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Battleships - Backend Documentation

This is the backend documentation for the Battleships project.

Table of Contents

Introduction

The backend server is a RESTful API that provides the functionality for the Battleships game. It is written in Kotlin in a JVM gradle project.

The JVM application is a simple Spring Boot application, built with Spring Initializr. Some used dependencies are:

  • Spring Web - for the REST API;
  • Spring Data JPA - for the database access, using PostgreSQL;
  • JSON Web Token - for the authentication.

To read more about the JVM application structure, see the Application Architecture section.

Modeling the Database

Conceptual Model

The following diagram holds the Entity-Relationship model for the information managed by the system.

Entity Relationship Diagram

We highlight the following aspects:

  • A Player is a weak entity of the Userand Game entities, since a player can be only assigned to a user and a game;
  • The GameState and GameConfig entities have a weak connection through the Game entity, since it is a 1:1 relationship in both cases;
  • All entities are identified through an attribute idwhich will be used as primary key in every table;
  • The coordinate attribute is a complex value comprised of col and row sub-attributes;
  • The Game entity has a 1:2 relationship with the Player entity, so one game can host up to two players;
  • The User entity has a points attribute that will be used to store the points of the user in the ranking; this attribute should not be mistaken with the points attribute of the Player entity, which is used to store the points in the current game;
  • The GameConfig entity has a relationship with the ShipType entity, so that the types of ships that can be used in the game are defined by the game creator.

The conceptual model has the following restrictions:

  • User entity:

    • The username and email attributes should be unique;
    • The username attribute length should be between 3 and 40 characters;
    • The email attribute needs to follow the following pattern: ^[A-Za-z0-9+_.-]+@(.+)$;
    • The points attrinbute (also present in the Player entity) should not have negative values;

  • Game, GameState and GameConfig entities:

    • The name attribute should have a length between 1 and 40 characters;
    • The phase attribute only accepts the following values: WAITING_FOR_PLAYERS, GRID_LAYOUT, IN_PROGRESS , FINISHED;
    • The grid_size attribute should be between 7 and 18;
    • The max_time_for_layout_phase should be between 10 and 120 seconds;
    • The max_time_per_round should be between 50 and 120 seconds;
    • The shots_per_round should be between 1 and 5;
    • The round attribute (also present in the Shot entity) should be greater than 0;

  • Ship and ShipType entities:

    • The ship_name attribute should have a length between 1 and 40 characters;
    • The quantity attribute should be between 0 and 10;
    • The size attribute should be between 1 and 7;
    • The points attribute should be between 1 and 100;
    • The orientation attribute only accepts the following values: VERTICAL, HORIZONTAL:
    • The lives attribute should be between 0 and 7;

  • The col attribute should be a character between A and R;

  • The row attribute should be between 1 and 18;

  • The result attribute of the Shot entity only accepts the following values: HIT, MISS, SUNK.

Physical Model

The physical model of the database is available in createSchema.sql.

To implement and manage the database PostgreSQL was used.

The code/sql folder contains all SQL scripts developed:

We highlight the following aspects of this model:

  • All primary keys are represented by the id attribute and are SERIAL;
  • Other attributes that are not primary keys, but also identify the entity are UNIQUE;
  • All the restrictions described in the Conceptual Model chapter are reflected in the physical model, using check constraints.

Application Architecture

System Architecture

The JVM application is organized as follows:

  • /domain - contains the domain classes of the application, implemented using Spring Data JPA;
  • /http - contains the HTTP layer of the application, implemented using Spring Web MVC;
  • /repository - contains the repository layer of the application, implemented using Spring Data JPA;
  • /service - contains the services that manage the business logic of the application;
  • /utils - contains utility classes used by the application;
  • BattleshipsApplication.kt - contains the entry point of the application.

Presentation Layer

The presentation layer is responsible for receiving the requests from the client, processing them, sending them to the service layer and returning the responses to the client. The data representation used in these requests and responses are DTOs (Data Transfer Objects).

This layer is implemented using Spring Web MVC.

The presentation layer is organized as follows:

  • /controllers - contains the controllers that manage the HTTP requests;
  • /pipeline - contains the pipeline that process the HTTP requests;
  • /media - contains the classes that represent the media types used in the application, such as application/vnd.siren+json and application/problem+json;
  • /utils - contains some utility objects, like:
    • Uris - object that contains the URIs of the application used by the controllers;
    • Actions - object that contains the actions used by the controllers;
    • Uris - object that contains the Uris used by the controllers.

Business Logic Layer

The service layer is responsible for managing the business logic of the application, receiving the requests from the presentation layer, processing them, sending them to the data access layer and returning the responses to the presentation layer. The data representation used in these requests and responses are DTOs (Data Transfer Objects).

Each service is responsible for managing a specific group of requests, and each service is implemented as a @Service class, which is injected into the controller that manages the requests.

Each service has an interface that defines the methods that it implements, and the implementation of the interface. The services are the following:

  • UsersService - manages the requests related to the User entity;
  • GamesService - manages the requests related to the Game entity;
  • PlayersService - manages the requests related to the Player entity.

To authenticate a user inside a service, we implemented the AuthenticatedService abstract class, which has a method that returns the authenticated user, given a token. This class is extended by the GamesService and PlayersService.

Data Access Layer

The data access layer is implemented using Spring Data JPA.

This layer is responsible for managing the data access of the application, receiving the requests from the service layer, processing them, sending them to the database and returning the responses to the service layer. The data representation used in these requests and responses is the domain classes, which are implemented using Spring Data JPA.

Data Representation

There are three types of data representation in the application:

  • Models;
  • DTOs (Data Trasfer Objects);
  • Entities/Domain Classes.

The models are used in the presentation layer to represent the request and response bodies. This representation is used to validate the request and response bodies, and to convert them to the DTOs, which are used in the service layer. The validation is done using Jakaarta Bean Validation.

The DTOs are the data representation used in the communication between the presentation layer and the service layer. The DTOs are a "raw" representation of the data, and they are converted to the entities/domain classes.

The entities/domain classes are the data representation used in the communication between the service layer and the data access layer. The entities are implemented using Spring Data JPA.

Authentication

To perform some requests, like creating a game, the client needs to be authenticated. The authentication mechanism is implemented using JSON Web Tokens. The authentication process is the following:

  • Each user has an Access Token and one or more Refresh Tokens;
  • The access token is used to authenticate the user in the application, and it expires after a short period of time, like an hour;
  • The refresh token is used to generate a new access token, and it expires after a long period of time, like a day; these tokens are stored in the database;
  • When the access token expires, the client sends a request to the /user/refresh endpoint, sending the refresh token in the body of the request; the server generates a new access token and returns it to the client in the response.

To authenticate a user inside a controller, we implemented the @Authenticated annotation, which is used in controllers and handlers, indicating that the user must be authenticated to access the resource.

The AuthenticationInterceptor class implements the HandlerInterceptor interface, and is responsible for intercepting the requests and checking if the user is authenticated, if the @Authenticated annotation is present. This class has an override of the preHandle method, which is called before the request is processed by the controller.

Error Handling

To handle errors/exceptions, we implemented the ExceptionHandler class, which is annotated with @ControllerAdvice, and is responsible for intercepting the exceptions and returning the appropriate response, with the appropriate status code and message.

The service layer throws the specific exceptions that are handled by the ErrorHandler class, present in the /exceptions package. We decided to use the specific exceptions, instead of the generic exceptions (e.g. IllegalArgumentException), because we wanted to have more control over the error messages and status codes.

Generic exceptions are thrown when there's an illegal state or an unexpected error in the application, like a nullable game ID, or a database error. These exceptions are considered application errors, and need to be fixed by the developers.

The response format of the errors is the Problem Details for HTTP APIs. The response body is a JSON object with the following properties:

  • type - a URI that identifies the problem type;
  • title - a short, human-readable summary of the problem type;
  • status - the HTTP status code for this occurrence of the problem;
  • detail - a human-readable explanation specific to this occurrence of the problem;
  • instance - a URI reference that identifies the specific occurrence of the problem.

Running the Application

To run the application, you need to have Docker desktop installed, and run the following command:

docker-compose up

Conclusions - Critical Evaluation

The main goal of this phase was to implement the API of the Battleships application, and we think that we achieved this goal with success. It is concluded that all the requirements of the project were met and that the API is fully functional.

We faced some difficulties during the development of the data model, since we had to rethink and refactor it several times to achieve the desired result. We also had to refactor the http layer several times, since we had to change the format of the responses to the Siren hypermedia format.

The next steps for this project are:

  • Implement the web client of the Battleships application, that will be implemented in the next phase;
  • Change GameConfig weak entity to a strong entity to improve the data model, saving memory by not having to store the same data in multiple tables, like the default game configuration that is used in the most of the games;
  • Implement surrendering a game and the game's history;
  • Improve the error handling;
  • Improve token mechanism, by implementing a blacklist of revoked access tokens;
  • Implement private session mechanism.