diff --git a/.vscode/launch.json b/.vscode/launch.json
index 61b1623..073c8b8 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -5,13 +5,15 @@
     "version": "0.2.0",
     "configurations": [
         {
+            "name": "Launch Program with debugger",
             "type": "node",
             "request": "launch",
-            "name": "Launch Program",
-            "skipFiles": [
-                "<node_internals>/**"
+            "cwd": "${workspaceFolder}",
+            "runtimeExecutable": "npm",
+            "runtimeArgs": [
+                "start"
             ],
-            "program": "${workspaceFolder}/src/js/app.js"
+            "port": 5858
         }
     ]
 }
\ No newline at end of file
diff --git a/src/js/configuration.js b/src/js/configuration.js
index b7f3720..ab6666c 100644
--- a/src/js/configuration.js
+++ b/src/js/configuration.js
@@ -3,14 +3,4 @@ const yaml = require('yaml');
 
 const configurationFile = fs.readFileSync('src/resources/application.yml', 'utf8');
 const configuration = yaml.parse(configurationFile);
-console.log(configuration);
-module.exports = configuration;
-
-// module.exports = {
-//     database: {
-//         url: 'mongodb://localhost:27017',
-//         username: 'express-user',
-//         password: 'P@ssword1',
-//         database: 'express-test'
-//     }
-// };
\ No newline at end of file
+module.exports = configuration;
\ No newline at end of file
diff --git a/src/js/controller/userCtrl.js b/src/js/controller/userCtrl.js
index 6bc57d0..c5cd1c7 100644
--- a/src/js/controller/userCtrl.js
+++ b/src/js/controller/userCtrl.js
@@ -2,15 +2,16 @@ const router = require('express').Router();
 const tokenService = require('../service/tokenService');
 const userService = require('../service/userService');
 
-// Develop routes here
 router.post('/login', (request, response) => {
     const loginRequest = request.body;
 
-    if (!!loginRequest && userService.areCredentialsValid(loginRequest.login, loginRequest.password)) {
-        const tokenPayload = { login: loginRequest.login };
-        response.json(tokenService.build(tokenPayload));
-    } else {
+    if (!loginRequest) {
         response.status(403).send();
+    } else {
+        userService.checkCredentials(loginRequest.login, loginRequest.password, () => {
+            const tokenPayload = { login: loginRequest.login };
+            response.json(tokenService.build(tokenPayload));
+        }, () => response.status(403).send());
     }
 });
 
diff --git a/src/js/repository/mongoClient.js b/src/js/repository/mongoClient.js
index 40ed8f5..2024f2e 100644
--- a/src/js/repository/mongoClient.js
+++ b/src/js/repository/mongoClient.js
@@ -14,14 +14,15 @@ class MongoClient {
         });
     }
 
-    find(collectionName, query, callback) {
+    find(collectionName, query, onSuccess, onError) {
         this.db.collection(collectionName).find(query).toArray()
             .then(results => {
                 console.log(`Entities ${collectionName} founded.`);
-                callback(results);
+                onSuccess(results);
             })
             .catch(error => {
-                throw new Error(`Unable to find entities in collection ${collectionName}: ${error}`);
+                console.error(`Unable to find entities in collection ${collectionName}: ${error}`);
+                onError(error);
             });
     }
 
diff --git a/src/js/repository/repository.js b/src/js/repository/repository.js
index f17bf90..44de763 100644
--- a/src/js/repository/repository.js
+++ b/src/js/repository/repository.js
@@ -23,11 +23,11 @@ module.exports = class Repository {
     /**
      * Returns the entities that matches criteria in {@code query}.
      * @param {*} query The query which contains criteria to find some entities.
-     * @param {*} callback The function to execute after getting entities.
+     * @param {*} onSuccess The function to execute after getting entities.
      */
-    find(query, callback) {
+    find(query, onSuccess, onError) {
         convertIdToMongodbFormat(query);
-        this.mongoClient.find(this.collectionName, query, callback);
+        this.mongoClient.find(this.collectionName, query, onSuccess, onError);
     }
 
     /**
diff --git a/src/js/service/userService.js b/src/js/service/userService.js
index 6a04100..d8cfdb6 100644
--- a/src/js/service/userService.js
+++ b/src/js/service/userService.js
@@ -4,21 +4,44 @@ const passwordService = require('./passwordService');
 const userRepository = new Repository('users');
 
 class UserService {
-    getUser(login) {
-        return login === 'toto'
-            ? {login: 'toto', password: passwordService.hashPassword('pwd')}
-            : undefined;
+    /**
+     * Get a user from database by its login.
+     * @param {String} login User login.
+     * @param {Function} onSuccess Callback function to execute if a user exists with this login.
+     * @param {Function} onError Callback function to execute if not any user exists with this login.
+     */
+    getUser(login, onSuccess, onError) {
+        userRepository.find({login: login}, results => onSuccess(results[0]), onError);
     }
 
+    /**
+     * 
+     */
     isAuthenticated() {
         return false;
     }
 
-    areCredentialsValid(login, password) {
-        const user = this.getUser(login);
-        // If login is incorrect, the "getUser" function will return "undefined".
-        // So if "user" is not "undefined", this proofs that login is correct.
-        return !!user && passwordService.areSamePasswords(password, user.password);
+    /**
+     * 
+     * @param {String} login User login.
+     * @param {String} password User password, in plain text.
+     * @param {Function} onSuccess Callback function to execute if a user exists with this login.
+     * @param {Function} onError Callback function to execute if not any user exists with this login.
+     */
+    checkCredentials(login, password, onSuccess, onError) {
+        this.getUser(
+            login,
+            dbUser => {
+                if (!!dbUser && passwordService.areSamePasswords(password, dbUser.password)) {
+                    onSuccess();
+                } else {
+                    onError();
+                }
+            },
+            // If login is incorrect, the "getUser" function will return "undefined".
+            // So if "user" is "undefined", this proofs that login is incorrect
+            onError
+        );
     }
 }