#1284 Implement Version Number pattern

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

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+/*
+ * 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 {
+    long bookId = 1;
+
+    BookRepository bookRepository = new BookRepository();
+    Book 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 Book aliceBook = bookRepository.get(bookId);
+    final Book 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
+    }
+  }
+}