Absinthe - Subscriptions

Guide to implementing real-time GraphQL subscriptions with Absinthe and Phoenix.

Key Concepts

Basic Setup

In your Phoenix endpoint

defmodule MyAppWeb.Endpoint do use Phoenix.Endpoint, otp_app: :my_app use Absinthe.Phoenix.Endpoint

socket "/socket", MyAppWeb.UserSocket, websocket: true, longpoll: false end

Socket configuration

defmodule MyAppWeb.UserSocket do use Phoenix.Socket use Absinthe.Phoenix.Socket, schema: MyApp.Schema

def connect(params, socket, _connect_info) do current_user = get_user_from_token(params["token"]) socket = Absinthe.Phoenix.Socket.put_options(socket, context: %{current_user: current_user} ) {:ok, socket} end

def id(socket), do: "user_socket:#{socket.assigns.user_id}" end

Defining Subscriptions

defmodule MyApp.Schema.Subscriptions do use Absinthe.Schema.Notation

object :post_subscriptions do field :post_created, :post do config fn _args, _resolution -> {:ok, topic: "posts"} end

  trigger :create_post, topic: fn _post ->
    "posts"
  end
end

field :post_updated, :post do
  arg :id, non_null(:id)

  config fn %{id: id}, _resolution ->
    {:ok, topic: "post:#{id}"}
  end

  trigger :update_post, topic: fn post ->
    "post:#{post.id}"
  end
end

end end

Publishing from Mutations

defmodule MyApp.Resolvers.Post do def create_post(_parent, %{input: input}, _resolution) do case MyApp.Posts.create_post(input) do {:ok, post} -> # Publish to subscription Absinthe.Subscription.publish( MyAppWeb.Endpoint, post, post_created: "posts" ) {:ok, post} {:error, changeset} -> {:error, changeset} end end end

User-Specific Subscriptions

field :user_notification, :notification do config fn _args, %{context: %{current_user: user}} -> {:ok, topic: "user:#{user.id}:notifications"} end end

Publishing

Absinthe.Subscription.publish( MyAppWeb.Endpoint, notification, user_notification: "user:#{user_id}:notifications" )

Best Practices

• Scope subscriptions - Use topics to limit data exposure

• Authenticate connections - Verify users in socket connect

• Use triggers - Automatically publish on mutations

• Handle disconnections - Clean up resources on disconnect

• Rate limit subscriptions - Prevent abuse

PubSub Configuration

config/config.exs

config :my_app, MyAppWeb.Endpoint, pubsub_server: MyApp.PubSub

application.ex

children = [ {Phoenix.PubSub, name: MyApp.PubSub}, MyAppWeb.Endpoint, {Absinthe.Subscription, MyAppWeb.Endpoint} ]

Authorization in Subscriptions

field :private_messages, :message do config fn _args, %{context: context} -> case context do %{current_user: %{id: user_id}} -> {:ok, topic: "user:#{user_id}:messages"} _ -> {:error, "Unauthorized"} end end end

Anti-Patterns

• Don't publish sensitive data to broad topics

• Avoid subscriptions without authentication

• Don't skip connection-level authorization

• Avoid overly granular topics (performance impact)