cli: enable embedded-postgres via config instead of env var

Rather than requiring the `EXPERIMENTAL_DEV_DB` environment variable,
the embedded postgres server is now started automatically when
`backend.database.client` is set to `embedded-postgres` in the app
config. The CLI reads the config before spawning the backend and
injects the actual pg connection details via env override.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

.changeset/thin-elephants-joke.md

@@ -1,5 +1,6 @@
 ---
-'@backstage/cli': patch
+'@backstage/cli-module-build': patch
+'@backstage/backend-defaults': patch
 ---
 
-The `package start` command now supports an experimental `EXPERIMENTAL_DEV_DB` env flag that can be set to enable the use of `embedded-postgres` as the database for local development, rather than SQLite. For this to work the desired version of the `embedded-postgres` package must be installed in your project, typically as a `devDependency`.
+Added experimental support for using `embedded-postgres` as the database for local development. Set `backend.database.client` to `embedded-postgres` in your app config to enable this. The `embedded-postgres` package must be installed as an explicit dependency in your project.

packages/backend-defaults/config.d.ts

@@ -585,7 +585,7 @@ export interface Config {
     /** Database connection configuration, select base database type using the `client` field */
     database: {
       /** Default database client to use */
-      client: 'better-sqlite3' | 'sqlite3' | 'pg';
+      client: 'better-sqlite3' | 'sqlite3' | 'pg' | 'embedded-postgres';
       /**
        * Base database connection string, or object with individual connection properties
        * @visibility secret

packages/cli-module-build/src/commands/package/start/startBackend.ts

@@ -23,6 +23,7 @@ import { runBackend } from '../../../lib/runner';
 interface StartBackendOptions {
   targetDir: string;
   checksEnabled: boolean;
+  configPaths?: string[];
   inspectEnabled?: boolean | string;
   inspectBrkEnabled?: boolean | string;
   linkedWorkspace?: string;
@@ -33,6 +34,7 @@ export async function startBackend(options: StartBackendOptions) {
   const waitForExit = await runBackend({
     targetDir: options.targetDir,
     entry: 'src/index',
+    configPaths: options.configPaths,
     inspectEnabled: options.inspectEnabled,
     inspectBrkEnabled: options.inspectBrkEnabled,
     linkedWorkspace: options.linkedWorkspace,
@@ -56,6 +58,7 @@ export async function startBackendPlugin(options: StartBackendOptions) {
   const waitForExit = await runBackend({
     targetDir: options.targetDir,
     entry: 'dev/index',
+    configPaths: options.configPaths,
     inspectEnabled: options.inspectEnabled,
     inspectBrkEnabled: options.inspectBrkEnabled,
     require: options.require,

packages/cli-module-build/src/lib/runner/runBackend.ts

@@ -20,8 +20,13 @@ import { ctrlc } from 'ctrlc-windows';
 import { IpcServer, ServerDataStore } from '../ipc';
 import debounce from 'lodash/debounce';
 import { fileURLToPath } from 'node:url';
-import { isAbsolute as isAbsolutePath } from 'node:path';
+import {
+  isAbsolute as isAbsolutePath,
+  resolve as resolvePath,
+} from 'node:path';
 import { targetPaths } from '@backstage/cli-common';
+import { ConfigSources } from '@backstage/config-loader';
+import { ConfigReader } from '@backstage/config';
 
 import spawn from 'cross-spawn';
 import { startEmbeddedDb } from './startEmbeddedDb';
@@ -46,6 +51,8 @@ export type RunBackendOptions = {
   require?: string | string[];
   /** An external linked workspace to override module resolution towards */
   linkedWorkspace?: string;
+  /** Config file paths from --config flags */
+  configPaths?: string[];
 };
 
 export async function runBackend(options: RunBackendOptions) {
@@ -60,7 +67,8 @@ export async function runBackend(options: RunBackendOptions) {
 
   const extraEnv: Record<string, string> = {};
 
-  if (process.env.EXPERIMENTAL_DEV_DB) {
+  const dbClient = await readDatabaseClient(options.configPaths);
+  if (dbClient === 'embedded-postgres') {
     const db = await startEmbeddedDb();
     extraEnv.APP_CONFIG_backend_database = JSON.stringify({
       client: 'pg',
@@ -207,3 +215,28 @@ export async function runBackend(options: RunBackendOptions) {
 
   return () => exitPromise;
 }
+
+async function readDatabaseClient(
+  configPaths?: string[],
+): Promise<string | undefined> {
+  const rootDir = targetPaths.rootDir;
+  const source = ConfigSources.default({
+    rootDir,
+    allowMissingDefaultConfig: true,
+    argv: (configPaths ?? []).flatMap(p => [
+      '--config',
+      resolvePath(rootDir, p),
+    ]),
+  });
+
+  const abortController = new AbortController();
+  for await (const { configs } of source.readConfigData({
+    signal: abortController.signal,
+  })) {
+    abortController.abort();
+    return ConfigReader.fromConfigs(configs).getOptionalString(
+      'backend.database.client',
+    );
+  }
+  return undefined;
+}