Thursday, May 26, 2011

Deadlock in SQL Server

Deadlocks can be a pain to debug since they're so rare and unpredictable. The problem lies in repeating them in your dev environment. That's why it's crucial to have as much information about them from the production environment as possible.

Deadlocks occur when two (or more) processes are holding locks on resources and are waiting for locks on resources in such a way that they will never resolve.

Here I'm discussing a one of the way to monitor the Deadlocks.

How to Trace Deadlocks?
Get Info By ID


Deadlocks can be traced by turning on two specific flags:
dbcc traceon (1204, 3605, -1)
dbcc tracestatus(-1)

Deadlocks trace output can be examined in SQL Server log.

How to Avoid Deadlocks?
Here are some tips on how to avoid deadlocking on your SQL Server:
  • Ensure the database design is properly normalized.
  • Have the application access server objects in the same order each time.
  • During transactions, don’t allow any user input. Collect it before the transaction begins.
  • Avoid cursors.
  • Keep transactions as short as possible. One way to help accomplish this is to reduce the number of round trips between your application and SQL Server by using stored procedures or keeping transactions with a single batch. Another way of reducing the time a transaction takes to complete is to make sure you are not performing the same reads over and over again. If your application does need to read the same data more than once, cache it by storing it in a variable or an array, and then re-reading it from there, not from SQL Server.
  • Reduce lock time. Try to develop your application so that it grabs locks at the latest possible time, and then releases them at the very earliest time.
  • If appropriate, reduce lock escalation by using the ROWLOCK or PAGLOCK.
  • Consider using the NOLOCK hint to prevent locking if the data being locked is not modified often.
  • If appropriate, use as low of an isolation level as possible for the user connection running the transaction.  
  • Consider using bound connections.


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