Since the two streams are independent, the client and server can read and write messages in any order

the server is notified that the RPC has been invoked with the client’s metadata for this call, the method name, and the specified deadline if applicable.

protocol buffers as the Interface Definition Language (IDL) for describing both the service interface and the structure of the payload messages.

The gRPC infrastructure decodes incoming requests, executes service methods, and encodes service responses.

gRPC can use protocol buffers as both its Interface Definition Language (IDL) and as its underlying message interchange format

This improves serialization and deserialization times

gRPC 的 service 接口是基于 protobuf 定义的，我们可以非常方便的将 service 与 HTTP/2 关联起来。

一条连接可以包含多个 streams，多个 streams 发送的数据互相不影响。 Stream 可以被 client 和 server 单方面或者共享使用。 Stream 可以被任意一段关闭。 Stream 会确定好发送 frame 的顺序，另一端会按照接受到的顺序来处理。 Stream 用一个唯一 ID 来标识。

Stream： 一个双向流，一条连接可以有多个 streams。 Message： 也就是逻辑上面的 request，response。 Frame:：数据传输的最小单位。每个 Frame 都属于一个特定的 stream 或者整个连接。一个 message 可能有多个 frame 组成。