The Hedera Consensus Service (HCS) gRPC API is a public mirror node managed by Hedera. It offers the ability to subscribe to HCS topics and receive messages for the topic subscribed. API docs for the mirror nodes can be found here:
The non-production public mainnet mirror node serves to help developers build their applications without having to run their own mirror node. For production-ready mainnet mirror nodes please check out Arkhia, Dragonglass, or Ledger Works. When building your Hedera client via SDK, you can use setMirrorNetwork() and enter the public mainnet mirror node endpoint. The gRPC API requires TLS. The following SDK versions support TLS:
Requests are throttled at 100 requests per second (rps). This may change in the future depending upon performance or security considerations. At this time, no authentication is required.