SQL Server deadlock while inserting rows in a table

Question

Hi,

The topic of sql server deadlock has been discussed many times, however, I was unsure that even two simultaneous inserts on a table can end up in a deadlock situation.

Scenario: While testing our application (SQL Server 2005 as backend, ASP.net 3.5) we inserted records into a table simultaneously (simplified overview) and that resulted into a deadlock for more than 70% of users. I could not get a hang of this as how an insert is being deadlock as this is not a case of multiple resources. After a detailed analysis (of reproducing the bug by two users) I found that both the processes were holding a RangeS-S lock on the primary key index of the table and were trying to convert this into RangeI-N lock, that resulted into a deadlock and one transaction being killed.

Question: Can we avoid or reduce these kind of deadlocks as this is not a case of change in order of access of resources? Cant' we force the transaction to get exclusive lock initially so that it blocks the other process and avoid deadlock? What (adverse) effects that may have? Also could some one explain more about RangeI-N lock.

Isolation Level for this was "Serializable".

Any suggestion will be helpful.

Thanks, Gaurav

Answer 1

Change your ADO isolation level. Unless you have clear requirements for Serializable, you shouldn't use it. If you do use it, then you must clearly understand the consequences, and frequent deadlocks due to range locks are one of these consequences.

The isolation level of System.Transactions is controlled by the IsolationLevel property.

Answer 2

Use sp_getapplock to acquire a custom exclusive lock