qr code generator c# Understanding Deadlock and Blocking Scenarios in C#.NET

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Understanding Deadlock and Blocking Scenarios
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Because transactions at certain isolation levels hold locks until the transaction is completed, transactions can block each other from completing successfully. By default, transactions in SQL Server 2008 wait an indefinite amount of time for a resource to become available unless SQL Server recognizes that a deadlock situation has occurred. In a deadlock situation, two transactions are holding resources that each of the two transactions requires before completion. Because of this, neither transaction is ever able to complete successfully. Based on the estimated cost for SQL Server to roll back each transaction, the lock manager selects a victim of the deadlock situation and rolls back that transaction, issuing a 1205 error. Because this error does not attempt to restart the transaction or provide an informative message to users, all 1205 errors should be captured and handled appropriately.
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Lesson 3: Managing Transactions CHAPTER 2 71
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You can use the following best practices to reduce deadlock situations and blocking issues:
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Keep transactions short. Collect and verify input data from users before opening a transaction. Access resources in the same order whenever possible within transactions. Keep transactions in a single batch. Where appropriate, use a lower isolation level or row versioning based isolation level. Access the least amount of data possible in the transaction.
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To manage blocking issues further, database administrators can adjust the query wait times based on performance analysis. This can be accomplished through the advanced server properties. To locate transactions that are affected by deadlock situations, you can use SQL Server Profiler to produce an Extensible Markup Language (XML) representation of a deadlock chain of events, including the system process ID (SPID) of the transactions involved in the deadlock situation.
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Understanding Reports on Lock Status
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There are many options for viewing lock status within your computer running SQL Server. You can use SQL Profiler to capture lock and blocking information. You can use the System Monitor that is part of the performance console (perfmon) to capture statistics on lock wait times, locks per second, and so on. You can use the sys.dm_tran_locks dynamic management view (DMV) to gather information on locks being held by transactions. Finally, you can use the Activity Monitor in SSMS to see information on blocking processes. A representation of Activity Monitor showing a session being blocked by another session is shown in Figure 2-3.
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FiGURe 2-3 Activity Monitor
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Modifying Data The INSERT, UPDATE, DELETE, and MERGE Statements
The sys.dm_tran_locks DMV provides detailed information about each lock that is currently being held on the instance. You can then use the sys.dm_tran_active_transactions DMV to provide additional information about the blocked or blocking transaction. Figures 2-4 and 2-5 show the results of a query of the sys.dm_tran_locks and the sys.dm_tran_active_transactions DMVs.
FiGURe 2-4 sys.dm_tran_locks
FiGURe 2-5 sys.dm_tran_active_transactions
Lesson 3: Managing Transactions
In many of these reports, you see the locking method and the resource that is locked. You might see locks on the following types of resources:
Row identifier (RiD)
A row identifier used to define a lock on a single row located in
a heap
note
HeaPS
A heap is the storage method for a table without a clustered index. For more information about indexing, see 6, Techniques to Improve Query Performance.
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keY PaGe
The range of keys in an index used to define a lock on key ranges An 8-kilobyte (KB) page from tables or indexes A group of eight contiguous pages within a table or index An entire table, made up of both data and index pages An entire database file An application-specified resource A single allocation unit Used for metadata locks An entire database, including all data files A heap or a balanced tree (B-tree) index
eXtent Hobt tabLe FiLe
aPPLicatiOn MetaData
aLLOcatiOn_Unit DatabaSe
Using SQL Server Extended Events
SQL Server 2008 includes SQL Server Extended Events, such as FindBlocker and lock_count, which can be used in conjunction with Windows Event logs, SQL Profiler, or System Monitor.
More info
eXtenDeD eventS
Working with SQL Server Extended Events is beyond the scope of this book. For more information about Extended Events, see the article titled Advanced Troubleshooting with Extended Events at http://technet.microsoft.com/en-us/magazine/dd314391.aspx. To learn more about or to download a copy of the Extended Events Manager, a C# Microsoft.NET WinForms application that aids in creating and working with Extended Events sessions, see SQL Server 2008 Extended Events Manager at http://www.codeplex.com/ExtendedEventManager.
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