Resolving Deadlocks in SQL Server 2000

Your application can detect a deadlock and resubmit its transaction, but a better approach is to resolve a deadlock by changing the conditions that lead to it in the first place. In this article, you'll learn how SQL Server deadlocks arise, what types of deadlocks there are, and how you can resolve them.

When your application must handle complex transactions and multiple users, SQL Server deadlocking can be an annoying and difficult problem.

Deadlocking is not a problem that is unique to SQL Server. Any database system that relies on locking to ensure that user transactions do not interfere with each other is subject to deadlock conditions. In order to understand and resolve SQL Server deadlocks, it's important to understand the basic concepts underlying deadlocking in SQL Server.

Deadlocking Concepts

The key concept behind deadlocking is the transaction. To give your users a consistent view of the database, where either all changes in a transactional unit of work succeed or all fail, the database system must lock some resources while the work is being done. A SQL Server deadlock occurs when two or more processes have acquired locks on their respective resource, and they need to get an incompatible lock on the other's resource in order to finish a transaction. The result is mutual blocking: each waits on each other to acquire some resource that the other process already has.

The result is a situation where neither process can finish. SQL Server's lock manager will detect a deadlock cycle and end one of the transactions. Table 1 shows how, in general, a deadlock occurs.

Read the table from top to bottom, imagining time to progress from instance T1 through T7. By time T3, Transaction1 and Transaction2 have both been granted locks on some resource. At time T4, Transaction1 requests an incompatible lock on the resource already locked by Transaction2, and is blocked. At that point, Transaction1 goes into a wait state, waiting for the lock to be released.

At time T5, Transaction2 requests an incompatible lock on the resource that Transaction1 already has locked. At this point, Transaction2 also goes into a wait state, and each process is blocking the other. This is a deadlock cycle, and here is where SQL Server will detect the deadlock cycle and end one of the transactions.

Types of waits

According to SQL Server Books Online, SQL Server threads can wait on

• Locks
• Parallel query resources
• Other threads
• Memory resource
• Application events

Deadlocking can occur with locks, parallelism, threads, and application events. (Memory waits are resolved by query time-out.) The most frequent source of SQL Server deadlocking is resource locking where the resources are table or index objects.

Deadlocks Involving Locks

Lock-based deadlocks involve two or more threads, at least one transaction, and one or more resources. It's useful to view deadlocks as occurring in two stages. The first is a grant stage, where each thread is granted a lock on its resource. They could be the same resource, but it's much more common that they are different resources.

The second stage is a blocked request where each thread requests an incompatible lock on the other thread's resource. Each thread waits on the other to release its locks before it can complete. SQL Server detects the deadlocked state and rolls back one of the transactions.

Deadlocking is more than blocking. Blocking occurs when one thread is waiting on another, and some brief blocking is normal. Blocking is expected in any database system that uses locking to maintain transaction isolation. Only blocks with long durations should be considered a problem. In an active system, short periods of blocking may be happening quite often. Lock-based deadlocking is a special type of blocking where two or more threads mutually block each other, and that's what you need to avoid.

How SQL Server handles a Deadlock

In SQL Server 2000, the Lock Monitor thread detects the deadlock. It uses a periodic detection system, inspecting processes about every 5 seconds to determine if there are any deadlock cycles. When it finds one, it automatically chooses one thread as the deadlock victim. It then rolls back the victim thread's transaction, cancels its query, and returns error 1205 to its client.

The Lock Monitor generally chooses the least expensive transaction to roll back. You can override this somewhat using SET DEADLOCK_PRIORITY to LOW for a session. But whenever both threads have the same DEADLOCK_PRIORITY setting, the Lock Monitor will have to choose one of them as the victim.

The message delivered by error 1205 is mysterious or entertaining, depending on your point of view:

Error 1205 is not fatal to the connection; your connection is not terminated. SQL Server aborts the batch, rolls back the transaction, and cancels the query. As a result, error 1205 cannot be detected from within Transact-SQL by @@ERROR within the batch or from calling a stored procedure, because the entire calling batch is cancelled. It has a severity level of 13, correctable by the user, so the client can resubmit the query.

Unfortunately, error 1205 does not tell you the identity of the other participating spid or spids that your process deadlocked with. It also does not reveal statements at the grant or blocked request stage, so you are not sure what commands set up the deadlock conditions in the first place.

By: Ron Talmage

Ron Talmage is a mentor and co-founder of Solid Quality Mentors. He is a SQL Server MVP, a PASS Regional Mentor, and current president of the Pacific Northwest SQL Server Users Group. He has been writing white papers and articles on SQL Server since way back when.