mirror of https://github.com/grpc/grpc.git
The C based gRPC (C++, Python, Ruby, Objective-C, PHP, C#)
https://grpc.io/
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63 lines
3.4 KiB
63 lines
3.4 KiB
7 years ago
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# gRPC Concepts Overview
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Remote Procedure Calls (RPCs) provide a useful abstraction for building
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distributed applications and services. The libraries in this repository
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provide a concrete implementation of the gRPC protocol, layered over HTTP/2.
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These libraries enable communication between clients and servers using any
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combination of the supported languages.
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## Interface
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Developers using gRPC start with the description of an RPC service (a collection
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of methods), and generate client and server side interfaces. The server implements
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the service interface, which can be remotely invoked by the client interface.
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By default, gRPC uses [Protocol Buffers](https://github.com/google/protobuf) as the
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Interface Definition Language (IDL) for describing both the service interface
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and the structure of the payload messages. It is possible to use other
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alternatives if desired.
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### Invoking & handling remote calls
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Starting from an interface definition in a .proto file, gRPC provides
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Protocol Compiler plugins that generate Client- and Server-side APIs.
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gRPC users call into these APIs on the Client side and implement
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the corresponding API on the server side.
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#### Synchronous vs. asynchronous
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Synchronous RPC calls, that block until a response arrives from the server, are
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the closest approximation to the abstraction of a procedure call that RPC
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aspires to.
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On the other hand, networks are inherently asynchronous and in many scenarios,
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it is desirable to have the ability to start RPCs without blocking the current
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thread.
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The gRPC programming surface in most languages comes in both synchronous and
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asynchronous flavors.
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## Streaming
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gRPC supports streaming semantics, where either the client or the server (or both)
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send a stream of messages on a single RPC call. The most general case is
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Bidirectional Streaming where a single gRPC call establishes a stream where both
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the client and the server can send a stream of messages to each other. The streamed
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messages are delivered in the order they were sent.
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# Protocol
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The [gRPC protocol](doc/PROTOCOL-HTTP2.md) specifies the abstract requirements for communication between
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clients and servers. A concrete embedding over HTTP/2 completes the picture by
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fleshing out the details of each of the required operations.
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## Abstract gRPC protocol
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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`).
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## Implementation over HTTP/2
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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).
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## Flow Control
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gRPC uses the flow control mechanism in HTTP/2. This enables fine-grained control of memory used for buffering in-flight messages.
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