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/api-report.md

@@ -5,12 +5,15 @@
 ```ts
 import { Backend } from '@backstage/backend-app-api';
 import { ServiceFactoryOrFunction } from '@backstage/backend-plugin-api';
+import { SharedBackendEnvironment } from '@backstage/backend-plugin-api';
 
 // @public (undocumented)
 export function createBackend(options?: CreateBackendOptions): Backend;
 
 // @public (undocumented)
 export interface CreateBackendOptions {
+  // (undocumented)
+  env?: SharedBackendEnvironment;
   // (undocumented)
   services?: ServiceFactoryOrFunction[];
 }

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 });
 }

packages/backend-plugin-api/api-report.md

@@ -153,6 +153,15 @@ export function createServiceRef<TService>(
   config: ServiceRefConfig<TService, 'root'>,
 ): ServiceRef<TService, 'root'>;
 
+// @public
+export function createSharedEnvironment<
+  TOptions extends [options?: object] = [],
+>(
+  config:
+    | SharedBackendEnvironmentConfig
+    | ((...params: TOptions) => SharedBackendEnvironmentConfig),
+): (...options: TOptions) => SharedBackendEnvironment;
+
 // @public
 export interface DatabaseService {
   getClient(): Promise<Knex>;
@@ -382,6 +391,18 @@ export interface ServiceRefConfig<TService, TScope extends 'root' | 'plugin'> {
   scope?: TScope;
 }
 
+// @public (undocumented)
+export interface SharedBackendEnvironment {
+  // (undocumented)
+  $$type: 'SharedBackendEnvironment';
+}
+
+// @public (undocumented)
+export interface SharedBackendEnvironmentConfig {
+  // (undocumented)
+  services?: ServiceFactoryOrFunction[];
+}
+
 // @public
 export interface TokenManagerService {
   authenticate(token: string): Promise<void>;

packages/backend-plugin-api/src/wiring/createSharedEnvironment.ts

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { ServiceFactory, ServiceFactoryOrFunction } from '../services';
+
+/** @public */
+export interface SharedBackendEnvironmentConfig {
+  services?: ServiceFactoryOrFunction[];
+}
+
+// This type is opaque in order to allow for future API evolution without
+// cluttering the external API. For example we might want to add support
+// for more powerful callback based backend modifications.
+//
+// By making this opaque we also ensure that the type doesn't become an input
+// type that we need to care about, as it would otherwise be possible to pass
+// a custom environment definition to `createBackend`, which we don't want.
+/**
+ * @public
+ */
+export interface SharedBackendEnvironment {
+  $$type: 'SharedBackendEnvironment';
+}
+
+/**
+ * This type is NOT supposed to be used by anyone except internally by the backend-app-api package.
+ * @internal */
+export interface InternalSharedBackendEnvironment {
+  version: 'v1';
+  services?: ServiceFactory[];
+}
+
+/**
+ * Creates a shared backend environment which can be used to create multiple backends
+ * @public
+ */
+export function createSharedEnvironment<
+  TOptions extends [options?: object] = [],
+>(
+  config:
+    | SharedBackendEnvironmentConfig
+    | ((...params: TOptions) => SharedBackendEnvironmentConfig),
+): (...options: TOptions) => SharedBackendEnvironment {
+  const configCallback = typeof config === 'function' ? config : () => config;
+
+  return (...options) => {
+    const actualConfig = configCallback(...options);
+    const services = actualConfig?.services?.map(sf =>
+      typeof sf === 'function' ? sf() : sf,
+    );
+
+    return { services } as unknown as SharedBackendEnvironment;
+  };
+}

packages/backend-plugin-api/src/wiring/index.ts

@@ -14,6 +14,11 @@
  * limitations under the License.
  */
 
+export { createSharedEnvironment } from './createSharedEnvironment';
+export type {
+  SharedBackendEnvironment,
+  SharedBackendEnvironmentConfig,
+} from './createSharedEnvironment';
 export type {
   BackendModuleConfig,
   BackendPluginConfig,