TDDL生成全局ID原理

TDDL 在分布式下的SEQUENCE原理

TDDL大家应该很熟悉了，淘宝分布式数据层。很好的为我们实现了分库分表、Master/Salve、动态数据源配置等功能。

那么分布式之后，数据库自增序列肯定用不了了，如何方便快捷的解决这个问题呢？TDDL也提供了SEQUENCE的解决方案。

总述

在数据库中创建 sequence 表，用于记录，当前已被占用的id最大值。

每台客户端主机取一个id区间（比如 1000~2000）缓存在本地，并更新 sequence 表中的id最大值记录。

客户端主机之间取不同的id区间，用完再取，使用乐观锁机制控制并发。

第一步：创建一张sequence对应的表

CREATE TABLE `imp_sequence` (
  `BIZ_NAME` varchar(45) NOT NULL COMMENT '业务名称',
  `CURRENT_VALUE` int(11) NOT NULL COMMENT '当前最大值',
  `GMT_CREATE` datetime DEFAULT NULL COMMENT '创建时间',
  `GMT_MODIFIED` datetime DEFAULT NULL COMMENT '修改时间',
  PRIMARY KEY (`BIZ_NAME`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='数据序列表';

表名和字段可以按各自规则定义，定义之后需要与第二步DAO中的定义相对应！

几张逻辑表需要声明几个sequence。

第二步：配置sequenceDao

<bean id="sequenceDao" class="com.taobao.tddl.client.sequence.impl.DefaultSequenceDao">
        <!-- 数据源 -->
        <property name="dataSource"  ref="dataSource" />
        <!-- 步长-->
        <property name="step" value="1000" />
        <!-- 重试次数-->
        <property name="retryTimes" value="1" />
        <!-- sequence 表名-->
        <property name="tableName" value="gt_sequence" />
        <!-- sequence 名称-->
        <property name="nameColumnName" value="BIZ_NAME" />
        <!-- sequence 当前值-->
        <property name="valueColumnName" value="CURRENT_VALUE" />
        <!-- sequence 更新时间-->
        <property name="gmtModifiedColumnName" value="gmt_modified" />
</bean>

第三步：配置sequence生成器

<bean id="businessSequence"  class="com.taobao.tddl.client.sequence.impl.DefaultSequence">
        <property name="sequenceDao" ref="sequenceDao"/>
        <property name="name" value="business_sequence" />
</bean>

第四步：调用

public class IbatisSmDAO extends SqlMapClientDaoSupport implements SmDAO {

  /**smSequence*/
  private DefaultSequence   businessSequence;

    public int insert(SmDO sm) throws DataAccessException {
        if (sm == null) {
            throw new IllegalArgumentException("Can't insert a null data object into db.");
        }

        try {
            sm.setId((int)businessSequence.nextValue());
        } catch (SequenceException e) {
            throw new RuntimeException("Can't get primary key.");
        }

        getSqlMapClientTemplate().insert("MS-SM-INSERT", sm);

        return sm.getId();
    }
}

从调用配置中，我们可以发现其中涉及到二个重要类DefaultSequenceDao和DefaultSequence，这二个都是TDDL的默认实现。DefaultSequenceDao：序列DAO默认实现，JDBC方式。DefaultSequence：序列默认实现。

先来看DefaultSequenceDao，TDDL中提供了默认的表名，列名和步长等，第一步的建表可以参照默认方式。

private static final int MIN_STEP = 1;
    private static final int MAX_STEP = 100000;
    private static final int DEFAULT_STEP = 1000;
    private static final int DEFAULT_RETRY_TIMES = 150;

    private static final String DEFAULT_TABLE_NAME = "sequence";
    private static final String DEFAULT_NAME_COLUMN_NAME = "name";
    private static final String DEFAULT_VALUE_COLUMN_NAME = "value";
    private static final String DEFAULT_GMT_MODIFIED_COLUMN_NAME = "gmt_modified";

    private static final long DELTA = 100000000L;

    private DataSource dataSource;

    /**
     * 重试次数
     */
    private int retryTimes = DEFAULT_RETRY_TIMES;

    /**
     * 步长
     */
    private int step = DEFAULT_STEP;

    /**
     * 序列所在的表名
     */
    private String tableName = DEFAULT_TABLE_NAME;

    /**
     * 存储序列名称的列名
     */
    private String nameColumnName = DEFAULT_NAME_COLUMN_NAME;

    /**
     * 存储序列值的列名
     */
    private String valueColumnName = DEFAULT_VALUE_COLUMN_NAME;

    /**
     * 存储序列最后更新时间的列名
     */
    private String gmtModifiedColumnName = DEFAULT_GMT_MODIFIED_COLUMN_NAME;

接下来看一下nextRange方法：取得下一个可用的序列区间：

public SequenceRange nextRange(String name) throws SequenceException {
        if (name == null) {
            throw new IllegalArgumentException("序列名称不能为空");
        }

        long oldValue;
        long newValue;

        Connection conn = null;
        PreparedStatement stmt = null;
        ResultSet rs = null;

        for (int i = 0; i < retryTimes + 1; ++i) {
            try {
                conn = dataSource.getConnection();
                stmt = conn.prepareStatement(getSelectSql());
                stmt.setString(1, name);
                rs = stmt.executeQuery();
                rs.next();
                oldValue = rs.getLong(1);

                if (oldValue < 0) {
                    StringBuilder message = new StringBuilder();
                    message.append("Sequence value cannot be less than zero, value = ").append(oldValue);
                    message.append(", please check table ").append(getTableName());

                    throw new SequenceException(message.toString());
                }

                if (oldValue > Long.MAX_VALUE - DELTA) {
                    StringBuilder message = new StringBuilder();
                    message.append("Sequence value overflow, value = ").append(oldValue);
                    message.append(", please check table ").append(getTableName());

                    throw new SequenceException(message.toString());
                }

                newValue = oldValue + getStep();
            } catch (SQLException e) {
                throw new SequenceException(e);
            } finally {
                closeResultSet(rs);
                rs = null;
                closeStatement(stmt);
                stmt = null;
                closeConnection(conn);
                conn = null;
            }

            try {
                conn = dataSource.getConnection();
                stmt = conn.prepareStatement(getUpdateSql());
                stmt.setLong(1, newValue);
                stmt.setTimestamp(2, new Timestamp(System.currentTimeMillis()));
                stmt.setString(3, name);
                stmt.setLong(4, oldValue);
                int affectedRows = stmt.executeUpdate();
                if (affectedRows == 0) {
                    // retry
                    continue;
                }

                return new SequenceRange(oldValue + 1, newValue);
            } catch (SQLException e) {
                throw new SequenceException(e);
            } finally {
                closeStatement(stmt);
                stmt = null;
                closeConnection(conn);
                conn = null;
            }
        }

        throw new SequenceException("Retried too many times, retryTimes = " + retryTimes);
    }

通过getSelectSql查询最新的value值，然后加上步点，通过getUpdateSql更新到数据库中

private String getSelectSql() {
        if (selectSql == null) {
            synchronized (this) {
                if (selectSql == null) {
                    StringBuilder buffer = new StringBuilder();
                    buffer.append("select ").append(getValueColumnName());
                    buffer.append(" from ").append(getTableName());
                    buffer.append(" where ").append(getNameColumnName()).append(" = ?");

                    selectSql = buffer.toString();
                }
            }
        }

        return selectSql;
    }

    private String getUpdateSql() {
        if (updateSql == null) {
            synchronized (this) {
                if (updateSql == null) {
                    StringBuilder buffer = new StringBuilder();
                    buffer.append("update ").append(getTableName());
                    buffer.append(" set ").append(getValueColumnName()).append(" = ?, ");
                    buffer.append(getGmtModifiedColumnName()).append(" = ? where ");
                    buffer.append(getNameColumnName()).append(" = ? and ");
                    buffer.append(getValueColumnName()).append(" = ?");

                    updateSql = buffer.toString();
                }
            }
        }

        return updateSql;
    }

有一个特殊需要说明的，在update语句中，where需要把之前的value当成条件传入。实现了类型version的乐观锁操作。如果同一个时间AB二台机器同时请求获取到相同的value，进行update操作只有可能一条成功。失败的会按retryTimes进行重试。

接下来看DefaultSequence，比较简单，就不说明了

public class DefaultSequence implements Sequence {
    private final Lock lock = new ReentrantLock();

    private SequenceDao sequenceDao;

    /**
     * 序列名称
     */
    private String name;

    private volatile SequenceRange currentRange;

    public long nextValue() throws SequenceException {
        if (currentRange == null) {
            lock.lock();
            try {
                if (currentRange == null) {
                    currentRange = sequenceDao.nextRange(name);
                }
            } finally {
                lock.unlock();
            }
        }

        long value = currentRange.getAndIncrement();
        if (value == -1) {
            lock.lock();
            try {
                for (;;) {
                    if (currentRange.isOver()) {
                        currentRange = sequenceDao.nextRange(name);
                    }

                    value = currentRange.getAndIncrement();
                    if (value == -1) {
                        continue;
                    }

                    break;
                }
            } finally {
                lock.unlock();
            }
        }

        if (value < 0) {
            throw new SequenceException("Sequence value overflow, value = " + value);
        }

        return value;
    }

    public SequenceDao getSequenceDao() {
        return sequenceDao;
    }

    public void setSequenceDao(SequenceDao sequenceDao) {
        this.sequenceDao = sequenceDao;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}