Our applications will have to send heartbeat signals to the registry service to notify the registry that the application is alive and running. The registry services rely on this heartbeat signal to register and deregister the application. That is, the existence of the application is determined with the help of this heartbeat ping. This is what will happen in the renew phase.
However, when the Eureka server is booting up, it will try to get all the information about instance registries from the nearby service. If the nearby service fails for any reason, then it will try to connect to all of its peers to fetch the information. If the Eureka server was able to fetch the information for all the servers, then it will set the renewal threshold based on the information received. Based on this information, JHipster Registry will hold the information on whether the current level is below the renewal threshold specified and notify users in the UI.