of the data. There is no requirement that both databases must have the RCSI option enabled. There are no IN_TRANSITION states here. Only ON and OFF states persist.
The IN_TRANSITION versioning states do not persist. Only the ON and OFF states are remembered on disk.
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