CONCEPTS.md - platform/external/grpc-grpc - Git at Google

 # gRPC Concepts Overview

 Remote Procedure Calls (RPCs) provide a useful abstraction for building
 distributed applications and services. The libraries in this repository
 provide a concrete implementation of the gRPC protocol, layered over HTTP/2.
 These libraries enable communication between clients and servers using any
 combination of the supported languages.


 ## Interface

 Developers using gRPC start with a language agnostic description of an RPC service (a collection
 of methods). From this description, gRPC will generate client and server side interfaces
 in any of the supported languages. The server implements
 the service interface, which can be remotely invoked by the client interface.

 By default, gRPC uses [Protocol Buffers](https://github.com/google/protobuf) as the
 Interface Definition Language (IDL) for describing both the service interface
 and the structure of the payload messages. It is possible to use other
 alternatives if desired.

 ### Invoking & handling remote calls
 Starting from an interface definition in a .proto file, gRPC provides
 Protocol Compiler plugins that generate Client- and Server-side APIs.
 gRPC users call into these APIs on the Client side and implement
 the corresponding API on the server side.

 #### Synchronous vs. asynchronous
 Synchronous RPC calls, that block until a response arrives from the server, are
 the closest approximation to the abstraction of a procedure call that RPC
 aspires to.

 On the other hand, networks are inherently asynchronous and in many scenarios,
 it is desirable to have the ability to start RPCs without blocking the current
 thread.

 The gRPC programming surface in most languages comes in both synchronous and
 asynchronous flavors.


 ## Streaming

 gRPC supports streaming semantics, where either the client or the server (or both)
 send a stream of messages on a single RPC call. The most general case is
 Bidirectional Streaming where a single gRPC call establishes a stream in which both
 the client and the server can send a stream of messages to each other. The streamed
 messages are delivered in the order they were sent.


 # Protocol

 The [gRPC protocol](doc/PROTOCOL-HTTP2.md) specifies the abstract requirements for communication between
 clients and servers. A concrete embedding over HTTP/2 completes the picture by
 fleshing out the details of each of the required operations.

 ## Abstract gRPC protocol
 A 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`).

 ## Implementation over HTTP/2
 The 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).

 ## Flow Control
 gRPC uses the flow control mechanism in HTTP/2. This enables fine-grained control of memory used for buffering in-flight messages.
	# gRPC Concepts Overview

	Remote Procedure Calls (RPCs) provide a useful abstraction for building
	distributed applications and services. The libraries in this repository
	provide a concrete implementation of the gRPC protocol, layered over HTTP/2.
	These libraries enable communication between clients and servers using any
	combination of the supported languages.


	## Interface

	Developers using gRPC start with a language agnostic description of an RPC service (a collection
	of methods). From this description, gRPC will generate client and server side interfaces
	in any of the supported languages. The server implements
	the service interface, which can be remotely invoked by the client interface.

	By default, gRPC uses [Protocol Buffers](https://github.com/google/protobuf) as the
	Interface Definition Language (IDL) for describing both the service interface
	and the structure of the payload messages. It is possible to use other
	alternatives if desired.

	### Invoking & handling remote calls
	Starting from an interface definition in a .proto file, gRPC provides
	Protocol Compiler plugins that generate Client- and Server-side APIs.
	gRPC users call into these APIs on the Client side and implement
	the corresponding API on the server side.

	#### Synchronous vs. asynchronous
	Synchronous RPC calls, that block until a response arrives from the server, are
	the closest approximation to the abstraction of a procedure call that RPC
	aspires to.

	On the other hand, networks are inherently asynchronous and in many scenarios,
	it is desirable to have the ability to start RPCs without blocking the current
	thread.

	The gRPC programming surface in most languages comes in both synchronous and
	asynchronous flavors.


	## Streaming

	gRPC supports streaming semantics, where either the client or the server (or both)
	send a stream of messages on a single RPC call. The most general case is
	Bidirectional Streaming where a single gRPC call establishes a stream in which both
	the client and the server can send a stream of messages to each other. The streamed
	messages are delivered in the order they were sent.


	# Protocol

	The [gRPC protocol](doc/PROTOCOL-HTTP2.md) specifies the abstract requirements for communication between
	clients and servers. A concrete embedding over HTTP/2 completes the picture by
	fleshing out the details of each of the required operations.

	## Abstract gRPC protocol
	A 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`).

	## Implementation over HTTP/2
	The 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).

	## Flow Control
	gRPC uses the flow control mechanism in HTTP/2. This enables fine-grained control of memory used for buffering in-flight messages.