feat(payments): add initial NestJS service load on heartbeat

Because:

* We want to load/cache the NestJS services on the first request to the
  app, so that the first request is not slow.
* We should use proper NextJS project organization.

This commit:

* Adds a new route to the app that the loadbalancers utilize, to ensure
  that the NestJS services are loaded before the first request to the
  app.
* Moves internal implementation that shouldn't be routed to, behind the
  underscore directory prefix.

apps/payments/next/app/%5F_heartbeat__/route.ts

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { NextResponse } from 'next/server';
+
+import { app } from '../_nestapp/app';
+
+export const dynamic = 'force-dynamic';
+
+export async function GET(request: Request) {
+  await app.getApp();
+  const resp = new NextResponse('{}');
+  resp.headers.set('Content-Type', 'application/json');
+  return resp;
+}

apps/payments/next/app/%5F_lbheartbeat__/route.ts

@@ -0,0 +1,16 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { NextResponse } from 'next/server';
+
+import { app } from '../_nestapp/app';
+
+export const dynamic = 'force-dynamic';
+
+export async function GET(request: Request) {
+  await app.getApp();
+  const resp = new NextResponse('{}');
+  resp.headers.set('Content-Type', 'application/json');
+  return resp;
+}

apps/payments/next/app/[offeringId]/checkout/page.tsx

@@ -1,13 +1,14 @@
-import { CartService } from '@fxa/payments/cart';
 import { PurchaseDetails, TermsAndPrivacy } from '@fxa/payments/ui/server';
 
-import { getCartData, getContentfulContent } from '../../lib/apiClient';
-import { getApp } from '../../nestapp/app';
+import { getCartData, getContentfulContent } from '../../_lib/apiClient';
+import { app } from '../../_nestapp/app';
 
 interface CheckoutParams {
   offeringId: string;
 }
 
+export const dynamic = 'force-dynamic';
+
 export default async function Index({ params }: { params: CheckoutParams }) {
   // TODO - Fetch Cart ID from cookie
   // https://nextjs.org/docs/app/api-reference/functions/cookies
@@ -21,7 +22,7 @@ export default async function Index({ params }: { params: CheckoutParams }) {
   const cartData = getCartData(cartId);
   const [contentful, cart] = await Promise.all([contentfulData, cartData]);
   /* eslint-disable @typescript-eslint/no-unused-vars */
-  const cartService = (await getApp()).get(CartService);
+  const cartService = await app.getCartService();
 
   return (
     <>

apps/payments/next/app/nestapp/app.module.ts → apps/payments/next/app/_nestapp/app.module.ts

@@ -5,7 +5,7 @@
 import { dotenvLoader, fileLoader, TypedConfigModule } from 'nest-typed-config';
 
 import { CartManager, CartService } from '@fxa/payments/cart';
-import { AccountDatabaseFactory } from '@fxa/shared/db/mysql/account';
+import { AccountDatabaseNestFactory } from '@fxa/shared/db/mysql/account';
 import { Module } from '@nestjs/common';
 
 import { RootConfig } from './config';
@@ -21,6 +21,6 @@ import { RootConfig } from './config';
     }),
   ],
   controllers: [],
-  providers: [AccountDatabaseFactory, CartService, CartManager],
+  providers: [AccountDatabaseNestFactory, CartService, CartManager],
 })
 export class AppModule {}

apps/payments/next/app/_nestapp/app.ts

@@ -0,0 +1,28 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import 'server-only';
+
+import { CartService } from '@fxa/payments/cart';
+import { NestFactory } from '@nestjs/core';
+
+import { AppModule } from './app.module';
+
+class AppSingleton {
+  private app!: Awaited<
+    ReturnType<typeof NestFactory.createApplicationContext>
+  >;
+
+  async getApp() {
+    if (this.app) return this.app;
+    this.app = await NestFactory.createApplicationContext(AppModule);
+    return this.app;
+  }
+
+  async getCartService() {
+    return (await this.getApp()).get(CartService);
+  }
+}
+
+export const app = new AppSingleton();

libs/shared/db/mysql/account/src/index.ts

@@ -8,4 +8,4 @@ export { CartFactory } from './lib/factories';
 export { setupAccountDatabase, AccountDbProvider } from './lib/setup';
 export { testAccountDatabaseSetup } from './lib/tests';
 export type { AccountDatabase } from './lib/setup';
-export { AccountDatabaseFactory } from './lib/account.provider';
+export { AccountDatabaseNestFactory } from './lib/account.provider';

libs/shared/db/mysql/account/src/lib/account.provider.ts

@@ -11,7 +11,7 @@ import {
   setupAccountDatabase,
 } from './setup';
 
-export const AccountDatabaseFactory: Provider<AccountDatabase> = {
+export const AccountDatabaseNestFactory: Provider<AccountDatabase> = {
   provide: AccountDbProvider,
   useFactory: (mysqlConfig: MySQLConfig) => {
     return setupAccountDatabase(mysqlConfig);