![]() The database engine deadlock monitor periodically checks for tasks that are in a deadlock. Each session owns a resource that the other process needs in order to continue.":::Īll transactions in a deadlock will wait indefinitely unless one of the participating transactions is rolled back, for example, because its session was terminated. :::image type="content" source="media/analyze-prevent-deadlocks/deadlock-overview.png" alt-text="A diagram showing two sessions in a deadlock. Session A is blocked by Session B on SalesLT.ProductDescription. Session A continues its transaction, and now runs an update against the SalesLT.ProductDescription table.Session B needs a shared lock on rows on the table SalesLT.Product, including the row that is locked by Session A.Session B acquires an update (U) lock on 72 rows on the SalesLT.ProductDescription table.The update statement joins to the SalesLT.Product table to find the correct rows to update. Session B runs an update statement that modifies the SalesLT.ProductDescription table.Session A begins an explicit transaction and runs an update statement that acquires an update (U) lock on one row on table SalesLT.Product that is converted to an exclusive (X) lock.A deadlock is also called a cyclic dependency: in the case of a two-task deadlock, transaction A has a dependency on transaction B, and transaction B closes the circle by having a dependency on transaction A. This may increase blocking and deadlocks.Ī deadlock occurs when two or more tasks permanently block one another because each task has a lock on a resource the other task is trying to lock. RCSI may be disabled, causing the database to use shared (S) locks to protect SELECT statements run under the read committed isolation level.Isolation levels may be specified via client library methods, query hints, or SET statements in Transact-SQL. Queries may run under isolation levels that increase blocking.Queries that modify data may block one another.However, blocking and deadlocks may still occur in databases in Azure SQL Database because: Blocking between sessions reading data and sessions writing data is minimized under RCSI, which uses row versioning to increase concurrency. How deadlocks occur in Azure SQL DatabaseĮach new database in Azure SQL Database has the read committed snapshot (RCSI) database setting enabled by default. ![]() Learn more about other types of deadlocks in resources that can deadlock. This article focuses on identifying and analyzing deadlocks due to lock contention. ![]() This article teaches you how to identify deadlocks in Azure SQL Database, use deadlock graphs and Query Store to identify the queries in the deadlock, and plan and test changes to prevent deadlocks from reoccurring. Analyze and prevent deadlocks in Azure SQL Database
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