Version Number
- Concurrency
名字 / 分类
版本号
或称
实体版本控制，乐观锁。
目的
解决多个客户端尝试同时更新同一实体时的并发冲突。
解释
现实世界的例子
爱丽丝（Alice）和鲍勃（Bob）正在管理书，该书存储在数据库中。 我们的英雄们正在同时进行更改，我们需要某种机制来防止他们相互覆盖。
通俗地说
版本号模式可防止对同一实体进行并发更新。
维基百科说
乐观并发控制假设多个事务可以频繁完成而不会互相干扰。 在运行时，事务使用数据资源而不获取这些资源的锁。 在提交之前，每个事务都将验证没有其他事务修改了已读取的数据。如果检查发现有冲突的修改，则提交的事务将回滚并可以重新启动。
程序示例
我们有Book 已版本化的实体，它有一个复制构造函数。
public class Book {
private long id;
private String title = "";
private String author = "";
private long version = 0; // version number
public Book(Book book) {
this.id = book.id;
this.title = book.title;
this.author = book.author;
this.version = book.version;
}
// getters and setters are omitted here
}
我们还有一个 BookRepository, 它实现了并发控制。
public class BookRepository {
private final Map<Long, Book> collection = new HashMap<>();
public void update(Book book) throws BookNotFoundException, VersionMismatchException {
if (!collection.containsKey(book.getId())) {
throw new BookNotFoundException("Not found book with id: " + book.getId());
}
var latestBook = collection.get(book.getId());
if (book.getVersion() != latestBook.getVersion()) {
throw new VersionMismatchException(
"Tried to update stale version " + book.getVersion()
+ " while actual version is " + latestBook.getVersion()
);
}
// update version, including client representation - modify by reference here
book.setVersion(book.getVersion() + 1);
// save book copy to repository
collection.put(book.getId(), new Book(book));
}
public Book get(long bookId) throws BookNotFoundException {
if (!collection.containsKey(bookId)) {
throw new BookNotFoundException("Not found book with id: " + bookId);
}
// return copy of the book
return new Book(collection.get(bookId));
}
}
这是实践中的并发控制：
var bookId = 1;
// 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
}
程序输出：
Alice updates the book with new version 1
Bob tries to update the book with his version 0
Exception: Tried to update stale version 0 while actual version is 1
类图
适用性
将版本号用于：
- 解决对数据的并发写访问
- 强的数据一致性
教程
已知用途
意义
版本号模式允许实现并发控制，通常通过乐观离线锁模式来完成。