Health check calls to the resource continue constantly while resources are online.
LooksAlive is a quick light lightweight check that happens every 5 seconds by default.The most interesting in most cases where resources go unresponsive and you see clustering need to recover is with the LooksAlive and IsAlive which is a health check to the resource. You can find the full list of resource DLL entry-point functions here. These VM actions from the user map to entry point calls that RHS makes to resources, such as Online, Offline, IsAlive, and LooksAlive. Resources all run in a component of the Failover Clustering feature called the Resource Hosting Subsystem (RHS).
The “Virtual Machine” resource and its associated resource DLL communicates with the VMMS service and tells the VM when to start, when to stop, and it also does health checks to ensure the VM is ok. When a Virtual Machine is clustered, there is a cluster “Virtual Machine” resource created which controls that VM. For the sake of simplicity I will use a Virtual Machine as an example throughout this blog, but the logic is generic and applies to all workloads. In this blog I will discuss how Failover Clustering communicates with cluster resources, along with how clustering detects and recovers when something goes wrong.
