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How a Witness Affects Database Availability

Hi All, Where does the witness server keep the Information that pricipal or Mirror Server is UP.. or how does witness server knows the Pricipal or Mirror Server is fail. thanks
database-mirroring
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You can basically understand it that the witness opens a connection to the principal and secondary and it watches to see if the principal dies. If so, and the secondary has problems connecting to the principal too, the witness will then promote the secondary to the active member of the mirror.
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Is there any table Which Stores the Information about that..
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There is a dmv sys.database_mirroring_witnesses check BOL for more details.
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A useful SP is sp_dbmmonitorresults 'DB_NAME' http://msdn.microsoft.com/en-us/library/ms366320.aspx
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i had gone trhough the link given by Cyborg but i didn't understand how the Witness server sence that the Pricipal server fail
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