checkpoint towards including traces in federated service responses

packages/apollo-server-core/src/ApolloServer.ts

@@ -14,6 +14,8 @@ import {
   ValidationContext,
   FieldDefinitionNode,
   DocumentNode,
+  isObjectType,
+  isScalarType,
 } from 'graphql';
 import { GraphQLExtension } from 'graphql-extensions';
 import {
@@ -362,6 +364,11 @@ export class ApolloServerBase {
       );
       // Let's keep this extension second so it wraps everything, except error formatting
       this.extensions.push(() => this.engineReportingAgent!.newExtension());
+    } else if (engine !== false && this.schemaIsFederated()) {
+      // We haven't configured this app to use Engine directly. But it looks like
+      // we are a federated service backend, so we should be capable of including
+      // our trace in a response extension if we are asked to by the gateway.
+      // FIXME actually write the extension
     }
 
     if (extensions) {
@@ -504,6 +511,31 @@ export class ApolloServerBase {
     return false;
   }
 
+  // Returns true if it appears that the schema was returned from
+  // @apollo/federation's buildFederatedSchema. This strategy avoids depending
+  // explicitly on @apollo/federation or relying on something that might not
+  // survive transformations like monkey-patching a boolean field onto the
+  // schema.
+  //
+  // The only thing this is used for is determining whether traces should be
+  // added to responses if requested with an HTTP header; if there's a false
+  // positive, that feature can be disabled by specifying `engine: false`.
+  private schemaIsFederated(): boolean {
+    const serviceType = this.schema.getType('_Service');
+    if (!(serviceType && isObjectType(serviceType))) {
+      return false;
+    }
+    const sdlField = serviceType.getFields().sdl;
+    if (!sdlField) {
+      return false;
+    }
+    const sdlFieldType = sdlField.type;
+    if (!isScalarType(sdlFieldType)) {
+      return false;
+    }
+    return sdlFieldType.name == 'String';
+  }
+
   private ensurePluginInstantiation(plugins?: PluginDefinition[]): void {
     if (!plugins || !plugins.length) {
       return;