Flow control is relevant for streaming RPC calls.

The underlying layer will make the write wait when there is no space to write the next message. This causes the request stream to go into a not ready state and the method invocation waits.

In server case, gRPC will pause the server implementation that is sending the messages too fast. Server implementation is in server_flow_control_server.cc. It will write a specified number of responses of a specified size as fast as possible. As client-side buffer is filled, the write operation will block until the buffer is freed.

A client implementation in server_flow_control_client.cc will delay for 1s before starting a next read to simulate client that does not have resources for handling the replies.

Also see gRPC Flow Control Users Guide