Federation: Insert empty Subscription type definition

packages/apollo-federation/src/composition/__tests__/compose.test.ts

@@ -752,6 +752,78 @@ describe('composeServices', () => {
                   `);
     });
 
+    it('allows extension of the Subscription type with no base type definition', () => {
+      const serviceA = {
+        typeDefs: gql`
+          extend type Subscription {
+            postAdded: Post
+          }
+
+          type Post {
+            author: String!
+            comment: String!
+          }
+        `,
+        name: 'serviceA',
+      };
+
+      const serviceB = {
+        typeDefs: gql`
+          extend type Subscription {
+            postRemoved: ID
+          }
+        `,
+        name: 'serviceB',
+      };
+
+      const { schema, errors } = composeServices([serviceA, serviceB]);
+      expect(errors).toHaveLength(0);
+      expect(schema).toBeDefined();
+
+      expect(schema.getType('Subscription')).toMatchInlineSnapshot(`
+                        type Subscription {
+                          postAdded: Post
+                          postRemoved: ID
+                        }
+                  `);
+    });
+
+    it('treats root Subscriptions type definition as an extension, not base definitions', () => {
+      const serviceA = {
+        typeDefs: gql`
+          type Subscription {
+            postAdded: Post
+          }
+
+          type Post {
+            author: String!
+            comment: String!
+          }
+        `,
+        name: 'serviceA',
+      };
+
+      const serviceB = {
+        typeDefs: gql`
+          extend type Subscription {
+            postRemoved: ID
+          }
+        `,
+        name: 'serviceB',
+      };
+
+      const { schema, errors } = composeServices([serviceA, serviceB]);
+      expect(errors).toHaveLength(0);
+      expect(schema).toBeDefined();
+
+      expect(schema.getType('Subscription')).toMatchInlineSnapshot(`
+                        type Subscription {
+                          postAdded: Post
+                          postRemoved: ID
+                        }
+                  `);
+    });
+
     // TODO: not sure what to do here. Haven't looked into it yet :)
     it.skip('works with custom root types', () => {});
   });

packages/apollo-federation/src/composition/compose.ts

@@ -56,6 +56,12 @@ const EmptyMutationDefinition = {
   fields: [],
   serviceName: null,
 };
+const EmptySubscriptionDefinition = {
+  kind: Kind.OBJECT_TYPE_DEFINITION,
+  name: { kind: Kind.NAME, value: 'Subscription' },
+  fields: [],
+  serviceName: null,
+};
 
 // Map of all type definitions to eventually be passed to extendSchema
 interface TypeDefinitionsMap {
@@ -320,6 +326,8 @@ export function buildMapsFromServiceList(serviceList: ServiceDefinition[]) {
     typeDefinitionsMap.Query = [EmptyQueryDefinition];
   if (typeExtensionsMap.Mutation && !typeDefinitionsMap.Mutation)
     typeDefinitionsMap.Mutation = [EmptyMutationDefinition];
+  if (typeExtensionsMap.Subscription && !typeDefinitionsMap.Subscription)
+    typeDefinitionsMap.Subscription = [EmptySubscriptionDefinition];
 
   return {
     typeToServiceMap,