Adds support for grpc-web

[drowzy]

Adds support for grpc-web.

• Adds two new functions to the GRPC.Codec behaviour in order to base64 decode / encode the payload.

There's an example repo to see it in action.

closes #193

[sleipnir]

This is awesome

[AleksandarFilipov]

Coming from an embedded perspective, being able to use this saves a lot of cpu cycles compared to my previous proxies.

Since the client now can be a web browser the library need to handle CORS. To get this operational I needed to do quite a lot of coors catchup resulting in the following changes from this

grpc/lib/grpc/transport/http2.ex

Lines 11 to 17 in 5b4c66c

	def server_headers(%{codec: GRPC.Codec.WebText = codec}) do
	%{"content-type" => "application/grpc-web-#{codec.name}"}
	end
	def server_headers(%{codec: codec}) do
	%{"content-type" => "application/grpc+#{codec.name}"}
	end

to:

  def server_headers(%{codec: GRPC.Codec.WebText = codec}) do
    %{"content-type" => "application/grpc-web-#{codec.name}",
      "Access-Control-Allow-Origin" => "*",
      "Access-Control-Allow-Headers" => "Content-Type, x-grpc-web, x-user-agent",
    }
  end

  def server_headers(%{codec: codec}) do
    %{"content-type" => "application/grpc+#{codec.name}",
     "Access-Control-Allow-Origin" => "*",
     "Access-Control-Allow-Headers" => "Content-Type, x-grpc-web, x-user-agent",
  }
  end

I'm assuming there is already a mechanism in place on this but I can't wrap my around it so please advice me. Some conclusions.

• cowboy can be configured for CORS but i don't see how to get pass the configuration properly.
• middleware is supported, possibly this can be managed using middleware, for some reason it doesn't feel as "the right way"

@drowzy Possibly you have the answer up your sleeve :)

[sleipnir]

Coming from an embedded perspective, being able to use this saves a lot of cpu cycles compared to my previous proxies.

Since the client now can be a web browser the library need to handle CORS. To get this operational I needed to do quite a lot of coors catchup resulting in the following changes from this
.....

I'm assuming there is already a mechanism in place on this but I can't wrap my around it so please advice me. Some conclusions.

* cowboy can be configured for CORS but i don't see how to get pass the configuration properly.

I think perhaps the best place to implement this is directly in Cowboy Handler https://github.com/elixir-grpc/grpc/blob/master/lib/grpc/adapter/cowboy/handler.ex

[sleipnir]

Any predictions for this to be approved? I think this library seems to have been abandoned by its creators

[polvalente]

This looks great overall! I've left some comments so we can iterate a bit :)

[AleksandarFilipov]

wonderful to see some activity here! Let's try and get some nice solution the the cors issue described above, possibly in a separate issue.

Short recap,
Adding this support suggested that web-client can access the services. Given the (cors) nature of web clients, they will likely be blocked if the accessing web-client is hosted separately. I've posted a solution, it caters for my needs, however it's not generic enough to be PR candidate.

We need to be able to provide headers in a configurable way. I've played with Interceptors (for other purposes), however I don't see a clean match since it will affect all services - regardless if they use grpc-web or not.

[polvalente]

@AleksandarFilipov I think we could use the interceptors in a similar way to how Tesla modifies the current response given the result of next.(stream, req)

https://github.com/elixir-tesla/tesla/blob/master/lib/tesla/middleware/compression.ex

In this example, they decide if the resp body needs to be decompressed based on the result tuple tag. We can provide a custom interceptor that injects response headers based on a predicate MFA tuple (that receives the stream and returns true if the headers should be injected). Something like:

defmodule MyServer do
  use GRPC.Endpoint

  intercept GRPC.Interceptors.ResponseHeaders, should_inject: {__MODULE__, :"add_cors?", 1}, headers: [{"Access-Control-Allow-Origin", "*"}, {"Access-Control-Allow-Headers", "Content-Type, x-grpc-web, x-user-agent"}]

  def add_cors?(...), do: ...
end

The one thing that might need to change (I'm not that familiar with the code base yet) is that this only works if intercepts are called before stream_reply.

I'm almost sure that intercepts are only called after the reply, though. If so, we'll need to introduce something like reply_intercept that will be called right before the reply, but the overall idea remains the same.
This idea would allow us to remove the Codec from the server_headers match as well, while still allowing us to modify the headers (the whole connection, really), as we see fit.

Shall we move this into a new issue?

[polvalente]

Suggested change

|> List.wrap()

We don't need this because a binary is also a valid iodata instance :)

[polvalente]

https://hexdocs.pm/elixir/1.13/typespecs.html#built-in-types

[sleipnir]

Awesome work guys!