AlwaysOn availability groups introduce the new flexible failover policy for SQL Server instance health monitoring for the AlwaysOn availability group resource.
Legacy clustered SQL Server utilized a LooksAlive that performed a lightweight check of the SQL Server process health. The legacy IsAlive connected to SQL Server and executed a simple query.
AlwaysOn flexible failover policy offers a more comprehensive health monitoring model that is configurable. When creating or modifying an availability group, the failure_condition_level property can be set or adjusted. This property supports values of one to five, with one performing the most lightweight checks up to five which includes more comprehensive internal SQL Server health monitoring.
For more information on availability group flexible failover policy settings, see 'Flexible Failover Policy for Automatic Failover of an Availability Group (SQL Server)'
The following discussion gives greater detail on the implementation of Windows cluster LooksAlive and IsAlive by SQL Server 2012 AlwaysOn failover cluster instance (SQLFCI) and availability groups.
Implementation of health monitoring Using LooksAlive and IsAlive
Once an availability group is created, the host process of the SQL resource DLL sets up health monitoring with SQL Server and begins periodic LooksAlive and IsAlive operations to satisfy health monitoring.
The SQL resource DLL initiates the lease thread which begins communicating with a dedicated thread in SQL Server. For more information on the lease mechanism, see 'How It Works: SQL Server AlwaysOn Lease Timeout'