New instances can start up at any point in time

Existing instances can stop responding and eventually crash at any point in time

Some of the failing instances might be okay after a while and should start to receive traffic again, while others will not and should be removed from the service registry

Some microservice instances might take some time to start up; that is, just because they can receive HTTP requests doesn't mean that traffic should be routed to them

Unintended network partitioning and other network-related errors can occur at any time