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Blocking and Deadlocks
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Blocking and deadlocks are two events that can occur with concurrent transactions. Sometimes they are desirable because they help maintain data consistency, and sometimes they are not desirable because of possible performance degradation for users. Blocking and deadlocks both relate to locking. Blocking occurs when one transaction is holding a lock on a resource and a second transaction requires a conflicting lock type on that resource. The second transaction must wait for the first transaction to release its lock in other words, it is blocked by the first transaction. If a transaction holds a lock for an extended period, it can cause a chain of blocked
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Microsoft SQL Server 2005 Architecture and Features
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transactions that are waiting for the first transaction to finish so they can obtain their required locks, a condition referred to as chain blocking. Figure 17-1 shows the concept of blocking.
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Chain Blocking" Tran1UPDATE Table A Blocked by Tran1 Tran2SELECT Table A Blocked by Tran2 Tran3UPDATE Table A
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Figure 17-1 Blocking.
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A deadlock differs from a blocked transaction in that a deadlock involves two blocked transactions, each of which is waiting for the other to release a lock. For example, assume that one transaction is holding an exclusive lock on Table1 and a second transaction is holding an exclusive lock on Table2. Before either exclusive lock is released, the first transaction requires a lock on Table2 and the second transaction requires a lock on Table1. Now each transaction is waiting for the other to release its exclusive lock, yet neither transaction will release its exclusive lock until a commit or rollback occurs to complete the transaction. Neither transaction can be completed because it requires a lock held by the other transaction in order to continue. Thus, the two transactions are in a deadlock. Figure 17-2 illustrates a deadlock scenario. When a deadlock occurs, SQL Server chooses to terminate one of the transactions, called the victim, and that transaction will have to be run again. If transactions are long running, then locks on the data may also be held for the duration of the transaction. For example, a single UPDATE statement that updates a large number of rows in a table can take considerable time to complete, thus holding locks for that entire transaction duration. For another example, assume a transaction consists of multiple update and insert statements. In this case, the locks on all data modifications are held for the duration of the transaction, so the more work performed during the transaction, the more locks will be held and the longer it will take. This can cause blocking and
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Transactions and Locking
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deadlocking with other processes. Therefore, it is important to keep transactions as short and quick as possible to avoid long blocking times.
"Deadlock" Tran1 BEGIN TRAN UPDATE Table A; SELECT Table B; COMMIT
cke Blo d
Tran2 BEGIN TRAN UPDATE Table B; SELECT Table A; COMMIT
Figure 17-2 Deadlock.
As already mentioned in this chapter, there are several factors that affect blocking, including using locking hints and setting isolation levels. Isolation levels are discussed in the next section. More Info
For information about how to code to avoid deadlocks, look up the Minimizing Deadlocks topic in SQL Server Books Online.
Isolation Levels
SQL Server 2005 supports five isolation levels that affect the way locking behavior for read operations is handled. There is one new isolation level and one new option to an existing isolation level for SQL Server 2005 that are intended to enhance concurrency for online transaction processing (OLTP) applications: snapshot isolation and read committed snapshot. These depend on a new feature called row versioning that can be used to avoid reader-writer blocking scenarios. The transaction isolation level determines the level at which a transaction is allowed to read inconsistent data that is, the degree to which one transaction is isolated from another. A higher isolation level increases data accuracy, but it can reduce the number of concurrent transactions. On the other hand, a lower isolation level allows more concurrency but results in reduced data accuracy. These isolation levels are set at the SQL Server session, or connection, level and last for the duration of the session. Note that some of them correspond directly to locking hints, which can be set at the statement level. (See the previous section Locking Hints. ) The isolation level you specify for a SQL Server session, or the default if you do not specify a level, determines the locking behavior for all SELECT statements performed during that session until the isolation level is modified.
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