A service registry is a database of services, their instances and their locations. 

The interaction between a service and the registry is called registration. 

 Service instances are registered with the service registry on startup and deregistered on shutdown. Client of the service and/or routers query the service registry to find the available instances of a service.

 A service registry might invoke a service instance’s health check API to verify that it is able to handle requests

https://microservices.io/patterns/service-registry.html