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SQL Server Race Condition Question

(Note: this is for MS SQL Server)

Say you have a table ABC with a primary key identity column, and a CODE column. We want every row in here to have a unique, sequentially-generated code (based on some typical check-digit formula).

Say you have another table DEF with only one row, which stores the next available CODE (imagine a simple autonumber).

I know logic like below would present a race condition, in which two users could end up with the same CODE:

1) Run a select query to grab next available code from DEF
2) Insert said code into table ABC
3) Increment the value in DEF so it's not re-used.

I know that, two users could get stuck at Step 1), and could end up with same CODE in the ABC table.

What is the best way to deal with this situation? I thought I could just wrap a "begin tran" / "commit tran" around this logic, but I don't think that worked. I had a stored procedure like this to test, but I didn't avoid the race condition when I ran from two different windows in MS:

begin tran

declare @x int

select   @x= nextcode FROM  def

waitfor delay '00:00:15'

update def set nextcode = nextcode + 1

select @x

commit tran

Can someone shed some light on this? I thought the transaction would prevent another user from being able to access my NextCodeTable until the first transaction completed, but I guess my understanding of transactions is flawed.

EDIT: I tried moving the wait to after the "update" statement, and I got two different codes... but I suspected that. I have the waitfor statement there to simulate a delay so the race condition can be easily seen. I think the key problem is my incorrect perception of how transactions work.

like image 932
Hythloth Avatar asked Nov 05 '09 21:11

Hythloth


People also ask

What is race condition in SQL Server?

A race condition occurs when two threads access a shared variable at the same time. The first thread reads the variable, and the second thread reads the same value from the variable.

How do you avoid race condition in database?

To prevent the race conditions from occurring, you can lock shared variables, so that only one thread at a time has access to the shared variable.

What does it mean for a database to have a race condition?

Race conditions are most commonly associated with computer science and programming. They occur when two computer program processes, or threads, attempt to access the same resource at the same time and cause problems in the system. Race conditions are considered a common issue for multithreaded applications.


5 Answers

Set the Transaction Isolation Level to Serializable.
At lower isolation levels, other transactions can read the data in a row that is read, (but not yet modified) in this transaction. So two transactions can indeed read the same value. At very low isolation (Read Uncommitted) other transactions can even read data after it's been modified (but before committed)...

Review details about SQL Server Isolation Levels here

So bottom line is that the Isolation level is crtitical piece here to control what level of access other transactions get into this one.

NOTE. From the link, about Serializable
Statements cannot read data that has been modified but not yet committed by other transactions.
This is because the locks are placed when the row is modified, not when the Begin Trans occurs, So what you have done may still allow another transaction to read the old value until the point where you modify it. So I would change the logic to modify it in the same statement as you read it, thereby putting the lock on it at the same time.

begin tran
declare @x int
update def set @x= nextcode, nextcode += 1
waitfor delay '00:00:15'
select @x
commit tran
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Charles Bretana Avatar answered Sep 29 '22 17:09

Charles Bretana


As other responders have mentioned, you can set the transaction isolation level to ensure that anything you 'read' using a SELECT statement cannot change within a transaction.

Alternatively, you could take out a lock specifically on the DEF table by adding the syntax WITH HOLDLOCK after the table name, e.g.,

SELECT nextcode FROM DEF WITH HOLDLOCK

It doesn't make much difference here, as your transaction is small, but it can be useful to take out locks for some SELECTs and not others within a transaction. It's a question of 'repeatability versus concurrency'.

A couple of relavant MS-SQL docs.

  • Isolation levels
  • Table hints
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martin clayton Avatar answered Sep 29 '22 17:09

martin clayton


Late answer. You want to avoid a race condition...

"SQL Server Process Queue Race Condition"

like image 32
gbn Avatar answered Sep 29 '22 15:09

gbn


Recap:

  • You began a transaction. This doesn't actually "do" anything in and of itself, it modifies subsequent behavior
  • You read data from a table. The default isolation level is Read Committed, so this select statement is not made part of the transaction.
  • You then wait 15 seconds
  • You then issue an update. With the declared transaction, this will generate a lock until the transaction is committed.
  • You then commit the transaction, releasing the lock.

So, guessing you ran this simultaneously in two windows (A and B):

  • A read the "next" value from table def, then went into wait mode
  • B read the same "next" value from the table, then went into wait mode. (Since A only did a read, the transaction did not lock anything.)
  • A then updated the table, and probably commited the change before B exited the wait state.
  • B then updated the table, after A's write was committed.

Try putting the wait statement after the update, before the commit, and see what happens.

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Philip Kelley Avatar answered Sep 29 '22 16:09

Philip Kelley


It's not a real race condition. It's more a common problem with concurrent transactions. One solution is to set a read lock on the table and therefor have a serialization in place.

like image 32
Guido Avatar answered Sep 29 '22 15:09

Guido