support subscriptions.

README.md

@@ -5,16 +5,17 @@ GraphQL stitching composes a single schema from multiple underlying GraphQL reso
 ![Stitched graph](./docs/images/stitching.png)
 
 **Supports:**
+- All operation types: query, mutation, and subscription.
 - Merged object and abstract types.
-- Multiple and composite keys per merged type.
 - Shared objects, fields, enums, and inputs across locations.
+- Multiple and composite type keys.
 - Combining local and remote schemas.
 - File uploads via [multipart form spec](https://github.com/jaydenseric/graphql-multipart-request-spec).
 - Tested with all minor versions of `graphql-ruby`.
 
 **NOT Supported:**
 - Computed fields (ie: federation-style `@requires`).
-- Subscriptions, defer/stream.
+- Defer/stream.
 
 This Ruby implementation is a sibling to [GraphQL Tools](https://the-guild.dev/graphql/stitching) (JS) and [Bramble](https://movio.github.io/bramble/) (Go), and its capabilities fall somewhere in between them. GraphQL stitching is similar in concept to [Apollo Federation](https://www.apollographql.com/docs/federation/), though more generic. The opportunity here is for a Ruby application to stitch its local schemas together or onto remote sources without requiring an additional proxy service running in another language. If your goal is to build a purely high-throughput federated reverse proxy, consider not using Ruby.
 
@@ -445,6 +446,7 @@ The [Executor](./docs/executor.md) component builds atop the Ruby fiber-based im
 - [Modeling foreign keys for stitching](./docs/mechanics.md##modeling-foreign-keys-for-stitching)
 - [Deploying a stitched schema](./docs/mechanics.md#deploying-a-stitched-schema)
 - [Schema composition merge patterns](./docs/composer.md#merge-patterns)
+- [Subscriptions tutorial](./docs/subscriptions.md)
 - [Field selection routing](./docs/mechanics.md#field-selection-routing)
 - [Root selection routing](./docs/mechanics.md#root-selection-routing)
 - [Stitched errors](./docs/mechanics.md#stitched-errors)

docs/README.md

@@ -15,4 +15,5 @@ Major components include:
 Additional topics:
 
 - [Stitching mechanics](./mechanics.md) - more about building for stitching and how it operates.
+- [Subscriptions](./subscriptions.md) - explore how to stitch realtime event subscriptions.
 - [Federation entities](./federation_entities.md) - more about Apollo Federation compatibility.

docs/subscriptions.md

@@ -0,0 +1,206 @@
+## Stitching subscriptions
+
+Stitching is an interesting prospect for subscriptions because socket-based interactions can be isolated to their own schema/server with very little implementation beyond resolving entity keys. Then, entity data can be stitched onto subscription payloads from other locations.
+
+### Composing a subscriptions schema
+
+For simplicity, subscription resolvers should exist together in a single schema (multiple schemas with subscriptions aren't worth the confusion). This subscriptions schema may provide basic entity types that will merge with other locations. For example, here's a bare-bones subscriptions schema:
+
+```ruby
+class SubscriptionSchema < GraphQL::Schema
+  class Post < GraphQL::Schema::Object
+    field :id, ID, null: false
+  end
+
+  class Comment < GraphQL::Schema::Object
+    field :id, ID, null: false
+  end
+
+  class CommentAddedToPost < GraphQL::Schema::Subscription
+    argument :post_id, ID, required: true
+    field :post, Post, null: false
+    field :comment, Comment, null: true
+
+    def subscribe(post_id:)
+      { post: { id: post_id }, comment: nil }
+    end
+
+    def update(post_id:)
+      { post: { id: post_id }, comment: object }
+    end
+  end
+
+  class SubscriptionType < GraphQL::Schema::Object
+    field :comment_added_to_post, subscription: CommentAddedToPost
+  end
+
+  use GraphQL::Subscriptions::ActionCableSubscriptions
+  subscription SubscriptionType
+end
+```
+
+The above subscriptions schema can compose with other locations, such as the following that provides full entity types:
+
+```ruby
+class EntitiesSchema < GraphQL::Schema
+  class StitchingResolver < GraphQL::Schema::Directive
+    graphql_name "stitch"
+    locations FIELD_DEFINITION
+    argument :key, String, required: true
+    argument :arguments, String, required: false
+    repeatable true
+  end
+
+  class Comment < GraphQL::Schema::Object
+    field :id, ID, null: false
+    field :message, String, null: false
+  end
+
+  class Post < GraphQL::Schema::Object
+    field :id, ID, null: false
+    field :title, String, null: false
+    field :comments, [Comment, null: false], null: false
+  end
+
+  class QueryType < GraphQL::Schema::Object
+    field :posts, [Post, null: true] do
+      directive StitchingResolver, key: "id"
+      argument :ids, [ID], required: true
+    end
+
+    def posts(ids:)
+      Post.where(id: ids)
+    end
+
+    field :comments, [Comment, null: true] do
+      directive StitchingResolver, key: "id"
+      argument :ids, [ID], required: true
+    end
+
+    def comments(ids:)
+      Comment.where(id: ids)
+    end
+  end
+
+  query QueryType
+end
+```
+
+These schemas are composed as normal into a stitching client. The subscriptions schema _must be locally-executable_ while the other entity schema(s) may be served from anywhere:
+
+```ruby
+StitchedSchema = GraphQL::Stitching::Client.new(locations: {
+  subscriptions: {
+    schema: SubscriptionSchema, # << locally executable!
+  },
+  entities: {
+    schema: GraphQL::Schema.from_definition(entities_schema_sdl),
+    executable: GraphQL::Stitching::HttpExecutable.new("http://localhost:3001"),
+  },
+})
+```
+
+### Serving stitched subscriptions
+
+Once you've stitched a schema with subscriptions, it gets called as part of three workflows:
+
+1. Controller - handles normal query and mutation requests recieved via HTTP.
+2. Channel - handles subscription-create requests recieved through a socket connection.
+3. Plugin – handles subscription-update events pushed to the socket connection.
+
+#### Controller
+
+A controller will recieve basic query and mutation requests sent over HTTP, including introspection requests. Fulfill these using the stitched schema client.
+
+```ruby
+class GraphqlController < ApplicationController
+  skip_before_action :verify_authenticity_token
+  layout false
+
+  def execute
+    result = StitchedSchema.execute(
+      params[:query],
+      context: {}, 
+      variables: params[:variables],
+      operation_name: params[:operationName],
+    )
+
+    render json: result
+  end
+end
+```
+
+#### Channel
+
+A channel handles subscription requests initiated via websocket connection. This mostly follows the [GraphQL Ruby documentation example](https://graphql-ruby.org/api-doc/2.3.9/GraphQL/Subscriptions/ActionCableSubscriptions), except that `execute` uses the stitched schema client while `unsubscribed` uses the subscriptions subschema directly:
+
+```ruby
+class GraphqlChannel < ApplicationCable::Channel
+  def subscribed
+    @subscription_ids = []
+  end
+
+  def execute(params)
+    result = StitchedSchema.execute(
+      params["query"],
+      context: { channel: self },
+      variables: params["variables"],
+      operation_name: params["operationName"]
+    )
+
+    payload = {
+      result: result.to_h,
+      more: result.subscription?,
+    }
+
+    if result.context[:subscription_id]
+      @subscription_ids << result.context[:subscription_id]
+    end
+
+    transmit(payload)
+  end
+
+  def unsubscribed
+    @subscription_ids.each { |sid|
+      # Go directly through the subscriptions subschema 
+      # when managing/triggering subscriptions:
+      SubscriptionSchema.subscriptions.delete_subscription(sid)
+    }
+  end
+end
+```
+
+What happens behind the scenes here is that stitching filters the `execute` request down to just subscription selections, and passes those through to the subscriptions subschema where they register an event binding. The subscriber response gets stitched while passing back out through the stitching client.
+
+#### Plugin
+
+Lastly, update events trigger with the filtered subscriptions selection, and must get stitched before transmitting. The stitching client adds an update handler into request context for this purpose. A small patch to the subscriptions plugin class can call this handler on update event payloads before transmitting them:
+
+```ruby
+class StitchedActionCableSubscriptions < GraphQL::Subscriptions::ActionCableSubscriptions
+  def execute_update(subscription_id, event, object)
+    super(subscription_id, event, object).tap do |result|
+      result.context[:stitch_subscription_update]&.call(result)
+    end
+  end
+end
+
+class SubscriptionSchema
+  # switch the plugin on the subscriptions schema to use the patched class... 
+  use StitchedActionCableSubscriptions
+end
+```
+
+### Triggering subscriptions
+
+Subscription update events are triggered as normal directly through the subscriptions subschema:
+
+```ruby
+class Comment < ApplicationRecord
+  after_create :trigger_subscriptions
+
+  def trigger_subscriptions
+    SubscriptionsSchema.subscriptions.trigger(:comment_added_to_post, { post_id: post_id }, self)
+  end
+end
+```

lib/graphql/stitching.rb

@@ -4,6 +4,18 @@
 
 module GraphQL
   module Stitching
+    # scope name of query operations.
+    QUERY_OP = "query"
+
+    # scope name of mutation operations.
+    MUTATION_OP = "mutation"
+
+    # scope name of subscription operations.
+    SUBSCRIPTION_OP = "subscription"
+
+    # introspection typename field.
+    TYPENAME = "__typename"
+
     # @api private
     EMPTY_OBJECT = {}.freeze
 

lib/graphql/stitching/client.rb

@@ -33,28 +33,28 @@ def initialize(locations: nil, supergraph: nil, composer: nil)
         @on_error = nil
       end
 
-      def execute(query:, variables: nil, operation_name: nil, context: nil, validate: true)
+      def execute(raw_query = nil, query: nil, variables: nil, operation_name: nil, context: nil, validate: true)
         request = Request.new(
           @supergraph,
-          query,
+          raw_query || query, # << for parity with GraphQL Ruby Schema.execute
           operation_name: operation_name,
           variables: variables,
           context: context,
         )
 
         if validate
           validation_errors = request.validate
-          return error_result(validation_errors) if validation_errors.any?
+          return error_result(request, validation_errors) if validation_errors.any?
         end
 
         request.prepare!
         load_plan(request)
         request.execute
       rescue GraphQL::ParseError, GraphQL::ExecutionError => e
-        error_result([e])
+        error_result(request, [e])
       rescue StandardError => e
         custom_message = @on_error.call(request, e) if @on_error
-        error_result([{ "message" => custom_message || "An unexpected error occured." }])
+        error_result(request, [{ "message" => custom_message || "An unexpected error occured." }])
       end
 
       def on_cache_read(&block)
@@ -93,12 +93,12 @@ def load_plan(request)
         plan
       end
 
-      def error_result(errors)
+      def error_result(request, errors)
         public_errors = errors.map! do |e|
           e.is_a?(Hash) ? e : e.to_h
         end
 
-        { "errors" => public_errors }
+        GraphQL::Query::Result.new(query: request, values: { "errors" => public_errors })
       end
     end
   end

lib/graphql/stitching/composer.rb

@@ -40,6 +40,9 @@ class T < GraphQL::Schema::Object
       # @return [String] name of the Mutation type in the composed schema.
       attr_reader :mutation_name
 
+      # @return [String] name of the Subscription type in the composed schema.
+      attr_reader :subscription_name
+
       # @api private
       attr_reader :subgraph_types_by_name_and_location
 
@@ -49,6 +52,7 @@ class T < GraphQL::Schema::Object
       def initialize(
         query_name: "Query",
         mutation_name: "Mutation",
+        subscription_name: "Subscription",
         description_merger: nil,
         deprecation_merger: nil,
         default_value_merger: nil,
@@ -57,6 +61,7 @@ def initialize(
       )
         @query_name = query_name
         @mutation_name = mutation_name
+        @subscription_name = subscription_name
         @description_merger = description_merger || BASIC_VALUE_MERGER
         @deprecation_merger = deprecation_merger || BASIC_VALUE_MERGER
         @default_value_merger = default_value_merger || BASIC_VALUE_MERGER
@@ -97,7 +102,6 @@ def perform(locations_input)
         # "Typename" => "location" => subgraph_type
         @subgraph_types_by_name_and_location = schemas.each_with_object({}) do |(location, schema), memo|
           raise CompositionError, "Location keys must be strings" unless location.is_a?(String)
-          raise CompositionError, "The subscription operation is not supported." if schema.subscription
 
           introspection_types = schema.introspection_system.types.keys
           schema.types.each do |type_name, subgraph_type|
@@ -107,10 +111,13 @@ def perform(locations_input)
               raise CompositionError, "Query name \"#{@query_name}\" is used by non-query type in #{location} schema."
             elsif type_name == @mutation_name && subgraph_type != schema.mutation
               raise CompositionError, "Mutation name \"#{@mutation_name}\" is used by non-mutation type in #{location} schema."
+            elsif type_name == @subscription_name && subgraph_type != schema.subscription
+              raise CompositionError, "Subscription name \"#{@subscription_name}\" is used by non-subscription type in #{location} schema."
             end
 
             type_name = @query_name if subgraph_type == schema.query
             type_name = @mutation_name if subgraph_type == schema.mutation
+            type_name = @subscription_name if subgraph_type == schema.subscription
             @mapped_type_names[subgraph_type.graphql_name] = type_name if subgraph_type.graphql_name != type_name
 
             memo[type_name] ||= {}
@@ -154,6 +161,7 @@ def perform(locations_input)
           orphan_types(schema_types.values.select { |t| t.respond_to?(:kind) && t.kind.object? })
           query schema_types[builder.query_name]
           mutation schema_types[builder.mutation_name]
+          subscription schema_types[builder.subscription_name]
           directives builder.schema_directives.values
 
           own_orphan_types.clear
@@ -593,7 +601,7 @@ def extract_resolvers(type_name, types_by_location)
       # @!scope class
       # @!visibility private
       def select_root_field_locations(schema)
-        [schema.query, schema.mutation].tap(&:compact!).each do |root_type|
+        [schema.query, schema.mutation, schema.subscription].tap(&:compact!).each do |root_type|
           root_type.fields.each do |root_field_name, root_field|
             root_field_locations = @field_map[root_type.graphql_name][root_field_name]
             next unless root_field_locations.length > 1

lib/graphql/stitching/composer/resolver_config.rb

@@ -38,7 +38,7 @@ def extract_federation_entities(schema, location)
               memo[field_path] << new(
                 key: key.to_definition,
                 type_name: entity_type.graphql_name,
-                arguments: "representations: { #{key_fields.join(", ")}, __typename: $.__typename }",
+                arguments: "representations: { #{key_fields.join(", ")}, #{TYPENAME}: $.#{TYPENAME} }",
               )
             end
           end