Find more information over HCS service in the link below
HCS gRPC is a native API part of the infrastructure made available by the mirror node. It enables for Dapps to subscribe to HCS topics and receive messages broadcasted downstream among others.
Arkhia makes this API available to all of our users, although it's functionality is rate limited and not adequate for production based environments that would need high reliability /volume needs. If you do have streaming needs for production environments, please take a look at our augmented API Watchtower