java-design-patterns/version-number/src/main/java/com/iluwatar/versionnumber/App.java

/*
 * The MIT License
 * Copyright © 2014-2019 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package com.iluwatar.versionnumber;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * The Version Number pattern helps to resolve concurrency conflicts in applications.
 * Usually these conflicts arise in database operations, when multiple clients are trying
 * to update the same record simultaneously.
 * Resolving such conflicts requires determining whether an object has changed.
 * For this reason we need a version number that is incremented with each change
 * to the underlying data, e.g. database. The version number can be used by repositories
 * to check for external changes and to report concurrency issues to the users.
 *
 * <p>In this example we show how Alice and Bob will try to update the {@link Book}
 * and save it simultaneously to {@link BookRepository}, which represents a typical database.
 *
 * <p>As in real databases, each client operates with copy of the data instead of original data
 * passed by reference, that's why we are using {@link Book} copy-constructor here.
 */
public class App {
  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

  /**
   * Program entry point.
   *
   * @param args command line args
   */
  public static void main(String[] args) throws
      BookDuplicateException,
      BookNotFoundException,
      VersionMismatchException {
    var bookId = 1;

    var bookRepository = new BookRepository();
    var book = new Book();
    book.setId(bookId);
    bookRepository.add(book); // adding a book with empty title and author
    LOGGER.info("An empty book with version {} was added to repository", book.getVersion());

    // Alice and Bob took the book concurrently
    final var aliceBook = bookRepository.get(bookId);
    final var bobBook = bookRepository.get(bookId);

    aliceBook.setTitle("Kama Sutra"); // Alice has updated book title
    bookRepository.update(aliceBook); // and successfully saved book in database
    LOGGER.info("Alice updates the book with new version {}", aliceBook.getVersion());

    // now Bob has the stale version of the book with empty title and version = 0
    // while actual book in database has filled title and version = 1
    bobBook.setAuthor("Vatsyayana Mallanaga"); // Bob updates the author
    try {
      LOGGER.info("Bob tries to update the book with his version {}", bobBook.getVersion());
      bookRepository.update(bobBook); // Bob tries to save his book to database
    } catch (VersionMismatchException e) {
      // Bob update fails, and book in repository remained untouchable
      LOGGER.info("Exception: {}", e.getMessage());
      // Now Bob should reread actual book from repository, do his changes again and save again
    }
  }
}