Add @defer directive support

config/config.exs

@@ -4,6 +4,9 @@ use Mix.Config
 
 config :logger, level: :debug
 
+config :absinthe,
+  max_defer_time: 60000
+
 # This configuration is loaded before any dependency and is restricted
 # to this project. If another project depends on this project, this
 # file won't be loaded nor affect the parent project. For this reason,

lib/absinthe/blueprint/execution.ex

@@ -41,7 +41,10 @@ defmodule Absinthe.Blueprint.Execution do
     result: nil,
     acc: %{},
     context: %{},
-    root_value: %{}
+    root_value: %{},
+    deferred_fields: [],
+    deferring: false,
+    defer_topic: nil
   ]
 
   @type t :: %__MODULE__{

lib/absinthe/phase/document/execution/deferred_resolution.ex

@@ -0,0 +1,85 @@
+defmodule Absinthe.Phase.Document.Execution.DeferredResolution do
+  alias Absinthe.{Blueprint, Phase}
+  alias Absinthe.Phase.Document.Execution.Resolution
+  alias Absinthe.Resolution.DeferredField
+
+  use Absinthe.Phase
+
+  @spec run(Blueprint.t(), Keyword.t()) :: Phase.result_t()
+  def run(blueprint, options \\ [])
+
+  def run(%Blueprint{execution: %{deferred_fields: []}} = blueprint, _options) do
+    {:ok, blueprint}
+  end
+
+  def run(%Blueprint{execution: %{deferred_fields: fields}} = blueprint, _options) do
+    context = blueprint.execution.context
+    pubsub = ensure_pubsub!(context)
+
+    topic = "__absinthe__:deferred_fields-" <> to_string(:erlang.unique_integer([:positive]))
+    pubsub.subscribe(topic)
+
+    self = self()
+
+    Task.start(fn -> do_defers(fields, blueprint, pubsub, topic, self) end)
+
+    {:ok, put_in(blueprint.execution.defer_topic, topic)}
+  end
+
+  defp do_defers(fields, blueprint, pubsub, topic, pid) do
+    fields
+    |> Enum.map(fn f -> Task.async(fn -> resolve_field(f, blueprint, pubsub, topic) end) end)
+    |> Task.yield_many(Application.get_env(:absinthe, :max_defer_time))
+    |> Enum.map(fn {task, res} ->
+      # Shutdown the tasks that did not reply nor exit
+      res || Task.shutdown(task)
+    end)
+
+    pubsub.unsubscribe(topic, pid)
+  end
+
+  defp resolve_field(f, blueprint, pubsub, topic) do
+    {result, _exec} = perform_deferred_resolution(f)
+
+    data =
+      blueprint.execution.result
+      |> put_in(result)
+      |> Absinthe.Phase.Document.Result.process()
+      |> add_path(f.path)
+
+    pubsub.publish_deferred(topic, data)
+  end
+
+  defp perform_deferred_resolution(%DeferredField{} = field) do
+    Resolution.do_resolve_field(
+      field.resolution,
+      %{field.exec | deferring: true},
+      field.source,
+      field.path
+    )
+  end
+
+  defp add_path(result, path) do
+    Map.put(result, :path, Enum.reverse(make_path(path)))
+  end
+
+  defp make_path(path) do
+    path
+    |> Enum.map(& &1.name)
+    |> Enum.filter(fn e -> e != nil end)
+  end
+
+  defp ensure_pubsub!(context) do
+    case Absinthe.Subscription.extract_pubsub(context) do
+      {:ok, pubsub} ->
+        pubsub
+
+      _ ->
+        raise """
+        Pubsub not configured!
+
+        Deferred fields require a configured pubsub module.
+        """
+    end
+  end
+end

lib/absinthe/phase/document/execution/resolution.ex

@@ -104,6 +104,7 @@ defmodule Absinthe.Phase.Document.Execution.Resolution do
   defp resolve_fields(parent, exec, source, path) do
     # parent is the parent field, we need to get the return type of that field
     # that return type could be an interface or union, so let's make it concrete
+
     parent
     |> get_return_type
     |> get_concrete_type(source, exec)
@@ -154,7 +155,28 @@ defmodule Absinthe.Phase.Document.Execution.Resolution do
     do_resolve_fields(fields, exec, source, parent_type, path, [result | acc])
   end
 
-  defp do_resolve_fields([], exec, _, _, _, acc), do: {:lists.reverse(acc), exec}
+  defp do_resolve_fields([], exec, _, _, _, acc) do
+    acc =
+      acc
+      |> Enum.filter(fn e -> not is_nil(e) end)
+      |> Enum.reverse()
+    {acc, exec}
+  end
+
+  def resolve_field(%{flags: %{defer: _}} = field, %{deferring: false} = exec,
+                    source, parent_type, path) do
+    resolution = build_resolution_struct(exec, field, source, parent_type, path)
+    deferred_field = %Absinthe.Resolution.DeferredField{
+      field: update_in(field.flags, &Map.drop(&1, [:defer])),
+      resolution: resolution,
+      exec: exec,
+      source: source,
+      parent_type: parent_type,
+      path: path
+    }
+
+    {nil, %{exec | deferred_fields: [deferred_field | exec.deferred_fields]}}
+  end
 
   def resolve_field(field, exec, source, parent_type, path) do
     exec
@@ -163,7 +185,7 @@ defmodule Absinthe.Phase.Document.Execution.Resolution do
   end
 
   # bp_field needs to have a concrete schema node, AKA no unions or interfaces
-  defp do_resolve_field(res, exec, source, path) do
+  def do_resolve_field(res, exec, source, path) do
     res
     |> reduce_resolution
     |> case do

lib/absinthe/phase/document/result.ex

@@ -12,7 +12,7 @@ defmodule Absinthe.Phase.Document.Result do
     {:ok, %{bp | result: result}}
   end
 
-  defp process(blueprint) do
+  def process(blueprint) do
     result =
       case blueprint.execution do
         %{validation_errors: [], result: result} ->
@@ -22,7 +22,9 @@ defmodule Absinthe.Phase.Document.Result do
           {:validation_failed, errors}
       end
 
-    format_result(result)
+    result
+    |> format_result()
+    |> maybe_add_topic(blueprint.execution)
   end
 
   defp format_result(:execution_failed) do
@@ -47,6 +49,12 @@ defmodule Absinthe.Phase.Document.Result do
     %{errors: [format_error(error)]}
   end
 
+  defp maybe_add_topic(result, %{defer_topic: nil}), do: result
+
+  defp maybe_add_topic(result, %{defer_topic: topic}) do
+    Map.put(result, :defer_topic, topic)
+  end
+
   defp data(%{errors: [_ | _] = field_errors}, errors), do: {nil, field_errors ++ errors}
 
   # Leaf

lib/absinthe/pipeline.ex

@@ -102,6 +102,7 @@ defmodule Absinthe.Pipeline do
       # Execution
       {Phase.Subscription.SubscribeSelf, options},
       {Phase.Document.Execution.Resolution, options},
+      Phase.Document.Execution.DeferredResolution,
       # Format Result
       Phase.Document.Result
     ]

lib/absinthe/resolution/deferred_field.ex

@@ -0,0 +1,12 @@
+defmodule Absinthe.Resolution.DeferredField do
+
+  defstruct [
+    :field,
+    :resolution,
+    :exec,
+    :source,
+    :parent_type,
+    :path
+  ]
+
+end

lib/absinthe/subscription/pubsub.ex

@@ -44,6 +44,11 @@ defmodule Absinthe.Subscription.Pubsub do
 
           # other fields as needed
       }
+  Unsubscribe the supplied process from a given topic
+  """
+  @callback unsubscribe(topic :: binary, process :: pid) :: term
+
+  @doc """
 
   """
   @callback publish_mutation(
@@ -62,4 +67,9 @@ defmodule Absinthe.Subscription.Pubsub do
   only.
   """
   @callback publish_subscription(topic :: binary, data :: map) :: term
+
+  @doc """
+  Publish the result of a deferred field
+  """
+  @callback publish_deferred(topic :: binary, data :: term) :: term
 end

lib/absinthe/subscription/registry.ex

@@ -552,24 +552,23 @@ defmodule Absinthe.Subscription.Registry do
       []
 
   """
-  @spec unregister(registry, key) :: :ok
-  def unregister(registry, key) when is_atom(registry) do
-    self = self()
+  @spec unregister(registry, key, pid) :: :ok
+  def unregister(registry, key, pid \\ self()) when is_atom(registry) do
     {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
+    {key_partition, pid_partition} = partitions(kind, key, pid, partitions)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
     # Remove first from the key_ets because in case of crashes
     # the pid_ets will still be able to clean up. The last step is
     # to clean if we have no more entries.
-    true = :ets.match_delete(key_ets, {key, {self, :_}})
-    true = :ets.delete_object(pid_ets, {self, key, key_ets})
+    true = :ets.match_delete(key_ets, {key, {pid, :_}})
+    true = :ets.delete_object(pid_ets, {pid, key, key_ets})
 
-    unlink_if_unregistered(pid_server, pid_ets, self)
+    unlink_if_unregistered(pid_server, pid_ets, pid)
 
     for listener <- listeners do
-      Kernel.send(listener, {:unregister, registry, key, self})
+      Kernel.send(listener, {:unregister, registry, key, pid})
     end
 
     :ok

lib/absinthe/type/built_ins/directives.ex

@@ -39,4 +39,15 @@ defmodule Absinthe.Type.BuiltIns.Directives do
         Blueprint.put_flag(node, :include, __MODULE__)
     end
   end
+
+  directive :defer do
+    description """
+    Directs the executor that it may defer evaluation of this field and send
+    the response later.
+    """
+
+    on [:field, :fragment_spread, :inline_fragment]
+
+    expand fn _, node -> Blueprint.put_flag(node, :defer, __MODULE__) end
+  end
 end