SQL Server Extended Events（下面簡稱XEvent）是SQL Server 2008裡新加的事件處理系統，用來取代SQL Server原先的SQL Trace的跟蹤機制。事件處理系統對一個復雜服務器系統的排錯，調試是極為關鍵的。和SQL Server原來的事件處理系統相比較，XEvent具有下列的優勢：
下面是一個簡單的例子介紹如何使用XEvent來做系統排錯工作。
當前台應用性能下降的時候，數據庫管理員經常需要確定是否有資源鎖的申請沖突，既所謂我們說的blocking。在SQL Server 2008裡面，你可以查詢系統視圖sys.dm_exec_requests，並且能發現有一些進程正在等待資源鎖，比如像下面這個結果：
我們可以看到有個Session 54的狀態是Suspended,意味著一個查詢的操作被blocked, 但是我們看不到是哪一個Query占用了資源，以及查詢計劃和T-SQL Stack等等，如果我有下面的XEvent Session我們就能夠解決這個問題。（在下面的例子中我們假設XEvent Session在Block發生之前已經啟動好了）
1、在查詢編輯器執行下面的命令，這會創建一個XEvent Session來捕捉lock_acquired和lock_released，同時我們還會抓下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
-- 我們這邊假設是數據庫”DAA”裡出了block,所以我們會使用一個predicate去過濾掉其他數據庫的Block問題
--
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
--這裡我們加了lock_acquired的Event,並要求返回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然後我們可以執行下面的語句去找到那些block了別人的查詢語句：
--
-- 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'
上面的語句返回下面的結果，我們可以看到是一個Session 53的Update更新語句Block了Session 54的查詢：
有下面sql_text裡可以看到，整個更新語句begin tran Update BlockTest set Col1 = 'Updated' where ID = 1中，由於少了一個Commit tran，所以block了其他查詢對表BlockTest的操作，
XEvent還有很多強大的功能和更廣應用，有機會的話以後再為大家介紹。
