upc internet cennik Identifying Lock Resources in VB.NET

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Identifying Lock Resources
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When SQL Server tries to determine whether a requested lock can be granted, it checks the sys.dm_tran_locks view to determine whether a matching lock with a con icting lock mode already exists. It compares locks by looking at the database ID (resource_database_ID), the values in the resource_description and resource_associated_entity_id columns, and the type of resource locked. SQL Server knows nothing about the meaning of the resource description. It simply compares the strings identifying the lock resources to look for a match. If it nds a match with a request_status value of GRANT, it knows the resource is already locked; it then uses the lock compatibility matrix to determine whether the current lock is compatible with the one being requested. Table 10-5 shows many of the possible lock resources that are displayed in the rst column of the sys.dm_tran_locks view and the information in the resource_description column, which is used to de ne the actual resource locked.
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TABLE 10-5
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Lockable Resources in SQL Server
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Resource_Description
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None; the database is always indicated in the resource_ database_ID column for every locked resource. The object ID (which can be any database object, not necessarily a table) is reported in the resource_associated_ entity_id column. hobt_id is reported in the resource_associated_entity_id column. Used only when partition locking has been enabled for a table. File number:page number of the rst page of the extent. File number:page number of the actual table or index page. A hashed value derived from all the key components and the locator. For a nonclustered index on a heap, where columns c1 and c2 are indexed, the hash will contain contributions from c1, c2, and the RID. File number:page number:slot number of the actual row.
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Resource_Type
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DATABASE OBJECT
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Example
12 69575286
HOBT
72057594038779904
EXTENT PAGE KEY
1:96 1:104 ac0001a10a00
1:161:3
Note that key locks and key-range locks have identical resource descriptions because key range is considered a mode of locking, not a locking resource. When you look at output from the sys.dm_tran_locks view, you see that you can distinguish between these types of locks by the value in the lock mode column. Another type of lockable resource is METADATA. More than any other resource, METADATA resources are divided into multiple subtypes, which are described in the resource_subtype column of sys.dm_tran_locks. You might see dozens of subtypes of METADATA resources, but most of them are beyond the scope of this book. For some, however, even though SQL Server Books Online describes them as for internal use only, it is pretty obvious what they refer to. For example, when you change properties of a database, you can see a resource_type of METADATA and a resource_subtype of DATABASE. The value in the resource_description column of that row is database_id =<ID>, indicating the ID of the database whose metadata is currently locked.
Associated Entity ID
For locked resources that are part of a larger entity, the resource_associated_entity_id column in sys.dm_tran_locks displays the ID of that associated entity in the database. This can be an object ID, a partition ID, or an allocation unit ID, depending on the resource type. Of course, for some resources, such as DATABASE and EXTENT, there is no resource_associated_entity_id. An object ID value is given in this column for OBJECT resources, and an allocation unit ID is given for ALLOCATION_UNIT resources. A partition ID is provided for resource types PAGE, KEY, and RID.
Microsoft SQL Server 2008 Internals
There is no simple function to convert a partition ID value to an object name; you have to actually select from the sys.partitions view. The following query translates all the resource_ associated_entity_id values for locks in the current database by joining sys.dm_tran_locks to sys.partitions. For OBJECT resources, the object_name function is applied to the resource_ associated_entity_id column. For PAGE, KEY, and RID resources, I use the object_name function with the object_id value from the sys.partitions view. For other resources for which there is no resource_associated_entity_id, the code just returns n/a. Because the code references the sys.partitions view, which occurs in each database, this code is ltered to return only lock information for resources in the current database. The output is organized to re ect the information returned by the sp_lock procedure, but you can add any additional lters or columns that you need. I will use this query in many examples later in this chapter, so I create a VIEW based on the SELECT and call it DBlocks:
CREATE VIEW DBlocks AS SELECT request_session_id as spid, db_name(resource_database_id) as dbname, CASE WHEN resource_type = 'OBJECT' THEN object_name(resource_associated_entity_id) WHEN resource_associated_entity_id = 0 THEN 'n/a' ELSE object_name(p.object_id) END as entity_name, index_id, resource_type as resource, resource_description as description, request_mode as mode, request_status as status FROM sys.dm_tran_locks t LEFT JOIN sys.partitions p ON p.partition_id = t.resource_associated_entity_id WHERE resource_database_id = db_id();
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