grpc/doc/PROTOCOL-WEB.md

# Overview

gRPC-Web provides a JS client library that supports the same API
as gRPC-Node to access a gRPC service. Due to browser limitation,
the Web client library implements a different protocol than the
[native gRPC protocol](http://www.grpc.io/docs/guides/wire.html).
This protocol is designed to make it easy for a proxy to translate
between the protocols as this is the most likely deployment model.

This document lists the differences between the two protocols.
To help tracking future revisions, this document describes a delta
with the protocol details specified in the
[native gRPC protocol](http://www.grpc.io/docs/guides/wire.html).

# Design goals

For the gRPC-Web protocol, we have decided on the following design goals:

* adopt the same framing as “application/grpc” whenever possible
* decouple from HTTP/2 framing which is not, and will never, be directly
exposed by browsers
* support text streams (e.g. base64) in order to provide cross-browser
support (e.g. IE-10)

While the new protocol will be published/reviewed publicly, we also
intend to keep the protocol as an internal detail to gRPC-Web.
More specifically, we expect the protocol to

* evolve over time, mainly to optimize for browser clients or support
web-specific features such as CORS, XSRF
* become optional (in 1-2 years) when browsers are able to speak the native
gRPC protocol via the new [whatwg fetch/streams API](https://github.com/whatwg/fetch)

# Protocol differences vs [gRPC over HTTP2](http://www.grpc.io/docs/guides/wire.html)

Content-Type

1. application/grpc-web
  * e.g. application/grpc-web+[proto, json, thrift]
2. application/grpc-web-text
  * text-encoded streams of “application/grpc-web”

---

HTTP wire protocols

1. support any HTTP/*, with no dependency on HTTP/2 specific framing
2. use lower-case header/trailer names
3. use EOF (end of body) to close the stream

---

HTTP/2 related behavior (specified in [gRPC over HTTP2](http://www.grpc.io/docs/guides/wire.html))

1. stream-id is not supported or used
2. go-away is not supported or used

---

Message framing (vs. [http2-transport-mapping](http://www.grpc.io/docs/guides/wire.html#http2-transport-mapping))

1. Response status encoded as part of the response body
  * Key-value pairs encoded in the HTTP/2 [literal header format](https://tools.ietf.org/html/rfc7541#section-6.2) as a single header block.
2. 8th (MSB) bit of the 1st gRPC frame byte
  * 0: data
  * 1: trailers
3. Trailers must be the last message of the response, as enforced
by the implementation
4. Trailers-only responses: no change to the gRPC protocol spec.
Trailers will be sent together with response headers, with no message
in the body.

---

User Agent

* grpc-web-javascript/0.1

---

Text-encoded (response) streams

1. The client library should indicate to the server via the "Accept" header that
the response stream needs to be text encoded e.g. when XHR is used or due
to security policies with XHR
  * Accept: application/grpc-web-text
2. The default text encoding is base64
  * Text encoding may be specified with Content-Type or Accept headers as
      * application/grpc-web-text-base64
  * Note that “Content-Transfer-Encoding: base64” should not be used.
  Due to in-stream base64 padding when delimiting messages, the entire
  response body is not necessarily a valid base64-encoded entity
  * While the server runtime will always base64-encode and flush gRPC messages
  atomically the client library should not assume base64 padding always
  happens at the boundary of message frames.
3. For binary trailers, when the content-type is set to
application/grpc-web-text, the extra base64 encoding specified
in [gRPC over HTTP2](http://www.grpc.io/docs/guides/wire.html)
for binary custom metadata is skipped.

# Other features

Compression

* Full-body compression is supported and expected for all unary
requests/responses. The compression/decompression will be done
by browsers, using standard Content-Encoding headers
  * “grpc-encoding” header is not used
  * SDCH, Brotli will be supported
* Message-level compression for streamed requests/responses is not supported
because manual compression/decompression is prohibitively expensive using JS
  * Per-message compression may be feasible in future with wasm

---

Retries, caching

* Will spec out the support after their respective gRPC spec extensions
are finalized
  * Safe retries: PUT
  * Caching: header encoded request and/or a web specific spec

---

Security

* XSRF, XSS etc to be specified

---

CORS preflight

* The client library may support header overwrites to avoid preflight
  * https://github.com/whatwg/fetch/issues/210
* CSP support to be specified

---

Keep-alive

* HTTP/2 PING is not supported or used
* Will not support send-beacon (GET)

---

Bidi-streaming, with flow-control

* Pending on [whatwg fetch/streams](https://github.com/whatwg/fetch) to be
finalized and implemented in modern browsers
* gRPC-Web client will support the native gRPC protocol with modern browsers