Support external config schemas in the backend.

Signed-off-by: David Festal <dfestal@redhat.com>

plugins/app-backend/api-report.md

@@ -4,6 +4,7 @@
 
 ```ts
 import { Config } from '@backstage/config';
+import { ConfigSchemaPackageEntry } from '@backstage/config-loader';
 import express from 'express';
 import { Logger } from 'winston';
 import { PluginDatabaseManager } from '@backstage/backend-common';
@@ -13,6 +14,7 @@ export function createRouter(options: RouterOptions): Promise<express.Router>;
 
 // @public (undocumented)
 export interface RouterOptions {
+  additionalSchemas?: ConfigSchemaPackageEntry[];
   appPackageName: string;
   // (undocumented)
   config: Config;

plugins/app-backend/src/lib/config.ts

@@ -20,6 +20,7 @@ import { Logger } from 'winston';
 import { AppConfig, Config } from '@backstage/config';
 import { JsonObject } from '@backstage/types';
 import { loadConfigSchema, readEnvConfig } from '@backstage/config-loader';
+import { ConfigSchemaPackageEntry } from '@backstage/config-loader';
 
 type InjectOptions = {
   appConfigs: AppConfig[];
@@ -74,6 +75,7 @@ type ReadOptions = {
   env: { [name: string]: string | undefined };
   appDistDir: string;
   config: Config;
+  additionalSchemas?: ConfigSchemaPackageEntry[];
 };
 
 /**
@@ -90,7 +92,10 @@ export async function readConfigs(options: ReadOptions): Promise<AppConfig[]> {
     const serializedSchema = await fs.readJson(schemaPath);
 
     try {
-      const schema = await loadConfigSchema({ serialized: serializedSchema });
+      const schema = await loadConfigSchema({
+        serialized: serializedSchema,
+        additionalSchemas: options.additionalSchemas,
+      });
 
       const frontendConfigs = await schema.process(
         [{ data: config.get() as JsonObject, context: 'app' }],

plugins/app-backend/src/service/appPlugin.ts

@@ -19,6 +19,7 @@ import {
   coreServices,
   createBackendPlugin,
 } from '@backstage/backend-plugin-api';
+import { schemaDiscoveryServiceRef } from '@backstage/backend-plugin-api/alpha';
 import { createRouter } from './router';
 import { loggerToWinstonLogger } from '@backstage/backend-common';
 import { staticFallbackHandlerExtensionPoint } from '@backstage/plugin-app-node';
@@ -49,8 +50,9 @@ export const appPlugin = createBackendPlugin({
         config: coreServices.rootConfig,
         database: coreServices.database,
         httpRouter: coreServices.httpRouter,
+        schemaDiscovery: schemaDiscoveryServiceRef,
       },
-      async init({ logger, config, database, httpRouter }) {
+      async init({ logger, config, database, httpRouter, schemaDiscovery }) {
         const appPackageName =
           config.getOptionalString('app.packageName') ?? 'app';
         const winstonLogger = loggerToWinstonLogger(logger);
@@ -61,6 +63,9 @@ export const appPlugin = createBackendPlugin({
           database,
           appPackageName,
           staticFallbackHandler,
+          additionalSchemas: (
+            await schemaDiscovery?.getAdditionalSchemas()
+          )?.schemas,
         });
         httpRouter.use(router);
       },

plugins/app-backend/src/service/router.ts

@@ -37,6 +37,7 @@ import {
   CACHE_CONTROL_NO_CACHE,
   CACHE_CONTROL_REVALIDATE_CACHE,
 } from '../lib/headers';
+import { ConfigSchemaPackageEntry } from '@backstage/config-loader';
 
 // express uses mime v1 while we only have types for mime v2
 type Mime = { lookup(arg0: string): string };
@@ -85,6 +86,17 @@ export interface RouterOptions {
    * This also disables configuration injection though `APP_CONFIG_` environment variables.
    */
   disableConfigInjection?: boolean;
+
+  /**
+   *
+   * Provides a list of additional config schemas, in addition to the serialized schemas
+   * generated during the application build.
+   * This is useful when additional plugins are dynamically loaded in the application at start,
+   * which were not part of the application build. This option allows feeding the corresponding
+   * JSON schemas.
+   *
+   */
+  additionalSchemas?: ConfigSchemaPackageEntry[];
 }
 
 /** @public */
@@ -121,6 +133,7 @@ export async function createRouter(
       config,
       appDistDir,
       env: process.env,
+      additionalSchemas: options.additionalSchemas,
     });
 
     injectedConfigPath = await injectConfig({ appConfigs, logger, staticDir });