gRPC transport
At a high level there are three distinct layers to the library: Stub, Channel, and Transport.
Stub
The Stub layer is what is exposed to most developers and provides type-safe bindings to whatever datamodel/IDL/interface you are adapting. gRPC comes with a plugin to the protocol-buffers compiler that generates Stub interfaces out of .proto files, but bindings to other datamodel/IDL are easy and encouraged.
Channel
The Channel layer is an abstraction over Transport handling that is suitable for interception/decoration and exposes more behavior to the application than the Stub layer. It is intended to be easy for application frameworks to use this layer to address cross-cutting concerns such as logging, monitoring, auth, etc.
Transport
The Transport layer does the heavy lifting of putting and taking bytes off the wire. The interfaces to it are abstract just enough to allow plugging in of different implementations. Note the transport layer API is considered internal to gRPC and has weaker API guarantees than the core API under package io.grpc.
gRPC comes with multiple Transport implementations:
- The Netty-based HTTP/2 transport is the main transport implementation based on Netty. It is not officially supported on Android. There is a "grpc-netty-shaded" version of this transport. It is generally preferred over using the Netty-based transport directly as it requires less dependency management and is easier to upgrade within many applications.
- The OkHttp-based HTTP/2 transport is a lightweight transport based on Okio and forked low-level parts of OkHttp. It is mainly for use on Android.
- The in-process transport is for when a server is in the same process as the client. It is used frequently for testing, while also being safe for production use.
- The Binder transport is for Android cross-process communication on a single device.