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10 Process A
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Transactions and Concurrency Process B
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Shared lock #1 Process A wants to convert shared lock on page 100 to exclusive but cannot because process B also has a shared lock on page 100.
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Shared lock #2 Process B wants to convert shared lock on page 100 to exclusive but cannot because process A also has a shared lock on page 100.
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FIGURE 10-6 A conversion deadlock resulting from two processes wanting to promote their locks on the same resource within a transaction
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In SQL Server, a separate thread called LOCK_MONITOR checks the system for deadlocks every ve seconds. As deadlocks occur, the deadlock detection interval is reduced and can go as low as 100 milliseconds. In fact, the rst few lock requests that cannot be satis ed after a deadlock has been detected will immediately trigger a deadlock search rather than wait for the next deadlock detection interval. If the deadlock frequency declines, the interval can go back to every ve seconds. This LOCK_MONITOR thread checks for deadlocks by inspecting the list of waiting locks for any cycles, which indicate a circular relationship between processes holding locks and processes waiting for locks. SQL Server attempts to choose as the victim the process that would be least expensive to roll back, considering the amount of work the process has already done. That process is killed and error message 1205 is sent to the corresponding client connection. The transaction is rolled back, meaning all its locks are released, so other processes involved in the deadlock can proceed. However, certain operations are marked as golden, or unkillable, and cannot be chosen as the deadlock victim. For example, a process involved in rolling back a transaction cannot be chosen as a deadlock victim because the changes being rolled back could be left in an indeterminate state, causing data corruption. Using the SET DEADLOCK_PRIORITY statement, a process can determine its priority for being chosen as the victim if it is involved in a deadlock. There are 21 different priority levels, from 10 to 10. You can also specify the value LOW, which is equivalent to 5, NORMAL, which is equivalent to 0, and HIGH, which is equivalent to 5. Which session is chosen as the deadlock victim depends on each session s deadlock priority. If the sessions have different deadlock
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Microsoft SQL Server 2008 Internals
priorities, the session with the lowest deadlock priority is chosen as the deadlock victim. If both sessions have set the same deadlock priority, SQL Server selects as the victim the session that is less expensive to roll back.
Note The lightweight latches and spinlocks used internally do not have deadlock detection
services. Instead, deadlocks on latches and spinlocks are avoided rather than resolved. Avoidance is achieved via strict programming guidelines used by the SQL Server development team. These lightweight locks must be acquired in a hierarchy, and a process must not have to wait for a regular lock while holding a latch or spinlock. For example, one coding rule is that a process holding a spinlock must never directly wait for a lock or call another service that might have to wait for a lock, and a request can never be made for a spinlock that is higher in the acquisition hierarchy. By establishing similar guidelines for your development team for the order in which SQL Server objects are accessed, you can go a long way toward avoiding deadlocks in the rst place.
In the example in Figure 10-5, the cycle deadlock could have been avoided if the processes had decided on a protocol beforehand for example, if they had decided always to access the Product table rst and the PurchaseOrderDetail table second. Then one of the processes gets the initial exclusive lock on the table being accessed rst, and the other process waits for the lock to be released. One process waiting for a lock is normal and natural. Remember, waiting is not a deadlock. You should always try to have a standard protocol for the order in which processes access tables. If you know that the processes might need to update the row after reading it, they should initially request an update lock, not a shared lock. If both processes request an update lock rather than a shared lock, the process that is granted an update lock is assured that the lock can later be promoted to an exclusive lock. The other process requesting an update lock has to wait. The use of an update lock serializes the requests for an exclusive lock. Other processes needing only to read the data can still get their shared locks and read. Because the holder of the update lock is guaranteed an exclusive lock, the deadlock is avoided. In many systems, deadlocks cannot be completely avoided, but if the application handles the deadlock appropriately, the impact on any users involved, and on the rest of the system, should be minimal. (Appropriate handling implies that when error 1205 occurs, the application resubmits the batch, which most likely succeeds on the second try. Once one process is killed, its transaction is aborted, and its locks are released, the other process involved in the deadlock can nish its work and release its locks, so the environment is not conducive to another deadlock.) Although you might not be able to avoid deadlocks completely, you can minimize their occurrence. For example, you should write your applications so that your processes hold locks for a minimal amount of time; in that way, other processes won t have to wait too long for locks to be released. Although you don t usually invoke locking directly, you can in uence locking by keeping transactions as short as possible. For example, don t ask for user input in the middle of a transaction. Instead, get the input rst and then quickly perform the transaction.
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