backend-plugin-api,backend-defaults: add createSharedEnvironment

Co-authored-by: blam <ben@blam.sh>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-defaults/src/CreateBackend.ts

@@ -33,7 +33,15 @@ import {
   urlReaderFactory,
   identityFactory,
 } from '@backstage/backend-app-api';
-import { ServiceFactoryOrFunction } from '@backstage/backend-plugin-api';
+import {
+  ServiceFactory,
+  ServiceFactoryOrFunction,
+  SharedBackendEnvironment,
+} from '@backstage/backend-plugin-api';
+
+// Internal import of the type to avoid needing to export this.
+// eslint-disable-next-line monorepo/no-internal-import
+import type { InternalSharedBackendEnvironment } from '@backstage/backend-plugin-api/src/wiring/createSharedEnvironment';
 
 export const defaultServiceFactories = [
   cacheFactory(),
@@ -57,6 +65,7 @@ export const defaultServiceFactories = [
  * @public
  */
 export interface CreateBackendOptions {
+  env?: SharedBackendEnvironment;
   services?: ServiceFactoryOrFunction[];
 }
 
@@ -64,15 +73,33 @@ export interface CreateBackendOptions {
  * @public
  */
 export function createBackend(options?: CreateBackendOptions): Backend {
+  const services = new Array<ServiceFactory>();
+
+  // Highest priority: Services passed directly to createBackend
   const providedServices = (options?.services ?? []).map(sf =>
     typeof sf === 'function' ? sf() : sf,
   );
-  const providedIds = new Set(providedServices.map(sf => sf.service.id));
-  const neededDefaultFactories = defaultServiceFactories.filter(
-    sf => !providedIds.has(sf.service.id),
-  );
+  services.push(...providedServices);
 
-  return createSpecializedBackend({
-    services: [...neededDefaultFactories, ...providedServices],
-  });
+  // Middle priority: Services from the shared environment
+  if (options?.env) {
+    const env = options.env as unknown as InternalSharedBackendEnvironment;
+    if (env.version !== 'v1') {
+      throw new Error(`Unsupported shared environment version: ${env.version}`);
+    }
+
+    const environmentServices =
+      env.services?.filter(
+        sf => !services.some(({ service }) => sf.service.id === service.id),
+      ) ?? [];
+    services.push(...environmentServices);
+  }
+
+  // Lowest priority: Default services that are not already provided by environment or directly to createBackend
+  const defaultServices = defaultServiceFactories.filter(
+    sf => !services.some(({ service }) => service.id === sf.service.id),
+  );
+  services.push(...defaultServices);
+
+  return createSpecializedBackend({ services });
 }