From f389e5267afe7a7f735fd7e4c2f923640d2f72c5 Mon Sep 17 00:00:00 2001 From: Jan Tattermusch Date: Tue, 12 Jun 2018 17:26:31 +0200 Subject: overhaul of top-level .md files --- CONCEPTS.md | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) create mode 100644 CONCEPTS.md (limited to 'CONCEPTS.md') diff --git a/CONCEPTS.md b/CONCEPTS.md new file mode 100644 index 0000000000..6855d43d39 --- /dev/null +++ b/CONCEPTS.md @@ -0,0 +1,62 @@ +# 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 the description of an RPC service (a collection +of methods), and generate client and server side interfaces. 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 where 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. -- cgit v1.2.3