1# gRPC Concepts Overview 2 3Remote Procedure Calls (RPCs) provide a useful abstraction for building 4distributed applications and services. The libraries in this repository 5provide a concrete implementation of the gRPC protocol, layered over HTTP/2. 6These libraries enable communication between clients and servers using any 7combination of the supported languages. 8 9 10## Interface 11 12Developers using gRPC start with a language agnostic description of an RPC service (a collection 13of methods). From this description, gRPC will generate client and server side interfaces 14in any of the supported languages. The server implements 15the service interface, which can be remotely invoked by the client interface. 16 17By default, gRPC uses [Protocol Buffers](https://github.com/google/protobuf) as the 18Interface Definition Language (IDL) for describing both the service interface 19and the structure of the payload messages. It is possible to use other 20alternatives if desired. 21 22### Invoking & handling remote calls 23Starting from an interface definition in a .proto file, gRPC provides 24Protocol Compiler plugins that generate Client- and Server-side APIs. 25gRPC users call into these APIs on the Client side and implement 26the corresponding API on the server side. 27 28#### Synchronous vs. asynchronous 29Synchronous RPC calls, that block until a response arrives from the server, are 30the closest approximation to the abstraction of a procedure call that RPC 31aspires to. 32 33On the other hand, networks are inherently asynchronous and in many scenarios, 34it is desirable to have the ability to start RPCs without blocking the current 35thread. 36 37The gRPC programming surface in most languages comes in both synchronous and 38asynchronous flavors. 39 40 41## Streaming 42 43gRPC supports streaming semantics, where either the client or the server (or both) 44send a stream of messages on a single RPC call. The most general case is 45Bidirectional Streaming where a single gRPC call establishes a stream in which both 46the client and the server can send a stream of messages to each other. The streamed 47messages are delivered in the order they were sent. 48 49 50# Protocol 51 52The [gRPC protocol](doc/PROTOCOL-HTTP2.md) specifies the abstract requirements for communication between 53clients and servers. A concrete embedding over HTTP/2 completes the picture by 54fleshing out the details of each of the required operations. 55 56## Abstract gRPC protocol 57A gRPC call comprises of a bidirectional stream of messages, initiated by the client. In the client-to-server direction, this stream begins with a mandatory `Call Header`, followed by optional `Initial-Metadata`, followed by zero or more `Payload Messages`. The server-to-client direction contains an optional `Initial-Metadata`, followed by zero or more `Payload Messages` terminated with a mandatory `Status` and optional `Status-Metadata` (a.k.a.,`Trailing-Metadata`). 58 59## Implementation over HTTP/2 60The abstract protocol defined above is implemented over [HTTP/2](https://http2.github.io/). gRPC bidirectional streams are mapped to HTTP/2 streams. The contents of `Call Header` and `Initial Metadata` are sent as HTTP/2 headers and subject to HPACK compression. `Payload Messages` are serialized into a byte stream of length prefixed gRPC frames which are then fragmented into HTTP/2 frames at the sender and reassembled at the receiver. `Status` and `Trailing-Metadata` are sent as HTTP/2 trailing headers (a.k.a., trailers). 61 62## Flow Control 63gRPC uses the flow control mechanism in HTTP/2. This enables fine-grained control of memory used for buffering in-flight messages. 64