backend-app-api: add support for adding module imports

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Co-authored-by: Camila Belo <camilaibs@gmail.com>
Co-authored-by: Philipp Hugenroth <philipph@spotify.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/thin-dryers-repeat.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+Add support for installing features as package imports, for example `backend.add(import('my-plugin'))`.

packages/backend-app-api/src/wiring/BackendInitializer.ts

@@ -46,6 +46,7 @@ export class BackendInitializer {
   #features = new Array<InternalBackendFeature>();
   #extensionPoints = new Map<string, { impl: unknown; pluginId: string }>();
   #serviceRegistry: ServiceRegistry;
+  #registeredFeatures = new Array<Promise<BackendFeature>>();
 
   constructor(defaultApiFactories: ServiceFactory[]) {
     this.#serviceRegistry = ServiceRegistry.create([...defaultApiFactories]);
@@ -90,11 +91,11 @@ export class BackendInitializer {
     return Object.fromEntries(result);
   }
 
-  add(feature: BackendFeature) {
+  add(feature: Promise<BackendFeature>) {
     if (this.#startPromise) {
       throw new Error('feature can not be added after the backend has started');
     }
-    this.#addFeature(feature);
+    this.#registeredFeatures.push(feature);
   }
 
   #addFeature(feature: BackendFeature) {
@@ -150,6 +151,10 @@ export class BackendInitializer {
   async #doStart(): Promise<void> {
     this.#serviceRegistry.checkForCircularDeps();
 
+    for (const feature of this.#registeredFeatures) {
+      this.#addFeature(await feature);
+    }
+
     const featureDiscovery = await this.#serviceRegistry.get(
       featureDiscoveryServiceRef,
       'root',

packages/backend-app-api/src/wiring/BackstageBackend.ts

@@ -25,8 +25,17 @@ export class BackstageBackend implements Backend {
     this.#initializer = new BackendInitializer(defaultServiceFactories);
   }
 
-  add(feature: BackendFeature | (() => BackendFeature)): void {
-    this.#initializer.add(typeof feature === 'function' ? feature() : feature);
+  add(
+    feature:
+      | BackendFeature
+      | (() => BackendFeature)
+      | Promise<{ default: BackendFeature | (() => BackendFeature) }>,
+  ): void {
+    if (isPromise(feature)) {
+      this.#initializer.add(feature.then(f => unwrapFeature(f.default)));
+    } else {
+      this.#initializer.add(Promise.resolve(unwrapFeature(feature)));
+    }
   }
 
   async start(): Promise<void> {
@@ -37,3 +46,18 @@ export class BackstageBackend implements Backend {
     await this.#initializer.stop();
   }
 }
+
+function isPromise<T>(value: unknown | Promise<T>): value is Promise<T> {
+  return (
+    typeof value === 'object' &&
+    value !== null &&
+    'then' in value &&
+    typeof value.then === 'function'
+  );
+}
+
+function unwrapFeature(
+  feature: BackendFeature | (() => BackendFeature),
+): BackendFeature {
+  return typeof feature === 'function' ? feature() : feature;
+}

packages/backend-app-api/src/wiring/types.ts

@@ -25,7 +25,12 @@ import {
  * @public
  */
 export interface Backend {
-  add(feature: BackendFeature | (() => BackendFeature)): void;
+  add(
+    feature:
+      | BackendFeature
+      | (() => BackendFeature)
+      | Promise<{ default: BackendFeature | (() => BackendFeature) }>,
+  ): void;
   start(): Promise<void>;
   stop(): Promise<void>;
 }

packages/backend-defaults/src/CreateBackend.test.ts

@@ -18,27 +18,19 @@ import {
   coreServices,
   createServiceFactory,
 } from '@backstage/backend-plugin-api';
+import { mockServices } from '@backstage/backend-test-utils';
 import { createBackend } from './CreateBackend';
 
 describe('createBackend', () => {
-  it('should not throw when overriding a default service implementation', () => {
+  it('should not throw when overriding a default service implementation', async () => {
     const backend = createBackend();
 
-    expect(() => {
-      backend.add(
-        createServiceFactory({
-          service: coreServices.rootLifecycle,
-          deps: {},
-          factory: async () => ({
-            addStartupHook: () => {},
-            addShutdownHook: () => {},
-          }),
-        }),
-      );
-    }).not.toThrow();
+    backend.add(mockServices.rootConfig.factory());
+
+    await expect(backend.start()).resolves.toBe(undefined);
   });
 
-  it('should throw on duplicate service implementations', () => {
+  it('should throw on duplicate service implementations', async () => {
     const backend = createBackend();
 
     backend.add(
@@ -51,34 +43,34 @@ describe('createBackend', () => {
         }),
       }),
     );
-
-    expect(() => {
-      backend.add(
-        createServiceFactory({
-          service: coreServices.rootLifecycle,
-          deps: {},
-          factory: async () => ({
-            addStartupHook: () => {},
-            addShutdownHook: () => {},
-          }),
+    backend.add(
+      createServiceFactory({
+        service: coreServices.rootLifecycle,
+        deps: {},
+        factory: async () => ({
+          addStartupHook: () => {},
+          addShutdownHook: () => {},
         }),
-      );
-    }).toThrow(
+      }),
+    );
+
+    await expect(backend.start()).rejects.toThrow(
       'Duplicate service implementations provided for core.rootLifecycle',
     );
   });
 
-  it('should throw when providing a plugin metadata service implementation', () => {
+  it('should throw when providing a plugin metadata service implementation', async () => {
     const backend = createBackend();
+    backend.add(
+      createServiceFactory({
+        service: coreServices.pluginMetadata,
+        deps: {},
+        factory: () => ({ getId: () => 'test' }),
+      }),
+    );
 
-    expect(() =>
-      backend.add(
-        createServiceFactory({
-          service: coreServices.pluginMetadata,
-          deps: {},
-          factory: async () => ({ getId: () => 'test' }),
-        }),
-      ),
-    ).toThrow('The core.pluginMetadata service cannot be overridden');
+    await expect(backend.start()).rejects.toThrow(
+      'The core.pluginMetadata service cannot be overridden',
+    );
   });
 });