docs: added troubleshooting component (#4255)

docs/content/development/endpoints/example-logged-in-user.mdx

@@ -0,0 +1,220 @@
+---
+description: 'In this document, you’ll see an example of how you can use middlewares and endpoints to register the logged-in user in the dependency container of your commerce application.'
+addHowToData: true
+---
+
+import Troubleshooting from '@site/src/components/Troubleshooting'
+import ServiceLifetimeSection from '../../troubleshooting/awilix-resolution-error/_service-lifetime.md'
+import CustomRegistrationSection from '../../troubleshooting/awilix-resolution-error/_custom-registration.md'
+
+# Example: Access Logged-In User
+
+In this document, you’ll see an example of how you can use middlewares and endpoints to register the logged-in user in the dependency container of your commerce application. You can then access the logged-in user in other resources, such as services.
+
+This guide showcases how to register the logged-in admin user, but you can apply the same steps if you want to register the current customer.
+
+This documentation does not explain the basics of [middlewares](./add-middleware.mdx) and [endpoints](./create.md). You can refer to their respective guides for more details about each.
+
+## Step 1: Create the Middleware
+
+Create the file `src/api/middlewares/logged-in-user.ts` with the following content:
+
+```ts title=src/api/middlewares/logged-in-user.ts
+import { User, UserService } from "@medusajs/medusa"
+
+export async function registerLoggedInUser(req, res, next) {
+  let loggedInUser: User | null = null
+
+  if (req.user && req.user.userId) {
+    const userService = 
+      req.scope.resolve("userService") as UserService
+    loggedInUser = await userService.retrieve(req.user.userId)
+  }
+
+  req.scope.register({
+    loggedInUser: {
+      resolve: () => loggedInUser,
+     },
+   })
+  
+  next()
+}
+```
+
+This retrieves the ID of the current user to retrieve an instance of it, then registers it in the scope under the name `loggedInUser`.
+
+---
+
+## Step 2: Apply Middleware on Endpoint
+
+If you don't have the `cors` package installed, make sure to install it first:
+
+```bash npm2yarn
+npm install cors
+```
+
+Then, create the file `src/api/routes/create-product.ts` with the following content:
+
+```ts title=src/api/routes/create-product.ts
+import cors from "cors"
+import { Router } from "express"
+import { 
+  registerLoggedInUser,
+} from "../middlewares/logged-in-user"
+import 
+  authenticate 
+from "@medusajs/medusa/dist/api/middlewares/authenticate"
+
+const router = Router()
+
+export default function (adminCorsOptions) {
+  // This router will be applied before the core routes. 
+  // Therefore, the middleware will be executed
+  // before the create product handler is hit
+  router.use(
+    "/admin/products", 
+    cors(adminCorsOptions), 
+    authenticate(),
+    registerLoggedInUser
+  )
+  return router
+}
+```
+
+In the example above, the middleware is applied on the `/admin/products` core endpoint. However, you can apply it on any other endpoint. You can also apply it to custom endpoints.
+
+For endpoints that require Cross-Origin Resource Origin (CORS) options, such as core endpoints, you must pass the CORS options to the middleware as well since it will be executed before the underlying endpoint.
+
+:::tip
+
+In the above code snippet, the `authenticate` middleware imported from `@medusajs/medusa` is used to ensure that the user is logged in first. If you're implementing this for middleware to register the logged-in customer, make sure to use the [customer's authenticate middleware](./create.md#protect-store-routes).
+
+:::
+
+---
+
+## Step 3: Register Endpoint in the API
+
+Create the file `src/api/index.ts` with the following content:
+
+```ts title=src/api/index.ts
+import configLoader from "@medusajs/medusa/dist/loaders/config"
+import createProductRouter from "./routes/create-product"
+
+export default function (rootDirectory: string) {
+  const config = configLoader(rootDirectory)
+
+  const adminCors = {
+    origin: config.projectConfig.admin_cors.split(","),
+    credentials: true,
+  }
+
+  const productRouters = [
+    createProductRouter(adminCors),
+  ]
+
+  return [...productRouters]
+}
+```
+
+This exports an array of endpoints, one of them being the product endpoint that you applied the middleware on in the second step. You can export more endpoints as well.
+
+---
+
+## Step 4: Use in a Service
+
+You can now access the logged-in user in a service. For example, to access it in a custom service:
+
+<!-- eslint-disable prefer-rest-params -->
+
+```ts
+import { Lifetime } from "awilix"
+import { 
+  TransactionBaseService, 
+  User,
+} from "@medusajs/medusa"
+
+class HelloService extends TransactionBaseService {
+
+  protected readonly loggedInUser_: User | null
+
+  constructor(container, options) {
+    super(...arguments)
+
+    try {
+      this.loggedInUser_ = container.loggedInUser
+    } catch (e) {
+      // avoid errors when backend first runs
+    }
+  }
+
+  // ...
+}
+
+export default HelloService
+```
+
+If you're accessing it in an extended core service, it’s important to change the lifetime of the service to `Lifetime.SCOPED`. For example:
+
+<!-- eslint-disable prefer-rest-params -->
+
+```ts
+import { Lifetime } from "awilix"
+import { 
+  ProductService as MedusaProductService, 
+  User, 
+} from "@medusajs/medusa"
+
+// extend core product service
+class ProductService extends MedusaProductService {
+  // The default life time for a core service is SINGLETON
+  static LIFE_TIME = Lifetime.SCOPED
+
+  protected readonly loggedInUser_: User | null
+
+  constructor(container, options) {
+    super(...arguments)
+
+    this.loggedInUser_ = container.loggedInUser
+  }
+}
+
+export default ProductService
+```
+
+You can learn more about the importance of changing the service lifetime in the [Middlewares documentation](./add-middleware.mdx#note-about-services-lifetime).
+
+---
+
+## Step 5: Test it Out
+
+To test out your implementation, run the following command in the root directory of the Medusa backend to transpile your changes:
+
+```bash npm2yarn
+npm run build
+```
+
+Then, run your backend with the following command:
+
+```bash npm2yarn
+npx @medusajs/medusa-cli develop
+```
+
+If you try accessing the endpoints you added the middleware to, you should see your implementation working as expected.
+
+---
+
+## Troubleshooting
+
+<Troubleshooting 
+  sections={[
+    {
+      title: 'AwilixResolutionError: Could Not Resolve X',
+      content: <ServiceLifetimeSection />
+    },
+    {
+      title: 'AwilixResolutionError: Could Not Resolve X (Custom Registration)',
+      content: <CustomRegistrationSection />
+    }
+  ]}
+/>