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The above sequence diagram shows the normal application ping and storage heartbeat being sent by the hypervisor 1 and 2. At some point, the application ping falls behind and the monitor informs CloudStack that the application ping has fallen behind. CloudStack then launches an investigation. On the first timeout, CloudStack was still able to contact the host so it leaves the host's status unchanged. On the second timeout, CloudStack was no longer able to contact the host, so it asks the investigator to check on the host. The first investigator, a PingTestInvestigator, was inconclusive because it timed out on the network ping. The second investigator, the XenServerInvestigator, finds that the storage heartbeat was no longer being written to. At this stage CloudStack hands off to HAManager to HA all of the VMs.

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The above sequence diagram shows HA Manager's investigation process.