diff --git a/src/data.js b/src/data.js
index 5dd8c35..7c75b95 100644
--- a/src/data.js
+++ b/src/data.js
@@ -51,6 +51,10 @@ const products = [{
   storeId: '5f2919aa-333a-4745-8166-3002ab30de0e',
 }];
 
+const reservations = [];
+
+let reservationProducts = [];
+
 function createStore({ city, name, number, postalCode, street }) {
   const newStore = {
     city,
@@ -74,11 +78,21 @@ function getStores() {
 
 function getStoreProducts(storeId) {
   return products.filter((p) => p.storeId === storeId)
-} 
+}
+
+function getReservationProducts(reservationId) {
+  return [];
+}
+
+function createReservation(reservation) {
+  return {};
+}
 
 module.exports = {
   createStore,
+  createReservation,
   getStore,
   getStores,
-  getStoreProducts
+  getStoreProducts,
+  getReservationProducts
 };
\ No newline at end of file
diff --git a/src/index.js b/src/index.js
index 66f7b5d..f34c9a4 100644
--- a/src/index.js
+++ b/src/index.js
@@ -11,21 +11,20 @@ const typeDefs = require('./typeDefs');
 // a store, we get a list of products (query) of that store.
 // We pick some products, pick a quantity, and we make a reservation (mutation)
 
-// 🏪  Exercise 3
+// 🏪  Exercise 4
 // --------------
 
-// Now we want to display the products for a store, we can select the quantity of each product,
-// and make a reservation.
+// Now we will focus on creating a reservation, which looks like this:
+// { id, date, reservationProducts: { product: { id, name price }, quantity })).
 
-// 1) Create a Query `store` to retrieve a store by id. Extend the data store to find the right store.
-// 2) Create a list of products with a reference (storeId).
-// 3) Extend the Store type with a list of products. A product has a price, name and description.
-//    `products` becomes a field of the store. Add a resolver function to get the products of a store.
-// 4) Go to the GraphQL Playground and use a directive to query a store with or without products,
-//    name the variable `withProducts`.
-
-// 1) Check: https://graphql.org/learn/queries/#variables
-// 4) products @include(if: $withProducts) {
+// 1) Create a Mutation to create a reservation `createReservation`
+//    that takes an object as input. The input type is named `ReservationInput`.
+//    ReservationInput contains a list of { productId, quantity } (input type is named `ReservationProductInput`),
+//    named `reservationProducts` (field of `ReservationInput`).
+// 2) Now create a resolver function for the mutation and insert it into our in-memory database.
+// 3) Create a query field reservationProducts under Reservation, to get the reservationProducts [{ product, quantity }].
+//    Similar as stores under Query.
+// 4) Go to the GraphQL Playground and try out the createReservation mutation!
 
 // In the most basic sense, the ApolloServer can be started
 // by passing type definitions (typeDefs) and the resolvers
diff --git a/src/resolvers.js b/src/resolvers.js
index 648c124..402b2c5 100644
--- a/src/resolvers.js
+++ b/src/resolvers.js
@@ -2,6 +2,8 @@ const { UserInputError } = require('apollo-server');
 const yup = require('yup');
 const data = require('./data');
 
+const uuidSchema = yup.string().min(36).max(36);
+
 const createStoreSchema = yup.object()
   .shape({
     city: yup.string().max(255),
@@ -15,9 +17,19 @@ const createStoreSchema = yup.object()
     street: yup.string().max(255),
   });
 
-const getStoreSchema =  yup.object()
+const getStoreSchema = yup.object()
   .shape({
-    id: yup.string().min(36).max(36),
+    id: uuidSchema,
+  });
+
+const createReservationSchema = yup.object()
+  .shape({
+    reservationProducts: yup.array(
+      yup.object().shape({
+        productId: uuidSchema,
+        quantity: yup.number().required().positive().integer(),
+      })
+    ),
   });
 
 // Resolvers define the technique for fetching the types in the
@@ -49,6 +61,6 @@ module.exports = {
       }
       const { city, name, number, postalCode, street } = input;
       return data.createStore({ city, name, number, postalCode, street });
-    }
+    },
   }
 };
\ No newline at end of file
diff --git a/src/typeDefs.js b/src/typeDefs.js
index 8dd3584..25b2345 100644
--- a/src/typeDefs.js
+++ b/src/typeDefs.js
@@ -32,7 +32,7 @@ module.exports = gql`
 
   input StoreInput {
     city: String
-    name: String
+    name: String!
     number: Int
     postalCode: String
     street: String