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+# 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.