Health checking is a key function of an application delivery controller (ADC) or load balancer, ensuring that traffic is only routed to servers that are able to respond effectively. To perform a health check, the application delivery controller (ADC) polls the members of the server cluster to test whether an application is available. If a server responds successfully to a given number of probes, it is considered healthy, and remains eligible to receive new requests. If the server fails to respond successfully, it is considered unhealthy, removed from the cluster, and flagged for attention by IT.
The load balancing function can then redistribute traffic across the remaining members of the cluster to ensure high availability and optimal service. Once IT has addressed the underlying issue in the server and brought it back to health, it can be restored to the cluster and begin delivering apps again, with the load balancer redistributing traffic accordingly.
Load balancing health checks are enabled by a full-featured application delivery controller (ADC), and are also provided by cloud providers such as Google, Amazon, Microsoft, and Oracle. Indicators used to measure the ongoing health of servers can include such metrics as time series of total bytes in and out from each server; time series of traffic rates (in Mbps) in and out from each server; percent of error traffic over range; number of good SSL connections; average application server latency by service; client-side latency; and the response time for specific SQL database queries.
The A10 Networks Thunder® ADC application delivery and load balancing solution ensures high availability and rapid failover with little or no downtime by continuously monitoring server health. The solution’s health checking functionality is part of a rich set of server load balancing and global server load balancing capabilities.< Back to Glossary of Terms
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