Windows Cluster Service use the “heartbeat” process to check whether each node is alive at both the OS level and SQL Server level. Heartbeat signals works on UDP.
Microsoft Windows Cluster service perform 2 health checks to perform node & resource availability check.
LooksAlive Health Check
1) LooksAlive is a quick lightweight health check
2) Run at interval of 5 Seconds
3) Does not impact performance but does not perform a thorough check
4) The check will succeed if the service appears to be running even though it might not be operational
5) If Fails calls “IsAlive” health check
6) Polling interval can be changed by adjusting LooksAlivePollInterval property of Cluster service
IsAlive Health Check
1) Run at interval of each 60 Second
2) Perform more detail check then LooksAlive
3) Run @@SERVERNAME to ensure that SQL Server is responding to queries
4) Does not ensure that all user databases are operational
5) Retired 5 times & then try to reconnect SQL Server
6) If all 5 retry fails, the SQL Server resource fails & Windows Cluster service will try to bring it online on other node as per configuration
7) Polling interval can be changed by adjusting IsAlivePollInterval property of Cluster service
Reference : Rohit Garg (http://mssqlfun.com/)
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