Quiescence, in a nutshell, is your mom telling you to “finish what you’re doing but don’t start anything new, we’re getting ready to go”.
It’s an integral capability of load balancers (of enterprise-class load balancers, at least) that enables the graceful shutdown of application instances for a variety of purposes (patches, scheduled maintenance, etc… ).
In more modern architectures this capability forms the foundation for non-disruptive (and thus live) migration of virtual machines.During the process the VM is moved and launched in location Y, the load balancer continues to send requests to the same VM in location X. Once the VM is available in location Y, the load balancer will no longer send new requests to location X but will continue to manage existing connections until they are complete.
Cloud bursting, too, is enabled by the ability of a load balancer to quiesce connections at a global layer (virtual pattern) and at the local layer (bridged pattern).
Load balancers must be able to support a “finish what’s been started but don’t start anything new” mode of operation on any given application.