diff --git a/README.md b/README.md
index 7cc01a7..6b3bae1 100644
--- a/README.md
+++ b/README.md
@@ -8,22 +8,58 @@ Step 1: CD to the project root directory `cart_api`
 
 Step 2: Run `./mvnw spring-boot:run` command
 
-## Lesson Overview (6 Mar 2023, Mon)
+## Lesson Overview (9 Mar 2023, Fri)
 
 Lesson Coverage:
-- Use of `@OneToOne` and `@JoinColumn` annotation
-- Start implementing new set of APIs
-
-## New APIs
-
-|URL|Status|Response|
-|-|-|-|
-|GET /carts|200|A list of records in cart table|
-||404|When no records are found in cart table|
-|POST /carts/add/{productId}?quantity=1|200|<p>If `productId` does not exist, create a new record with `quantity` set to 1.</p><p>When `quantity` is provided, set the quantity of the specified `productId` to the specified value.</p><p>When `quantity` is not provided, increment the quantity of the specified `productId` by 1.</p>|
-||400|When the specified `productId` does not exist in the product table.
-|POST /carts/decrement/{productId}|200|<p>If the specified `productId` exist and the quantity is 1, delete the record from cart table.</p><p>If the specified `productId` exist and the quantity is greater than 1, decrement the quantity by 1.</p>|
-||400|When the specified `productId` does not exist in the `cart` table|
-|POST /carts/clear|200|If there are records in the cart table, delete all records in the cart table|
-||304|If there are no records in the cart table, do nothing.|
 
+1. Move complex logics in `/carts` to a `@Service` class
+2. Adding `User` entity
+3. Use of `UserID` header as a mock authentication header
+
+> Firebase SDK has a major shift and no longer support `signInWithEmailAndPassword` from the backend `Firebase Admin SDK` using Java.
+
+## 1. Move complex logics to @Service class
+
+Refer to the `add` and `decrement` methods in [CartService.java](./cart_api/src/main/java/sg/edu/ntu/cart_api/service/CartService.java) where complex logics is moved into.
+
+Refer to [CartController.java](./cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java) where the code mainly focuses on handling http request and response (status).
+
+A custom exception called [NotFoundException.java](./cart_api/src/main/java/sg/edu/ntu/cart_api/exception/NotFoundException.java) is created to help us identify scenario where the `productId` is not found.
+
+## 2. Adding the `User` Entity
+
+Let's refer to our ERD again while creating our `User` Entity class.
+
+<img src="./assets/erd.png" />
+
+The SQL Statements required to generate the `user` table:
+
+```sql
+create table user (
+    id int not null auto_increment,
+    email varchar(255) not null,
+    created_at timestamp not null default current_timestamp,
+    primary key (id)
+);
+
+alter table cart add column user_id int;
+alter table cart add constraint fk_cart_user foreign key (user_id) references user(id);
+```
+
+If you need to check all foreign keys in the DB, use the following command:
+
+```sql
+SELECT TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_SCHEMA = 'cartdb';
+```
+
+## 3. Using `@RequestHeader` to capture value from HTTP Header
+
+```java
+public ResponseEntity<List<Cart>> findAll(@RequestHeader("user-id") int userId)
+```
+
+See [CartController.java](./cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java) for full details.
+
+We should also implement this for every methods in `CartController.java`. This will eventually affect `CartService.java` as well. All request to `/carts` must contains a `user-id` header.
+
+End
\ No newline at end of file
diff --git a/assets/erd.png b/assets/erd.png
new file mode 100644
index 0000000..3288370
Binary files /dev/null and b/assets/erd.png differ
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java
index 882af3f..7e62183 100644
--- a/cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/controller/CartController.java
@@ -8,6 +8,7 @@
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.RequestHeader;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.bind.annotation.RequestParam;
@@ -34,16 +35,16 @@ public class CartController {
     CartService service;
 
     @RequestMapping(method=RequestMethod.GET)
-    public ResponseEntity<List<Cart>> findAll(){
-        List<Cart> cartItems = (List<Cart>)repo.findAll();
+    public ResponseEntity<List<Cart>> findAll(@RequestHeader("user-id") int userId){
+        List<Cart> cartItems = (List<Cart>)repo.findByUserId(userId);
         if(cartItems.size() > 0) return ResponseEntity.ok().body(cartItems);
         return ResponseEntity.notFound().build();
     }
 
     @RequestMapping(value="/add/{productId}", method=RequestMethod.POST)
-    public ResponseEntity add(@PathVariable int productId, @RequestParam Optional<Integer> quantity){
+    public ResponseEntity add(@PathVariable int productId, @RequestParam Optional<Integer> quantity, @RequestHeader("user-id") int userId){
         try{
-            service.add(productId, quantity);
+            service.add(productId, quantity, userId);
             return ResponseEntity.ok().build(); // When no exception is thrown means operation is successful.
         }catch(NotFoundException nfe){
             nfe.printStackTrace();
@@ -56,10 +57,10 @@ public ResponseEntity add(@PathVariable int productId, @RequestParam Optional<In
     }
 
     @RequestMapping(value="/decrement/{productId}", method=RequestMethod.POST)
-    public ResponseEntity decrement(@PathVariable int productId){
+    public ResponseEntity decrement(@PathVariable int productId, @RequestHeader("user-id") int userId){
 
         try{
-            service.decrement(productId);
+            service.decrement(productId, userId);
             return ResponseEntity.ok().build(); // When no exception is thrown means operation is successful.
         }catch(NotFoundException nfe){
             nfe.printStackTrace();
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Cart.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Cart.java
index 837e754..937e86a 100644
--- a/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Cart.java
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Cart.java
@@ -29,6 +29,16 @@ public class Cart {
     @Column(name="created_at", updatable= false)
     Timestamp createdAt = new Timestamp(new Date().getTime());
 
+    @Column(name="user_id")
+    Integer userId;
+
+    public Integer getUserId() {
+        return userId;
+    }
+    public void setUserId(Integer userId) {
+        this.userId = userId;
+    }
+    
     public Integer getId() {
         return id;
     }
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Product.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Product.java
index dad7ec6..6a9f872 100644
--- a/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Product.java
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/Product.java
@@ -29,7 +29,7 @@ public class Product {
 
     @Column(name="created_at", updatable= false)
     Timestamp createdAt = new Timestamp(new Date().getTime());
-
+    
     public Integer getId() {
         return id;
     }
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/User.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/User.java
new file mode 100644
index 0000000..861f170
--- /dev/null
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/entity/User.java
@@ -0,0 +1,49 @@
+package sg.edu.ntu.cart_api.entity;
+
+import java.sql.Timestamp;
+import java.util.Date;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+import javax.persistence.Table;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Column;
+
+@Entity
+@Table(name="user")
+public class User {
+
+    @Id
+    @GeneratedValue(strategy=GenerationType.IDENTITY)
+    Integer id; 
+
+    String email;
+
+    @Column(name="created_at", updatable= false)
+    Timestamp createdAt = new Timestamp(new Date().getTime());
+
+    public Integer getId() {
+        return id;
+    }
+
+    public void setId(Integer id) {
+        this.id = id;
+    }
+
+    public String getEmail() {
+        return email;
+    }
+
+    public void setEmail(String email) {
+        this.email = email;
+    }
+
+    public Timestamp getCreatedAt() {
+        return createdAt;
+    }
+
+    public void setCreatedAt(Timestamp createdAt) {
+        this.createdAt = createdAt;
+    }
+}
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/repository/CartRepository.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/repository/CartRepository.java
index 4f6bd35..db598b5 100644
--- a/cart_api/src/main/java/sg/edu/ntu/cart_api/repository/CartRepository.java
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/repository/CartRepository.java
@@ -1,6 +1,7 @@
 package sg.edu.ntu.cart_api.repository;
 
 import java.util.Optional;
+import java.util.List;
 
 import org.springframework.data.repository.CrudRepository;
 import org.springframework.stereotype.Repository;
@@ -10,5 +11,7 @@
 @Repository
 public interface CartRepository extends CrudRepository<Cart, Integer> {
     
-    Optional<Cart> findByProductId(int productId);
+    Optional<Cart> findByProductIdAndUserId(int productId, int userId);
+
+    List<Cart> findByUserId(int userId);
 }
diff --git a/cart_api/src/main/java/sg/edu/ntu/cart_api/service/CartService.java b/cart_api/src/main/java/sg/edu/ntu/cart_api/service/CartService.java
index fa81f22..f4d22e2 100644
--- a/cart_api/src/main/java/sg/edu/ntu/cart_api/service/CartService.java
+++ b/cart_api/src/main/java/sg/edu/ntu/cart_api/service/CartService.java
@@ -19,8 +19,8 @@ public class CartService {
     @Autowired
     ProductRepository productRepo;
     
-    public void add(int productId, Optional<Integer> quantity) throws NotFoundException {
-        Optional<Cart> optionalCartItem = repo.findByProductId(productId);
+    public void add(int productId, Optional<Integer> quantity, int userId) throws NotFoundException {
+        Optional<Cart> optionalCartItem = repo.findByProductIdAndUserId(productId, userId);
         
         if(optionalCartItem.isPresent()){
 
@@ -39,6 +39,7 @@ public void add(int productId, Optional<Integer> quantity) throws NotFoundExcept
                 Cart cartItem = new Cart();
                 cartItem.setProduct(product);
                 cartItem.setQuantity(quantity.orElseGet(() -> 1));
+                cartItem.setUserId(userId);
                 repo.save(cartItem);
             }else{
                 throw new NotFoundException("Product ID not found");
@@ -46,8 +47,8 @@ public void add(int productId, Optional<Integer> quantity) throws NotFoundExcept
         }
     }
 
-    public void decrement(int productId) throws NotFoundException{
-        Optional<Cart> optionalCartItem = repo.findByProductId(productId);        
+    public void decrement(int productId, int userId) throws NotFoundException{
+        Optional<Cart> optionalCartItem = repo.findByProductIdAndUserId(productId, userId);        
         if(optionalCartItem.isPresent()){
 
             int currentQuantity = 0;