ステイル読み取り

public class BookDAO {

    // Omit some code...

    public List<Book> getTop5LatestBooks() throws SQLException {
        List<Book> books = new ArrayList<>();
        try (Connection conn = ds.getConnection()) {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("""
            SELECT id, title, type, price FROM books ORDER BY published_at DESC LIMIT 5;
            """);
            while (rs.next()) {
                Book book = new Book();
                book.setId(rs.getLong("id"));
                book.setTitle(rs.getString("title"));
                book.setType(rs.getString("type"));
                book.setPrice(rs.getDouble("price"));
                books.add(book);
            }
        }
        return books;
    }

    public void updateBookPriceByID(Long id, Double price) throws SQLException {
        try (Connection conn = ds.getConnection()) {
            PreparedStatement stmt = conn.prepareStatement("""
            UPDATE books SET price = ? WHERE id = ?;
            """);
            stmt.setDouble(1, price);
            stmt.setLong(2, id);
            int affects = stmt.executeUpdate();
            if (affects == 0) {
                throw new SQLException("Failed to update the book with id: " + id);
            }
        }
    }

    public List<Book> getTop5LatestBooksWithStaleRead(Integer seconds) throws SQLException {
        List<Book> books = new ArrayList<>();
        try (Connection conn = ds.getConnection()) {
            PreparedStatement stmt = conn.prepareStatement("""
            SELECT id, title, type, price FROM books AS OF TIMESTAMP NOW() - INTERVAL ? SECOND ORDER BY published_at DESC LIMIT 5;
            """);
            stmt.setInt(1, seconds);
            ResultSet rs = stmt.executeQuery();
            while (rs.next()) {
                Book book = new Book();
                book.setId(rs.getLong("id"));
                book.setTitle(rs.getString("title"));
                book.setType(rs.getString("type"));
                book.setPrice(rs.getDouble("price"));
                books.add(book);
            }
        } catch (SQLException e) {
            if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 1105) {
                System.out.println("WARN: cannot set read timestamp to a future time.");
            } else if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 9006) {
                System.out.println("WARN: GC life time is shorter than transaction duration.");
            } else {
                throw e;
            }
        }
        return books;
    }
}

List<Book> top5LatestBooks = bookDAO.getTop5LatestBooks();

if (top5LatestBooks.size() > 0) {
    System.out.println("The latest book price (before update): " + top5LatestBooks.get(0).getPrice());

    Book book = top5LatestBooks.get(0);
    bookDAO.updateBookPriceByID(book.getId(), book.price + 10);

    top5LatestBooks = bookDAO.getTop5LatestBooks();
    System.out.println("The latest book price (after update): " + top5LatestBooks.get(0).getPrice());

    // Use the stale read.
    top5LatestBooks = bookDAO.getTop5LatestBooksWithStaleRead(5);
    System.out.println("The latest book price (maybe stale): " + top5LatestBooks.get(0).getPrice());

    // Try to stale read the data at the future time.
    bookDAO.getTop5LatestBooksWithStaleRead(-5);

    // Try to stale read the data before 20 minutes.
    bookDAO.getTop5LatestBooksWithStaleRead(20 * 60);
}

次の結果は、 ステイル読み取りによって返された価格が更新前の値である 100.00 であることを示しています。

The latest book price (before update): 100.00
The latest book price (after update): 150.00
The latest book price (maybe stale): 100.00
WARN: cannot set read timestamp to a future time.
WARN: GC life time is shorter than transaction duration.

トランザクションのヘルパー クラスを定義して、トランザクション レベルでステイル読み取り を有効にするコマンドをヘルパー メソッドとしてカプセル化することができます。

public static class StaleReadHelper {

    public static void startTxnWithStaleRead(Connection conn, Integer seconds) throws SQLException {
        conn.setAutoCommit(false);
        PreparedStatement stmt = conn.prepareStatement(
            "START TRANSACTION READ ONLY AS OF TIMESTAMP NOW() - INTERVAL ? SECOND;"
        );
        stmt.setInt(1, seconds);
        stmt.execute();
    }

}

次に、 BookDAOクラスのトランザクションを通じてステイル読み取り機能を有効にするメソッドを定義します。クエリ ステートメントにAS OF TIMESTAMP追加する代わりに、メソッドを使用してクエリを実行します。

public class BookDAO {

    // Omit some code...

    public List<Book> getTop5LatestBooksWithTxnStaleRead(Integer seconds) throws SQLException {
        List<Book> books = new ArrayList<>();
        try (Connection conn = ds.getConnection()) {
            // Start a read only transaction.
            TxnHelper.startTxnWithStaleRead(conn, seconds);

            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("""
            SELECT id, title, type, price FROM books ORDER BY published_at DESC LIMIT 5;
            """);
            while (rs.next()) {
                Book book = new Book();
                book.setId(rs.getLong("id"));
                book.setTitle(rs.getString("title"));
                book.setType(rs.getString("type"));
                book.setPrice(rs.getDouble("price"));
                books.add(book);
            }

            // Commit transaction.
            conn.commit();
        } catch (SQLException e) {
            if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 1105) {
                System.out.println("WARN: cannot set read timestamp to a future time.");
            } else if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 9006) {
                System.out.println("WARN: GC life time is shorter than transaction duration.");
            } else {
                throw e;
            }
        }
        return books;
    }
}

List<Book> top5LatestBooks = bookDAO.getTop5LatestBooks();

if (top5LatestBooks.size() > 0) {
    System.out.println("The latest book price (before update): " + top5LatestBooks.get(0).getPrice());

    Book book = top5LatestBooks.get(0);
    bookDAO.updateBookPriceByID(book.getId(), book.price + 10);

    top5LatestBooks = bookDAO.getTop5LatestBooks();
    System.out.println("The latest book price (after update): " + top5LatestBooks.get(0).getPrice());

    // Use the stale read.
    top5LatestBooks = bookDAO.getTop5LatestBooksWithTxnStaleRead(5);
    System.out.println("The latest book price (maybe stale): " + top5LatestBooks.get(0).getPrice());

    // After the stale read transaction is committed.
    top5LatestBooks = bookDAO.getTop5LatestBooks();
    System.out.println("The latest book price (after the transaction commit): " + top5LatestBooks.get(0).getPrice());
}

結果は以下のようになります。

The latest book price (before update): 100.00
The latest book price (after update): 150.00
The latest book price (maybe stale): 100.00
The latest book price (after the transaction commit): 150

トランザクションのヘルパー クラスを定義して、トランザクション レベルでステイル読み取り を有効にするコマンドをヘルパー メソッドとしてカプセル化することができます。

public static class TxnHelper {

    public static void setTxnWithStaleRead(Connection conn, Integer seconds) throws SQLException {
        PreparedStatement stmt = conn.prepareStatement(
            "SET TRANSACTION READ ONLY AS OF TIMESTAMP NOW() - INTERVAL ? SECOND;"
        );
        stmt.setInt(1, seconds);
        stmt.execute();
    }

}

次に、 BookDAOクラスのトランザクションを通じてステイル読み取り機能を有効にするメソッドを定義します。クエリ ステートメントにAS OF TIMESTAMP追加する代わりに、メソッドを使用してクエリを実行します。

public class BookDAO {

    // Omit some code...

    public List<Book> getTop5LatestBooksWithTxnStaleRead2(Integer seconds) throws SQLException {
        List<Book> books = new ArrayList<>();
        try (Connection conn = ds.getConnection()) {
            StaleReadHelper.setTxnWithStaleRead(conn, seconds);

            // Start a read only transaction.
            conn.setAutoCommit(false);

            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("""
            SELECT id, title, type, price FROM books ORDER BY published_at DESC LIMIT 5;
            """);
            while (rs.next()) {
                Book book = new Book();
                book.setId(rs.getLong("id"));
                book.setTitle(rs.getString("title"));
                book.setType(rs.getString("type"));
                book.setPrice(rs.getDouble("price"));
                books.add(book);
            }

            // Commit transaction.
            conn.commit();
        } catch (SQLException e) {
            if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 1105) {
                System.out.println("WARN: cannot set read timestamp to a future time.");
            } else if ("HY000".equals(e.getSQLState()) && e.getErrorCode() == 9006) {
                System.out.println("WARN: GC life time is shorter than transaction duration.");
            } else {
                throw e;
            }
        }
        return books;
    }
}

public static class StaleReadHelper{

    public static void enableStaleReadOnSession(Connection conn, Integer seconds) throws SQLException {
        PreparedStatement stmt = conn.prepareStatement(
            "SET @@tidb_read_staleness= ?;"
        );
        stmt.setString(1, String.format("-%d", seconds));
        stmt.execute();
    }

    public static void disableStaleReadOnSession(Connection conn) throws SQLException {
        PreparedStatement stmt = conn.prepareStatement(
            "SET @@tidb_read_staleness=\"\";"
        );
        stmt.execute();
    }

}