基于Mysql的Sequence实现

团队更换新框架。新的业务全部使用新的框架，甚至是新的数据库--Mysql。
这边之前一直是使用oracle，各种订单号、流水号、批次号啥的，都是直接使用oracle的sequence提供的数字序列号。现在数据库更换成Mysql了，显然以前的老方法不能适用了。
需要新写一个：

• 分布式场景使用
• 满足一定的并发要求
  找了一些相关的资料，发现mysql这方面的实现，原理都是一条数据库记录，不断update它的值。然后大部分的实现方案，都用到了函数。
  贴一下网上的代码：

基于mysql函数实现

表结构

CREATE TABLE `t_sequence` (
`sequence_name`  varchar(64) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '序列名称' ,
`value`  int(11) NULL DEFAULT NULL COMMENT '当前值' ,
PRIMARY KEY (`sequence_name`)
)
ENGINE=InnoDB
DEFAULT CHARACTER SET=utf8 COLLATE=utf8_general_ci
ROW_FORMAT=COMPACT
;

获取下一个值

CREATE DEFINER = `root`@`localhost` FUNCTION `nextval`(sequence_name varchar(64))
 RETURNS int(11)
BEGIN
    declare current integer;
    set current = 0;
    
    update t_sequence t set t.value = t.value + 1 where t.sequence_name = sequence_name;
    select t.value into current from t_sequence t where t.sequence_name = sequence_name;

    return current;
end;

并发场景有可能会出问题，虽然可以在业务层加锁，但分布式场景就无法保证了，�然后效率应该也不会高。
自己实现一个，java版
原理：

• 读取一条记录，缓存一个数据段，如：0-100，将记录的当前值从0修改为100
• 数据库乐观锁更新，允许重试
• 读取数据从缓存中读取，用完再读取数据库
  不废话，上代码：

基于java实现

表结构

每次update，都是将SEQ_VALUE设置为SEQ_VALUE+STEP

CREATE TABLE `t_pub_sequence` (
  `SEQ_NAME` varchar(128) CHARACTER SET utf8 NOT NULL COMMENT '序列名称',
  `SEQ_VALUE` bigint(20) NOT NULL COMMENT '目前序列值',
  `MIN_VALUE` bigint(20) NOT NULL COMMENT '最小值',
  `MAX_VALUE` bigint(20) NOT NULL COMMENT '最大值',
  `STEP` bigint(20) NOT NULL COMMENT '每次取值的数量',
  `TM_CREATE` datetime NOT NULL COMMENT '创建时间',
  `TM_SMP` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
  PRIMARY KEY (`SEQ_NAME`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='流水号生成表';

sequence接口

/**
 * <p></p>
 * @author coderzl
 * @Title MysqlSequence
 * @Description 基于mysql数据库实现的序列
 * @date 2017/6/6 23:03
 */
public interface MysqlSequence {
    /**
     * <p>
     * 获取指定sequence的序列号
     * </p>
     * @param  seqName sequence名
     * @return String 序列号
     */
    public String nextVal(String seqName);
}

序列区间

用于本地缓存一段序列，从min到max区间

/**
 * <p></p>
 *
 * @author coderzl
 * @Title SequenceRange
 * @Description 序列区间，用于缓存序列
 * @date 2017/6/6 22:58
 */
 @Data
public class SequenceRange {
    private final long       min;
    private final long       max;
    /**  */
    private final AtomicLong value;
    /** 是否超限 */
    private volatile boolean over = false;

    /**
     * 构造.
     *
     * @param min 
     * @param max 
     */
    public SequenceRange(long min, long max) {
        this.min = min;
        this.max = max;
        this.value = new AtomicLong(min);
    }

    /**
     * <p>Gets and increment</p>
     *
     * @return 
     */
    public long getAndIncrement() {
        long currentValue = value.getAndIncrement();
        if (currentValue > max) {
            over = true;
            return -1;
        }

        return currentValue;
    }

}   

BO

对应数据库记录

@Data
public class MysqlSequenceBo {
    /**
     * seq名
     */
    private String seqName;
    /**
     * 当前值
     */
    private Long seqValue;
    /**
     * 最小值
     */
    private Long minValue;
    /**
     * 最大值
     */
    private Long maxValue;
    /**
     * 每次取值的数量
     */
    private Long step;
    /**  */
    private Date tmCreate;
    /**  */
    private Date tmSmp;

    public boolean validate(){
      //一些简单的校验。如当前值必须在最大最小值之间。step值不能大于max与min的差
      if (StringUtil.isBlank(seqName) || minValue < 0 || maxValue <= 0 || step <= 0 || minValue >= maxValue || maxValue - minValue <= step ||seqValue < minValue || seqValue > maxValue ) {
            return false;
        }
        return true;  
    }
}    

DAO

增删改查，其实就用到了改和查

public interface MysqlSequenceDAO {
    /**
    * 
    */
    public int createSequence(MysqlSequenceBo bo);

    public int updSequence(@Param("seqName") String seqName, @Param("oldValue") long oldValue ,@Param("newValue") long newValue);

    public int delSequence(@Param("seqName") String seqName);

    public MysqlSequenceBo getSequence(@Param("seqName") String seqName);

    public List<MysqlSequenceBo> getAll();
}

Mapper

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.xxxxx.core.sequence.impl.dao.MysqlSequenceDAO" >
    <resultMap id="BaseResultMap" type="com.xxxxx.core.sequence.impl.MysqlSequenceBo" >
        <result column="SEQ_NAME" property="seqName" jdbcType="VARCHAR" />
        <result column="SEQ_VALUE" property="seqValue" jdbcType="BIGINT" />
        <result column="MIN_VALUE" property="minValue" jdbcType="BIGINT" />
        <result column="MAX_VALUE" property="maxValue" jdbcType="BIGINT" />
        <result column="STEP" property="step" jdbcType="BIGINT" />
        <result column="TM_CREATE" property="tmCreate" jdbcType="TIMESTAMP" />
        <result column="TM_SMP" property="tmSmp" jdbcType="TIMESTAMP" />
    </resultMap>
    <delete id="delSequence" parameterType="java.lang.String" >
        delete from t_pub_sequence
        where SEQ_NAME = #{seqName,jdbcType=VARCHAR}
    </delete>
    <insert id="createSequence" parameterType="com.xxxxx.core.sequence.impl.MysqlSequenceBo" >
        insert into t_pub_sequence (SEQ_NAME,SEQ_VALUE,MIN_VALUE,MAX_VALUE,STEP,TM_CREATE)
        values (#{seqName,jdbcType=VARCHAR}, #{seqValue,jdbcType=BIGINT},
        #{minValue,jdbcType=BIGINT}, #{maxValue,jdbcType=BIGINT}, #{step,jdbcType=BIGINT},
        now())
    </insert>
    <update id="updSequence" parameterType="com.xxxxx.core.sequence.impl.MysqlSequenceBo" >
        update t_pub_sequence
        set SEQ_VALUE = #{newValue,jdbcType=BIGINT}
        where SEQ_NAME = #{seqName,jdbcType=VARCHAR} and SEQ_VALUE = #{oldValue,jdbcType=BIGINT}
    </update>

    <select id="getAll" resultMap="BaseResultMap" >
        select SEQ_NAME, SEQ_VALUE, MIN_VALUE, MAX_VALUE, STEP
        from t_pub_sequence
    </select>

    <select id="getSequence" resultMap="BaseResultMap" >
        select SEQ_NAME, SEQ_VALUE, MIN_VALUE, MAX_VALUE, STEP
        from t_pub_sequence
        where SEQ_NAME = #{seqName,jdbcType=VARCHAR}
    </select>
</mapper>

接口实现

@Repository("mysqlSequence")
public class MysqlSequenceImpl implements MysqlSequence{

    @Autowired
    private MysqlSequenceFactory mysqlSequenceFactory;
    /**
     * <p>
     * 获取指定sequence的序列号
     * </p>
     *
     * @param seqName sequence名
     * @return String 序列号
     * @author coderzl
     */
    @Override
    public String nextVal(String seqName) {
        return Objects.toString(mysqlSequenceFactory.getNextVal(seqName));
    }
}

工厂

工厂只做了两件事

• 服务启动的时候，初始化数据库中所有sequence【完成序列区间缓存】
• 获取sequence的下一个值

@Component
public class MysqlSequenceFactory {

    private final Lock lock = new ReentrantLock();

    /**  */
    private Map<String,MysqlSequenceHolder> holderMap = new ConcurrentHashMap<>();

    @Autowired
    private MysqlSequenceDAO msqlSequenceDAO;
    /** 单个sequence初始化乐观锁更新失败重试次数 */
    @Value("${seq.init.retry:5}")
    private int initRetryNum;
    /** 单个sequence更新序列区间乐观锁更新失败重试次数 */
    @Value("${seq.get.retry:20}")
    private int getRetryNum;

    @PostConstruct
    private void init(){
        //初始化所有sequence
        initAll();
    }


    /**
     * <p> 加载表中所有sequence，完成初始化 </p>
     * @return void
     * @author coderzl
     */
    private void initAll(){
        try {
            lock.lock();
            List<MysqlSequenceBo> boList = msqlSequenceDAO.getAll();
            if (boList == null) {
                throw new IllegalArgumentException("The sequenceRecord is null!");
            }
            for (MysqlSequenceBo bo : boList) {
                MysqlSequenceHolder holder = new MysqlSequenceHolder(msqlSequenceDAO, bo,initRetryNum,getRetryNum);
                holder.init();
                holderMap.put(bo.getSeqName(), holder);
            }
        }finally {
            lock.unlock();
        }
    }


    /**
     * <p>  </p>
     * @param seqName
     * @return long
     * @author coderzl
     */
    public long getNextVal(String seqName){
        MysqlSequenceHolder holder = holderMap.get(seqName);
        if (holder == null) {
            try {
                lock.lock();
                holder = holderMap.get(seqName);
                if (holder != null){
                    return holder.getNextVal();
                }
                MysqlSequenceBo bo = msqlSequenceDAO.getSequence(seqName);
                holder = new MysqlSequenceHolder(msqlSequenceDAO, bo,initRetryNum,getRetryNum);
                holder.init();
                holderMap.put(seqName, holder);
            }finally {
               lock.unlock();
            }
        }
        return holder.getNextVal();
    }

}

单一sequence的Holder

• init() 初始化 其中包括参数校验，数据库记录更新，创建序列区间
• getNextVal() 获取下一个值

public class MysqlSequenceHolder {

    private final Lock lock                = new ReentrantLock();

    /** seqName */
    private String seqName;

    /** sequenceDao */
    private MysqlSequenceDAO sequenceDAO;

    private MysqlSequenceBo sequenceBo;
    /**  */
    private SequenceRange sequenceRange;
    /** 是否初始化 */
    private volatile boolean       isInitialize      = false;
    /** sequence初始化重试次数 */
    private int initRetryNum;
    /** sequence获取重试次数 */
    private int getRetryNum;

    /**
     * <p> 构造方法 </p>
     * @Title MysqlSequenceHolder
     * @param sequenceDAO 
     * @param sequenceBo
     * @param initRetryNum 初始化时，数据库更新失败后重试次数
     * @param getRetryNum 获取nextVal时，数据库更新失败后重试次数
     * @return
     * @author coderzl
     */
    public MysqlSequenceHolder(MysqlSequenceDAO sequenceDAO, MysqlSequenceBo sequenceBo,int initRetryNum,int getRetryNum) {
        this.sequenceDAO = sequenceDAO;
        this.sequenceBo = sequenceBo;
        this.initRetryNum = initRetryNum;
        this.getRetryNum = getRetryNum;
        if(sequenceBo != null)
            this.seqName = sequenceBo.getSeqName();
    }

    /**
     * <p> 初始化 </p>
     * @Title init
     * @param
     * @return void
     * @author coderzl
     */
    public void init(){
        if (isInitialize == true) {
            throw new SequenceException("[" + seqName + "] the MysqlSequenceHolder has inited");
        }
        if (sequenceDAO == null) {
            throw new SequenceException("[" + seqName + "] the sequenceDao is null");
        }
        if (seqName == null || seqName.trim().length() == 0) {
            throw new SequenceException("[" + seqName + "] the sequenceName is null");
        }
        if (sequenceBo == null) {
            throw new SequenceException("[" + seqName + "] the sequenceBo is null");
        }
        if (!sequenceBo.validate()){
            throw new SequenceException("[" + seqName + "] the sequenceBo validate fail. BO:"+sequenceBo);
        }
        // 初始化该sequence
        try {
            initSequenceRecord(sequenceBo);
        } catch (SequenceException e) {
            throw e;
        }
        isInitialize = true;
    }

    /**
     * <p> 获取下一个序列号 </p>
     * @Title getNextVal
     * @param
     * @return long
     * @author coderzl
     */
    public long getNextVal(){
        if(isInitialize == false){
            throw new SequenceException("[" + seqName + "] the MysqlSequenceHolder not inited");
        }
        if(sequenceRange == null){
            throw new SequenceException("[" + seqName + "] the sequenceRange is null");
        }
        long curValue = sequenceRange.getAndIncrement();

        if(curValue == -1){
            try{
                lock.lock();
                curValue = sequenceRange.getAndIncrement();
                if(curValue != -1){
                    return curValue;
                }
                sequenceRange = retryRange();
                curValue = sequenceRange.getAndIncrement();
            }finally {
                lock.unlock();
            }
        }
        return curValue;
    }

    /**
     * <p> 初始化当前这条记录 </p>
     * @Title initSequenceRecord
     * @Description
     * @param sequenceBo
     * @return void
     * @author coderzl
     */
    private void initSequenceRecord(MysqlSequenceBo sequenceBo){
        //在限定次数内，乐观锁更新数据库记录
        for(int i = 1; i < initRetryNum; i++){
            //查询bo
            MysqlSequenceBo curBo = sequenceDAO.getSequence(sequenceBo.getSeqName());
            if(curBo == null){
                throw new SequenceException("[" + seqName + "] the current sequenceBo is null");
            }
            if (!curBo.validate()){
                throw new SequenceException("[" + seqName + "] the current sequenceBo validate fail");
            }
            //改变当前值
            long newValue = curBo.getSeqValue()+curBo.getStep();
            //检查当前值
            if(!checkCurrentValue(newValue,curBo)){
                newValue = resetCurrentValue(curBo);
            }
            int result = sequenceDAO.updSequence(sequenceBo.getSeqName(),curBo.getSeqValue(),newValue);
            if(result > 0){
                sequenceRange = new SequenceRange(curBo.getSeqValue(),newValue - 1);
                curBo.setSeqValue(newValue);
                this.sequenceBo = curBo;
                return;
            }else{
                continue;
            }
        }
        //限定次数内，更新失败，抛出异常
        throw new SequenceException("[" + seqName + "]  sequenceBo update error");
    }

    /**
     * <p> 检查新值是否合法 新的当前值是否在最大最小值之间</p>
     * @param curValue
     * @param curBo
     * @return boolean
     * @author coderzl
     */
    private boolean checkCurrentValue(long curValue,MysqlSequenceBo curBo){
        if(curValue > curBo.getMinValue() && curValue <= curBo.getMaxValue()){
            return true;
        }
        return false;
    }

    /**
     * <p> 重置sequence当前值 ：当前sequence达到最大值时，重新从最小值开始 </p>
     * @Title resetCurrentValue
     * @param curBo
     * @return long
     * @author coderzl
     */
    private long resetCurrentValue(MysqlSequenceBo curBo){
        return curBo.getMinValue();
    }

    /**
     * <p> 缓存区间使用完毕时，重新读取数据库记录，缓存新序列段 </p>
     * @Title retryRange
     * @param SequenceRange
     * @author coderzl
     */
    private SequenceRange retryRange(){
        for(int i = 1; i < getRetryNum; i++){
            //查询bo
            MysqlSequenceBo curBo = sequenceDAO.getSequence(sequenceBo.getSeqName());
            if(curBo == null){
                throw new SequenceException("[" + seqName + "] the current sequenceBo is null");
            }
            if (!curBo.validate()){
                throw new SequenceException("[" + seqName + "] the current sequenceBo validate fail");
            }
            //改变当前值
            long newValue = curBo.getSeqValue()+curBo.getStep();
            //检查当前值
            if(!checkCurrentValue(newValue,curBo)){
                newValue = resetCurrentValue(curBo);
            }
            int result = sequenceDAO.updSequence(sequenceBo.getSeqName(),curBo.getSeqValue(),newValue);
            if(result > 0){
                sequenceRange = new SequenceRange(curBo.getSeqValue(),newValue - 1);
                curBo.setSeqValue(newValue);
                this.sequenceBo = curBo;
                return sequenceRange;
            }else{
                continue;
            }
        }
        throw new SequenceException("[" + seqName + "]  sequenceBo update error");

    }
}

总结

• 当服务重启或异常的时候，会丢失当前服务所缓存且未用完的序列
• 分布式场景，多个服务同时初始化，或者重新获取sequence时，乐观锁会保证彼此不冲突。A服务获取0-99，B服务会获取100-199，以此类推
• 当该sequence获取较为频繁时，增大step值，能提升性能。但同时服务异常时，损失的序列也较多
• 修改数据库里sequence的一些属性值，比如step，max等，再下一次从数据库获取时，会启用新的参数
• sequence只是提供了有限个序列号（最多max-min个），达到max后，会循环从头开始。
• 由于sequence会循环，所以达到max后，再获取，就不会唯一。建议使用sequence来做业务流水号时，拼接时间。如：20170612235101+序列号

业务id拼接方法

@Service
public class JrnGeneratorService {
  private static final String  SEQ_NAME = "T_SEQ_TEST";

  /** sequence服务 */
  @Autowired
  private MySqlSequence mySqlSequence;
    
    public String generateJrn() {
        try {
            String sequence = mySqlSequence.getNextValue(SEQ_NAME);
            sequence  = leftPadding(sequence,8);
            Calendar calendar = Calendar.getInstance();
            SimpleDateFormat sDateFormat = new SimpleDateFormat("yyyyMMddHHmmss");
            String nowdate = sDateFormat.format(calendar.getTime());
            nowdate.substring(4, nowdate.length());
            String jrn = nowdate + sequence + RandomUtil.getFixedLengthRandom(6);//10位时间＋8位序列 + 6位随机数＝24位流水号
            return jrn;
        } catch (Exception e) {
            //TODO
        }
    }
    
    private String leftPadding(String seq,int len){
        String res  ="";
        String str ="";
        if(seq.length()<len){
            for(int i=0;i<len-seq.length();i++){
                str +="0";  
            }           
        }
        res  =str+seq;
        return res;
        
    }

}