The stored procedure reports the status for the system, resource, IO subsystem, query The SQL Server Always on resource DLL uses the sp_server_diagnostics stored procedure for the The case of AG failover, SQL Server logs another entry to stop the lease renewal. If the lease timeout period elapsed while waiting for the signal, the AG resource goes into the resolving state. Resource host maintains a time-to-leave (TTL), and it gets updated each time threads wait up after a signal from In the SQL Server logs on the primary replica, you get a message from the lease worker. The lease worker thread and resource host work circularly, as shown in the below image. By default, it is set to 20000 milliseconds (20 seconds). In the properties, you get configured values for LeaseTimeout. Go to Windows Failover Cluster and right-click on the cluster resource. On the primary replica, SQL Server uses a dedicated lease worker thread. The Windows failover cluster uses the Looks-Alive mechanism for an integration between the cluster resource host and SQL process. This process is known as Looks-Alive and runs every 5 seconds. If due to any reason, the cluster could not determine the quorum majority, all nodes in a cluster go into the resolving state and all cluster resources are taken offline. Subsequently, it broadcasts a message to other cluster nodes, and a node takes over the cluster ownership based on the quorum votes and majority configuration. If it does not receive a heartbeat acknowledgment after a series of heartbeats, it assumes that the corresponding node is down. In a Windows failover cluster, the cluster service continuously transmits heartbeat to other cluster nodes and waits for the acknowledgment. In the below image, we see two levels of monitoring in an AG configuration. These DLLs are responsible for health monitoring and offline-online resource management. SQL Server uses resource DLLs for the resource monitor. The resource host acts as an interface between the cluster resource and service. The cluster requires cluster services on all participating nodes. The availability group feature requires an underlying Windows Failover Clusters. SQL Server failover diagnostics ( sp_server_diagnostics) logs Useful factors for monitoring and investigating the failures. It is an important aspect to determine the auto-failover causes in a production database environment. The flexible failover policy depends on the Health-check timeout threshold, Failure-Condition Level and Cluster timeouts. However, an automatic failover requires the satisfied conditions defined in the flexible failover policy. In the case of automatic failover, the availability group fails over to a synchronized secondary replica without any data loss. You can do forced failover with possible data loss We can have the following types of failover. Suppose you have an availability group configured for automatic failover. In the article, Session timeouts in SQL Server Always On Availability Groups, we explained the session timeout configurations to safeguard AG from soft errors. SQL Server Always On Availability Groups provides resilience for high availability and disaster recovery solution in a multi-node architecture. In the 42 nd article for SQL Server Always On Availability Groups, we are going to explore lease timeouts
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