SQL Server Extended Events（下面简称XEvent）是SQL Server 2008里新加的事件处理系统，用来取代SQL Server原先的SQL Trace的跟踪机制。事件处理系统对一个复杂服务器系统的排错，调试是极为关键的。和SQL Server原来的事件处理系统相比较，XEvent具有下列的优势：
消耗更少的系统资源，更适用于在产品服务器上的排错和调试。并且每收集一个系统事件所消耗的资源都是可预测的。
当前台应用性能下降的时候，数据库管理员经常需要确定是否有资源锁的申请冲突，既所谓我们说的blocking。在SQL Server 2008里面，你可以查询系统视图sys.dm_exec_requests，并且能发现有一些进程正在等待资源锁，比如像下面这个结果：
我们可以看到有个Session 54的状态是Suspended,意味着一个查询的操作被blocked, 但是我们看不到是哪一个Query占用了资源，以及查询计划和T-SQL Stack等等，如果我有下面的XEvent Session我们就能够解决这个问题。（在下面的例子中我们假设XEvent Session在Block发生之前已经启动好了）
1、在查询编辑器执行下面的命令，这会创建一个XEvent Session来捕捉lock_acquired和lock_released，同时我们还会抓下sql_text，tsql_stack和plan_handle等信息。
-- Perform cleanup.
IF EXISTS(SELECT * FROM sys.server_event_sessions WHERE name='FindBlockers')
DROP EVENT SESSION FindBlockers ON SERVER
GO
-- 我们这边假设是数据库”DAA”里出了block,所以我们会使用一个predicate去过滤掉其他数据库的Block问题
--
DECLARE @dbid int
SELECT @dbid = db_id('DAA')
IF @dbid IS NULL
BEGIN
RAISERROR('DAA is not installed. Install DAA before proceeding', 17, 1)
RETURN
END
DECLARE @sql nvarchar(1024)
SET @sql = '
CREATE EVENT SESSION FindBlockers ON SERVER
--这里我们加了lock_acquired的Event,并要求返回sql_text，database_id,tsql_stack, --plan_handle和session_id的信息
--
ADD EVENT sqlserver.lock_acquired
(action
( sqlserver.sql_text, sqlserver.database_id, sqlserver.tsql_stack,
sqlserver.plan_handle, sqlserver.session_id)
WHERE ( database_id=' + cast(@dbid as nvarchar) + ' AND resource_0!=0)
),
ADD EVENT sqlserver.lock_released
(WHERE ( database_id=' + cast(@dbid as nvarchar) + ' AND resource_0!=0 ))
ADD TARGET package0.pair_matching
( SET begin_event=''sqlserver.lock_acquired'',
begin_matching_columns=''database_id, resource_0, resource_1, resource_2, transaction_id, mode'',
end_event=''sqlserver.lock_released'',
end_matching_columns=''database_id, resource_0, resource_1, resource_2, transaction_id, mode'',
respond_to_memory_pressure=1)
WITH (max_dispatch_latency = 1 seconds)'
EXEC (@sql)
--
-- Create the metadata for the event session
-- Start the event session
--
ALTER EVENT SESSION FindBlockers ON SERVER
STATE = START
2、一旦XEVENT然后我们可以执行下面的语句去找到那些block了别人的查询语句：
--
-- The pair matching targets report current unpaired events using
-- the sys.dm_xe_session_targets dynamic management view (DMV)
-- in XML format.
-- The following query retrieves the data from the DMV and stores
-- key data in a temporary table to speed subsequent access and
-- retrieval.
--
SELECT
objlocks.value('(action/value)[5]', 'int')
AS session_id,
objlocks.value('(data/value)[5]', 'int')
AS database_id,
objlocks.value('(data/text)[1]', 'nvarchar(50)' )
AS resource_type,
objlocks.value('(data/value)[9]', 'bigint')
AS resource_0,
objlocks.value('(data/value)[10]', 'bigint')
AS resource_1,
objlocks.value('(data/value)[11]', 'bigint')
AS resource_2,
objlocks.value('(data/text)[2]', 'nvarchar(50)')
AS mode,
objlocks.value('(action/value)[1]', 'varchar(MAX)')
AS sql_text,
CAST(objlocks.value('(action/value)[4]', 'varchar(MAX)') AS xml)
AS plan_handle,
CAST(objlocks.value('(action/value)[3]', 'varchar(MAX)') AS xml)
AS tsql_stack
INTO #unmatched_locks
FROM (
SELECT CAST(xest.target_data as xml)
lockinfo
FROM sys.dm_xe_session_targets xest
JOIN sys.dm_xe_sessions xes ON xes.address = xest.event_session_address
WHERE xest.target_name = 'pair_matching' AND xes.name = 'FindBlockers'
) heldlocks
CROSS APPLY lockinfo.nodes('//event[@name="lock_acquired"]') AS T(objlocks)
--
-- Join the data acquired from the pairing target with other
-- DMVs to return provide additional information about blockers
--
SELECT ul.*
FROM #unmatched_locks ul
INNER JOIN sys.dm_tran_locks tl ON ul.database_id = tl.resource_database_id AND ul.resource_type = tl.resource_type
WHERE resource_0 IS NOT NULL
AND session_id IN
(SELECT blocking_session_id FROM sys.dm_exec_requests WHERE blocking_session_id != 0)
AND tl.request_status='wait'
上面的语句返回下面的结果，我们可以看到是一个Session 53的Update更新语句Block了Session 54的查询：
有下面sql_text里可以看到，整个更新语句begin tran Update BlockTest set Col1 = 'Updated' where ID = 1中，由于少了一个Commit tran，所以block了其他查询对表BlockTest的操作，
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