Task T1 has a lock on resource R1 (indicated by the arrow from R1 to T1) and has requested a lock on resource R2 (indicated by the arrow from T1 to R2).The following graph presents a high level view of a deadlock state where: Detect and ending 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. This type of deadlock can be avoided by setting LOCK_ESCALATION to TABLE although this setting will reduce concurrency by forcing large updates to a partition to wait for a table lock. However, when separate transactions hold partition locks in a table and want a lock somewhere on the other transactions partition, this causes a deadlock. When LOCK_ESCALATION is set to AUTO, concurrency increases by allowing the SQL Server Database Engine to lock table partitions at the HoBT level instead of at the table level. Because these dependencies form a cycle, there is a deadlock between transactions T1 and T2.ĭeadlocks can also occur when a table is partitioned and the LOCK_ESCALATION setting of ALTER TABLE is set to AUTO. Similarly, transaction T2 has a dependency on transaction T1 for the Supplier table lock resource. In the illustration, transaction T1 has a dependency on transaction T2 for the Part table lock resource. In an instance of the SQL Server Database Engine, sessions can deadlock when acquiring non-database resources, such as memory or threads. The waiting thread is said to have a dependency on the owning thread for that particular resource. 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.
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