If the resource being acquired is currently owned by another thread, the first thread may have to wait for the owning thread to release the target resource. For example, a thread in a multithreaded operating system might acquire one or more resources, such as blocks of memory. Deadlocks are sometimes called a deadly embrace.ĭeadlock is a condition that can occur on any system with multiple threads, not just on a relational database management system, and can occur for resources other than locks on database objects. Deadlocks are resolved almost immediately, whereas blocking can, in theory, persist indefinitely. Eventually, the owning transaction will complete and release the lock, and then the requesting transaction will be granted the lock and proceed. The requesting transaction is blocked, not deadlocked, because the requesting transaction has not done anything to block the transaction owning the lock. By default, SQL Server transactions do not time out, unless LOCK_TIMEOUT is set. When a transaction requests a lock on a resource locked by another transaction, the requesting transaction waits until the lock is released. The application with the transaction that terminated with an error can retry the transaction, which usually completes after the other deadlocked transaction has finished.ĭeadlocking is often confused with normal blocking. This allows the other task to complete its transaction. If the monitor detects a cyclic dependency, it chooses one of the tasks as a victim and terminates its transaction with an error. the SQL Server Database Engine deadlock monitor periodically checks for tasks that are in a deadlock. This condition is also called a cyclic dependency: Transaction A has a dependency on transaction B, and transaction B closes the circle by having a dependency on transaction A.īoth transactions in a deadlock will wait forever unless the deadlock is broken by an external process. Transaction A cannot complete until transaction B completes, but transaction B is blocked by transaction A. Transaction B now requests an exclusive lock on row 1, and is blocked until transaction A finishes and releases the shared lock it has on row 1.Transaction A now requests an exclusive lock on row 2, and is blocked until transaction B finishes and releases the shared lock it has on row 2.

Transaction B acquires a shared lock on row 2.Transaction A acquires a shared lock on row 1.Understand deadlocksĪ deadlock occurs when two or more tasks permanently block each other by each task having a lock on a resource that the other tasks are trying to lock. For more on transaction locking, see Transaction locking and row versioning guide.įor more specific information on identification and prevention of deadlocks in Azure SQL Database, see Analyze and prevent deadlocks in Azure SQL Database. Deadlocks are caused competing, concurrent locks in the database, often in multi-step transactions. This article discusses deadlocks in the SQL Server Database Engine in depth. Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Sql server deadlock error