feat: add an api to add items to a cart atomically

package.json

@@ -52,17 +52,17 @@
     "src"
   ],
   "dependencies": {
-    "@loopback/boot": "^0.13.2",
+    "@loopback/boot": "^0.13.5",
     "@loopback/context": "^0.12.8",
-    "@loopback/core": "^0.11.8",
+    "@loopback/core": "^0.11.9",
     "@loopback/dist-util": "^0.3.7",
-    "@loopback/openapi-v3": "^0.13.2",
-    "@loopback/repository": "^0.16.2",
-    "@loopback/rest": "^0.21.1",
+    "@loopback/openapi-v3": "^0.14.2",
+    "@loopback/repository": "^0.16.5",
+    "@loopback/rest": "^0.22.2",
     "bcryptjs": "^2.4.3",
     "isemail": "^3.1.3",
     "loopback-connector-kv-redis": "^3.0.0",
-    "loopback-connector-mongodb": "^3.6.0"
+    "loopback-connector-mongodb": "^3.7.1"
   },
   "devDependencies": {
     "@commitlint/cli": "^7.1.2",

src/controllers/shopping-cart.controller.ts

@@ -3,10 +3,18 @@
 // This file is licensed under the MIT License.
 // License text available at https://opensource.org/licenses/MIT
 
-import {put, get, del, param, requestBody, HttpErrors} from '@loopback/rest';
+import {
+  put,
+  get,
+  del,
+  param,
+  requestBody,
+  HttpErrors,
+  post,
+} from '@loopback/rest';
 import {repository} from '@loopback/repository';
 import {ShoppingCartRepository} from '../repositories';
-import {ShoppingCart} from '../models';
+import {ShoppingCart, ShoppingCartItem} from '../models';
 
 /**
  * Controller for shopping cart
@@ -59,4 +67,17 @@ export class ShoppingCartController {
   async remove(@param.path.string('userId') userId: string) {
     await this.shoppingCartRepository.delete(userId);
   }
+
+  /**
+   * Add an item to the shopping cart for a given user
+   * @param userId User id
+   * @param cart Shopping cart item to be added
+   */
+  @post('/shoppingCarts/{userId}/items')
+  async addItem(
+    @param.path.string('userId') userId: string,
+    @requestBody({description: 'shopping cart item'}) item: ShoppingCartItem,
+  ) {
+    await this.shoppingCartRepository.addItem(userId, item);
+  }
 }

src/models/shopping-cart.model.ts

@@ -9,11 +9,11 @@ import {Entity, model, property} from '@loopback/repository';
  * Item in a shopping cart
  */
 @model()
-export class ShoppingCartItem {
+export class ShoppingCartItem extends Entity {
   /**
    * Product id
    */
-  @property()
+  @property({id: true})
   productId: string;
   /**
    * Quantity
@@ -25,6 +25,10 @@ export class ShoppingCartItem {
    */
   @property()
   price?: number;
+
+  constructor(data?: Partial<ShoppingCartItem>) {
+    super(data);
+  }
 }
 
 @model()

src/repositories/shopping-cart.repository.ts

@@ -4,14 +4,69 @@
 // License text available at https://opensource.org/licenses/MIT
 
 import {DefaultKeyValueRepository} from '@loopback/repository';
-import {ShoppingCart} from '../models/shopping-cart.model';
+import {ShoppingCart, ShoppingCartItem} from '../models/shopping-cart.model';
 import {RedisDataSource} from '../datasources/redis.datasource';
 import {inject} from '@loopback/context';
+import {promisify} from 'util';
 
 export class ShoppingCartRepository extends DefaultKeyValueRepository<
   ShoppingCart
 > {
   constructor(@inject('datasources.redis') ds: RedisDataSource) {
     super(ShoppingCart, ds);
   }
+
+  /**
+   * Add an item to the shopping cart with optimistic lock to allow concurrent
+   * `adding to cart` from multiple devices
+   *
+   * @param userId User id
+   * @param item Item to be added
+   */
+  addItem(userId: string, item: ShoppingCartItem) {
+    const addItemToCart = (cart: ShoppingCart | null) => {
+      cart = cart || new ShoppingCart({userId});
+      cart.items = cart.items || [];
+      cart.items.push(item);
+      return cart;
+    };
+    return this.checkAndSet(userId, addItemToCart);
+  }
+
+  /**
+   * Use Redis WATCH and Transaction to check and set against a key
+   * See https://redis.io/topics/transactions#optimistic-locking-using-check-and-set
+   *
+   * Ideally, this method should be made available by `KeyValueRepository`.
+   *
+   * @param userId User id
+   * @param check A function that checks the current value and produces a new
+   * value. It returns `null` to abort.
+   */
+  async checkAndSet(
+    userId: string,
+    check: (current: ShoppingCart | null) => ShoppingCart | null,
+  ) {
+    const connector = this.kvModelClass.dataSource!.connector!;
+    // tslint:disable-next-line:no-any
+    const execute = promisify((cmd: string, args: any[], cb: Function) => {
+      return connector.execute!(cmd, args, cb);
+    });
+    /**
+     * - WATCH userId
+     * - GET userId
+     * - check(cart)
+     * - MULTI
+     * - SET userId
+     * - EXEC
+     */
+    await execute('WATCH', [userId]);
+    let cart: ShoppingCart | null = await this.get(userId);
+    cart = check(cart);
+    if (!cart) return null;
+    await execute('MULTI', []);
+    await this.set(userId, cart);
+    await execute('EXEC', []);
+    return cart;
+  }
 }