This project is a GraphQL adapter for Yggdrasil publisher/subscriber.

Small example

Let’s say we want to have the following GraphQL subscription:

subscription {
  events(channel: "my_channel") {
    content
  }
}

And we have a process in Elixir that, using Yggdrasil, generates the following event:

Yggdrasil.publish(
  %Yggdrasil.Channel{name: "my_channel"},
  %{content: "some message"}
)

Using Absinthe, our Schema would look like this:

defmodule MyAppWeb.Schema do
  use Absinthe.Schema

  object :message do
    field :content, :string
  end

  query do
  end

  subscription do
    field :events, :message do
      arg :channel, non_null(:string)

      config fn args, %{context: %{pubsub: endpoint}} ->
        channel = %Yggdrasil.Channel{name: args.channel}
        Yggdrasil.GraphQL.subscribe(endpoint, :events, channel)
      end
    end
  end
end

Phoenix setup

This is an extract from this guide modified slightly to fit this example.

1. Add the Absinthe libraries:

  {:absinthe, "~> 1.4"},
  {:absinthe_phoenix, "~> 1.4"},

2. Add the Phoenix.PubSub configuration for your endpoint:

  config :my_app, MyAppWeb.Endpoint,
    # ... other config
    pubsub: [
      name: MyApp.PubSub,
      adapter: Phoenix.PubSub.PG2
    ]

3. In your application supervisor, add a line after your existing endpoint supervision line:

  [
    # other children ...
    supervisor(MyAppWeb.Endpoint, []), # this line should already exist.
    supervisor(Absinthe.Subscription, [MyAppWeb.Endpoint]), # add this line
    # other children ...
  ]

Where MyAppWeb.Endpoint is the name of your application’s phoenix endpoint.

4. In your MyAppWeb.Endpoint module add:

  use Absinthe.Phoenix.Endpoint

5. In your socket add:

   • Phoenix 1.3

     use Absinthe.Phoenix.Socket, schema: MyAppWeb.Schema

   • Phoenix 1.2

     use Absinthe.Phoenix.Socket
     def connect(_params, socket) do
       socket = Absinthe.Phoenix.Socket.put_schema(socket, MyAppWeb.Schema)
       {:ok, socket}
     end

And that should be enough to have a working subscription setup.

GraphQL adapter

The GraphQL adapter has the following rules:

• The adapter name is identified by the atom :graphql.
• The channel name must be a tuple with the endpoint name, the subscription field and an Yggdrasil channel to any of the available adapters.
• The transformer must encode to a map. It is recommended to leave the encoding and decoding to the underlying adapter. Defaults to :default transformer.
• The backend is and always should be :graphql.

The function Yggdrasil.GraphQL.subscribe/3 is in charged of creating the channel and generating the topic forAbsinthe.

Installation

Using this GraphQL adapter with Yggdrasil is a matter of adding the available hex package to your mix.exs file e.g:

def deps do
  [{:yggdrasil_graphql, "~> 0.1"}]
end