What maintains a list of running instances of microservices?

The correct choice is the Service Registry, as it plays a crucial role in microservices architecture by maintaining a list of running instances of microservices. This component is essential for service discovery, allowing microservices to dynamically locate each other without the need for hardcoded addresses.

The Service Registry stores metadata about the services, including their location, health status, and other relevant information. This enables various microservices to communicate effectively, ensuring that requests can be routed to the appropriate service instance that is currently operational.

This dynamic nature of service registration and discovery helps in managing load balancing, failover, and overall resilience of the microservices ecosystem, making it a fundamental element in modern cloud-based architectures.

In contrast, while a Microservice Registry may sound similar, it typically refers to a domain-specific implementation that may not encompass the broader functionality of a Service Registry. The Instance Tracker, on the other hand, might suggest a focus on tracking individual service instances but lacks the comprehensive service discovery capabilities offered by the Service Registry.